11institutetext:  1Department of Physics and Astronomy, The Open University,
Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
2Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
3DFISTS, EPS, Universidad de Alicante, Carretera San Vicente del Raspeig s/n,
E-03690 San Vicente del Raspeig, Spain
4Departamento de Astrofísica, Centro de Astrobiología, (CSIC-INTA), Ctra.
Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
5UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill,
Edinburgh EH9 3HJ, UK

# Constraining the population of isolated massive stars within the Central
Molecular Zone. ††thanks: Based on observations made at the European Southern
Observatory, Paranal, Chile under programmes ESO 093.D-0168

J. S. Clark 11 L. R. Patrick 2233 F. Najarro 44 C. J. Evans 55 M. Lohr 11

###### Abstract

Context. Many galaxies host pronounced circumnuclear starbursts, fuelled by
infalling gas. Such activity is expected to drive the secular evolution of the
nucleus and generate super winds, while the intense radiation fields and
extreme gas and cosmic ray densities present may act to modify the outcome of
star formation with respect to more quiescent galactic regions.

Aims. The centre of the Milky Way is the only example of this phenomenon
where, by virtue of its proximity, individual stars may be resolved. Previous
studies reveal that it hosts a rich population of very massive stars; these
are located within three clusters, with an additional cohort dispersed
throughout the Central Molecular Zone. In this paper we investigate the size
and composition of the latter contingent.

Methods. We utilised the VLT+KMOS to obtain homogeneous, high signal-to-noise
ratio observations of known and candidate massive stars suitable for spectral
classification and quantitative analysis.

Results. Including previously identified examples, we find a total of 83
isolated massive stars within the Galactic Centre, which are strongly biased
towards objects supporting powerful stellar winds and/or extensive
circumstellar envelopes. No further stellar clusters, or their tidally
stripped remnants, were identified, although an apparent stellar overdensity
is found to be coincident with the Sgr B1 star forming region.

Conclusions. The cohort of isolated massive stars within the Central Molecular
Zone is directly comparable in size to that of the known clusters and, due to
observational biases, is likely highly incomplete at this time. Combining both
populations yields $\gtrsim 320$ spectroscopically classified stars that would
be expected to undergo core collapse within the next $\sim 20$Myr. Given that
this is almost certainly a substantial underestimate of the true number, the
population of massive stars associated with the Galactic Centre appears
unprecedented amongst star formation complexes within the Milky Way, and it
appears probable that they play a substantial role in the energetics and
evolution of the nuclear region.

###### Key Words.:

stars:evolution - stars:massive - Galaxy:nucleus

##  1 Introduction

While many galaxies host prominent (circum-)nuclear starbursts, the physics
governing their formation and subsequent contribution to the wider galactic
ecology and energy budget is currently opaque due to our inability to resolve
individual stars in such environments at extragalactic distances. Indeed there
is only one example of this phenomenon - the central region of our own Galaxy
- where this is currently possible. Multiwavelength observations have revealed
that physical conditions in the Galactic Centre (GC) are particularly extreme
with respect to the disc, bearing close resemblance to those anticipated for
high redshift starburst galaxies (Kruijssen & Longmore dkl (2013)), hence the
hope that the GC will act as a template for such objects. Specifically, the
mean temperature, density, pressure, and velocity dispersion of molecular
material; the magnetic field strength; the cosmic ray density; and the
ionisation rate are significantly greater than those found in the Galactic
disc, in some cases by orders of magnitude. As such, one might anticipate that
processes such as star formation proceed in a different manner than in more
quiescent regions of the Milky Way.

It is important to determine if this is the case. Encompassing the inner $\sim
500$pc of the GC, the Central Molecular Zone (CMZ) contains up to $\sim 10$%
of the molecular mass of the Galaxy ($\sim 2-6\times 10^{7}M_{\odot}$; Morris
& Serabyn MS (1996)), which in turn fuels the most extreme star forming region
within the Milky Way. Observations from sub-mm to radio wavelengths suggest
this activity is occurring at multiple locations within the H ii regions
populating the GC; within $\sim 6$pc of Sgr A∗ (Yusef-Zadeh et al. yz10
(2009)), Sgr B1+2 (e.g. Ginsburg et al. 2018a , Hankins et al. hankins
(2020)), and Sgr C (Kendrew et al. kendrew (2013)), as well as the dust ridge
linking Sgr A to Sgr B (Immer et al. 2012b ) and mid-IR hotspots associated
with ionised gas between Sgr A and C (Hankins et al. hankins (2020)).
Additional mid-IR surveys have identified numerous isolated point sources
distributed throughout the CMZ with properties that are consistent with young
stellar objects (Yusef-Zadeh et al. yz09 (2009), An et al. an (2011), Immer et
al. 2012a ). A compact aggregate of such sources $\sim 8$arcmin north of the
Sgr C H ii region is suggestive of cluster formation (Yusef Zadeh et al. yz09
(2009)); a conclusion that is buttressed by sub-mm observations of the Sgr B2
region, which imply a combination of clustered and distributed star formation
(Ginsburg et al. 2018a , Ginsburg & Kruijssen 2018b ).

However, despite this vigorous activity and presence of copious molecular
material, estimates of the star formation rate for the CMZ suggest that it is
at least an order of magnitude lower than expected based on observations of
nearby regions (Longmore et al. longmore13 (2013)). The physical cause of this
discrepancy is uncertain (Barnes et al. barnes (2017)), but it raises the
possibility that the resultant stellar population(s) may also show an
environmental dependance, possibly characterised by an anomalous initial mass
function (IMF). As a consequence, much effort has been expended in attempts to
characterise the young stellar population within the CMZ, with near-IR
observations revealing a rich population of massive stars located within
clusters - the Arches, Quintuplet, and Galactic Centre (e.g. Figer et al.
figer99 (1999), Paumard et al. paumard (2006)) - and distributed throughout
the CMZ in apparent isolation (Cotera et al. cotera96 (1996), cotera99 (1999),
Muno et al. 2006a , Mauerhan et al. mauerhan07 (2007), 2010b , 2010c , Dong et
al. dong15 (2015)).

Beyond constraining the star formation physics operating within the extreme
conditions of the GC, a determination of the properties of this stellar cohort
is of considerable importance for a number of other astrophysical topics. With
a subset born with masses $M_{\rm init}>100M_{\odot}$ (Lohr et al. lohr
(2018)), they provide vital observational data on the lifecycle of the most
massive stars that form in the local Universe, up to and including the point
of core-collapse. Constraining a robust evolutionary scheme for such stars is
essential if we are to predict both the nature and production rate of
relativistic remnants from this population, noting that a rich cohort of young
neutron stars and black holes deriving from such stars appears present within
the GC (Deneva et al. deneva (2009), Kennea et al. kennea (2013), Hailey et
al. hailey (2018)).

It is anticipated that massive stars also play an important role in driving
the evolution of the GC via the feedback of ionising radiation, mechanical
energy and chemically enriched material. Of particular interest is their role
in shaping the emergent high energy spectrum of the GC, which recent
observations suggest extends from soft X-rays ($kT\sim 1-10$keV; Ponti et al.
ponti (2015)) through to very high energy $\gamma$-rays ($kT>100$GeV;
Aharonian et al. aharonian06 (2006)). It appears likely that the diffuse, low
energy X-ray emission arises from a combination of unresolved low mass point
sources (pre-MS stars and Cataclysmic Variables), the winds of massive stars
and their supernova (SN) endpoints - acting both individually and in concert
in massive clusters such as Wd1 (Muno et al. 2006b ) - and pulsar wind nebulae
(Ponti et al. ponti (2015)).

The $\gamma$-ray component is thought to derive from the cosmic rays that
permeate the GC (Aharonian et al. aharonian06 (2006), H.E.S.S. collaboration
HESS16 (2016), HESS18 (2018)). An exceptional cosmic ray density may be
inferred by the abundance of H${}_{3}^{+}$ (produced via the ionisation of H2)
and has been suggested to play an important role in regulating the temperature
of the warm molecular material that suffuses the CMZ (Le Petit et al. lepetit
(2016), Oka et al. oka (2019)). Plausible sources for the production of cosmic
rays are the supermassive black hole Sgr A∗ and massive stars - the latter via
the interaction between their winds, cluster driven outflows and supernovae
(e.g. Aharonian et al. aharonian18 (2018), Bednarek et al. bed (2014), Bykov
et al. bykov (2015), Cesarsky & Montmerle CM (1983)). Unfortunately, in the
absence of a full stellar census the relative contributions of these channels
is currently uncertain.

Nevertheless both physical agents have been implicated in the initiation of
mass outflows - thought to be driven by a combination of cosmic ray and
thermal gas pressure (cf. Everett et al. everett (2008), Yusef-Zadeh & Wardle
yz19 (2019)) - that originate in the GC. These range in size from the $\sim
15$pc radio and X-ray lobes (Morris et al. morris03 (2003), Zhao et al. zhao
(2016)) through to the order of magnitude larger bipolar radio bubbles and
X-ray chimney (Heywood et al. heywood (2019) and Ponti et al. ponti19 (2019),
respectively), and ultimately the $\sim 50$kpc Fermi bubbles (Su et al. su
(2010)). An intriguing possibility is that the removal of material from the
CMZ via such winds may help quench star formation, leading to the low rate
currently observed.

The preceding discussion leads to the conclusion that a more complete
understanding of the population of massive stars within the GC is extremely
timely and well motivated for a multitude of reasons. With the Galactic Centre
cluster cohort well constrained (Paumard et al. paumard (2006), Bartko et al.
bartko (2010)) previous papers in this series have focused on a reappraisal of
the Arches and Quintuplet clusters (Clark et al. 2018a , 2018b , 2019b ). In
this work we concentrate on the apparently isolated massive stellar component
distributed throughout the GC, utilising extant surveys (Sect. 4.1) to compile
a target list which we observed with the K-band multi-object spectrograph
(KMOS) mounted on UT1 of the Very Large Telescope (VLT). The manuscript is
ordered as follows. Sect. 2 details data acquisition and reduction and the
classification criteria employed to characterise the resultant spectra. Sect.
3 provides a detailed breakdown of the resultant dataset by spectral subtype,
while we discuss survey completeness, the distribution of massive stars across
the CMZ and construct a complete stellar census for the CMZ in Sect. 4.
Finally we summarise our findings and highlight future prospects to advance
these research lines in Sect. 5.

##  2 Data acquisition, reduction, and classification

###  2.1 Acquisition and reduction

The VLT-KMOS (Sharples et al. sharples (2013)) data for this paper were
obtained under ESO programme 093.D-0306 (PI: Clark), with observations made
between 2014 August 02-13. KMOS is a multi-object, integral field
spectrograph, which has 24 configurable integral field units (IFUs) positioned
within a 6.7 arcminute field of view. The spectral resolution of the
observations is a function of rotator angles and the IFUs used (e.g. Patrick
et al. patrick15 (2015)), varying between ${\Delta}{\lambda}/{\lambda}\sim
3895-4600$. Each observing block consisted of 12$\times$30 s exposures in an
ABA observing pattern, where the first observation of each field used the more
rigorous 24-arm telluric standard star approach and all subsequent
observations of the same field used the standard 3-arm telluric approach. The
standard stars used for these observations were HIP 84846 (A0V), HIP 91137
(A0V), and HIP 3820 (B8V).

The data reduction methodology is identical to that of the KMOS data presented
in Clark et al. (2018b ). Science and standard star observations were
calibrated, reconstructed and combined using the KMOS/esorex pipeline (Davies
et al. kmos (2013)), employing the standard set of calibrations delivered by
the telescope. Clark et al. (2018b ) detail this procedure and discuss the
modifications made to the standard processes.

In the K-band, telluric correction is a fundamentally important part of the
data reduction process. Since the majority of the useful diagnostic lines for
these targets lie in regions of the K-band that are highly contaminated by
telluric absorption, we implemented a rigorous correction routine adapted from
Patrick et al. (patrick15 (2015), patrick17 (2017)) and further detailed in
Clark et al. (2018b ). Given the intrinsic shape of the telluric spectrum, the
continuum placement is vital to accurately recover the shape of the science
spectrum. This is typically done empirically, by selecting multiple continuum
points from the science and standard star spectra throughout the entire
spectral range and is highly non-linear.

For targets with particularly broad spectral features, such as the WN and WC
stars, identification of the continuum is a complicated process. This problem
is compounded when broad emission features coincide with strong telluric
absorption, as seen for the He ii 2.0379$\mu$m and 2.3799$\mu$m and He i
2.059$\mu$m features in the WN5-7 stars. For such stars continuum placement
was guided by comparison with a combination of published spectra - in
particular that of qF353E (WN6; Steinke et al. steinke (2016)) - and synthetic
examples computed with from the CMFGEN code (Hillier & Miller hillier98
(1998), hillier99 (1999)).

###  2.2 Spectral classification

A number of publications have been dedicated to the classification of post-MS
massive stars in the near-IR window: specifically O stars (Hanson et al.
hanson96 (1996), hanson05 (2005)), B-hypergiants (Clark et al. clark12 (2012),
2018b ), luminous blue variables (LBVs; Morris et al. morris (1996), Clark et
al. clark11 (2011)) supergiant B[e] stars (sgB[e]; Oksala et al. oksala
(2013)) and Wolf-Rayets (WRs; Figer et al. figer97 (1997), Crowther et al.
crowther06 (2006), Crowther & Walborn crowther11 (2011) and Rosslowe &
Crowther rosslowe (2018)).

We have employed - and expanded upon - these classification criteria in our
study of the Arches and Quintuplet (Clark et al. 2018a , 2018b ). We follow an
identical methodology here, referring the reader to these works for details
beyond those summarised in the relevant sections below. As in previous works,
given the uncertainty in the parameterisation of the spatially inhomogeneous
interstellar extinction along sightlines towards the GC, we prioritise
spectral rather than photometric data. However given that we are unable to
utilise cluster membership to locate target stars within the GC, we are forced
to employ the photometric datasets and analysis of Dong et al. (dong12 (2012))
to identify likely foreground interlopers. In doing so we choose to only make
use of ground based photometry to avoid issues of calibrating such data with
space-based observations, given the significant issues accounting for
convolving very different filter responses for intrinsically red photometric
sources (cf. Dong et al. dong12 (2012)).

For candidate massive stars previously identified in the literature but
without KMOS observations we used published spectra to reappraise their
classifications in light of this methodology. Where stars are reclassified on
this basis, if we were unable to obtain the relevant spectra we provide the
appropriate figure number in addition to the formal reference in the following
discussion.

In the remaining sections for conciseness, we abbreviate the [DWC2011]xxx
designation for stars in the primary list of Pa$\alpha$ emitters presented in
Dong et al. (dong11 (2011)) to a simple Pxxx. No recognised nomenclature
exists for those stars derived from the secondary list of Pa$\alpha$ emitters
from this paper; hence we choose to designate these simply as Sxxx sources.

##  3 Results

Observations were made of a total of 82 candidate and confirmed massive stars
derived from the list of Pa$\alpha$ excess sources of Dong et al. (dong11
(2011)) and other literatures sources (Mauerhan et al. mauerhan07 (2007),
2010b , 2010c ). Details of each target, including previous and new
classifications are provided in Table 1. The sample contains a diverse group
of objects including pre- and post-main sequence massive stars as well as a
large number of foreground high- and low-mass interlopers. Below we break down
this population by spectral sub-types and location along the sightline to the
GC, including discussion of relevant examples not included in our sample in
order to provide the basis for the construction of a comprehensive stellar
census.

###  3.1 OB supergiants

Spectra of OB supergiants are presented in Fig. 1. We are able to identify
three new candidates; S73, S152, and P95. The first two stars are clearly
mid-O supergiants given the presence of He ii 2.189$\mu$m absorption and C iv
2.069+2.078$\mu$m emission. The lack of a pronounced He i $\sim 2.112\mu$m
absorption feature in the He i+N iii+C iii blend of either star indicates
comparatively early spectral types (O4-5), while the relatively weak
Br$\gamma$ photospheric line signals significant mass loss, though not
sufficient to drive the line into emission as is seen in hypergiants (Fig. 2).
Three additional mid-O supergiants - CXOGC J174628.2-283920, 174703.1-285354,
and 174725.3-282523 - were observed by Mauerhan et al. (2010c ). These were
not observed with KMOS, but we include their published classifications in
Table 1 in order to compile a comprehensive census of massive stars in the GC
region.

Moving to later spectral sub-types and the presence of narrow photospheric
absorption in He i 2.059, 2.112, and 2.161$\mu$m, Br$\gamma$ and He ii
2.189$\mu$m indicates that P95 is a new $\sim$O9 supergiant via close
similarity to the spectral template HD154368 (Hanson et al. hanson05 (2005));
however the narrower Br$\gamma$ profile and stronger He i 2.112$\mu$m emission
suggests a stronger wind than this classification standard. Given the S/N of
our spectrum of CXOGC J174537.3-285354 around 2.19$\mu$m, we may not improve
on the previous O9-B0Ia classification, nor reassess the nature of P50 from
the spectrum presented in Mauerhan et al. (2010c ). Geballe et al. (geballe
(2019); their Fig. 6) identify 2MASS J17444501-2919307 as B2-3 Ia+; we prefer
a slightly more conservative B0-3 Ia classification. Finally De Witt et al.
(dewitt (2013)) propose a generic O star classification for XID 947; given the
low S/N of the published spectrum we are unable to improve on this.

Figure 1: Montage of spectra of OB supergiants. Template spectra with
classifications from Hanson et al. (hanson05 (2005)) and Clark et al. (2018a )
overplotted in red. It is important to note that hot pixels around 2.080$\mu$m
- nearly coincident with the C iv 2.079$\mu$m lines - in the spectrum of CXOGC
J174537.3-285354 were artificially removed; we suspect the anomalously narrow
component of the Br$\gamma$ photospheric profile is also spurious. Likewise
the broad absorption feature centred on $\sim 2.1\mu$m in the spectrum of P95
is also artificial.

###  3.2 O hypergiants and WN7-9ha stars

Consideration of the spectra of members of the Arches reveals the close
evolutionary and morphological similarities between $\sim$O4-8 hypergiants and
WN7-9ha Wolf-Rayets (Martins et al. martins08 (2008), Clark et al. 2018a ).
The former are delineated by systematically weaker Br$\gamma$ emission and a P
Cygni absorption component in the $\sim 2.11\mu$m He i+C iii+N iii+O iii
emission blend at later (O6-8) spectral subtypes. In contrast no absorption
component is present in the $\sim 2.11\mu$m feature of any WNLha star, while
He ii 2.189$\mu$m is in emission in the earlier ($<$WN7-8) spectral subtypes.

As a consequence we discuss both types of star here; presenting spectra of
three new examples in Figs. 2 and 3. Of these P15 and S131 are clearly new O
hypergiants by virtue of broad, pronounced Br$\gamma$ emission and He ii
2.189$\mu$m absorption. Strong C iv emission and a lack of He i 2.112$\mu$m
absorption indicates that P15 is an early O4-5 Ia+ star (Clark et al. 2018a ,
Hanson et al. hanson05 (2005)). Conversely the presence of He i 2.112$\mu$m
absorption and weak C iv emission in the spectrum of S131 indicates a later
(O7-8 Ia+) subtype; we examine the possible causes of the double peaked
Br$\gamma$ emission line profile below.

Of those stars with previous classifications, the detection of Br$\gamma$
emission in P36 and 114 marks them out as hypergiants rather than supergiants
and of early (O4-5) spectral type, given the lack of He i 2.112$\mu$m
absorption and consequent similarity to the O4-5 Ia+ Arches star F27 (Fig. 2).
The Br$\gamma$ emission line in the spectrum of P36 shows a central reversal,
which is also present in the O4 Ia+ spectroscopic template HD15570 and other
hypergiants considered here (see below); unfortunately the presence of a
narrow emission component of uncertain origin prevents interpretation of the
corresponding line profile of P114. The similarity of P100 & 107 to Arches F10
suggests a revision to slightly later spectral subtypes (O4-6 to O7-8; Fig.
2); the strength of Br$\gamma$ emission in both stars suggest they are close
to transitioning to a WNLha evolutionary phase. In terms of the strength of C
iv emission P97 is intermediate between these stars and P15; we assign an O6-7
subtype by comparison to Arches F15 (Fig. 2).

The similarity of P23 to P97 suggests a comparable classification, although
the former demonstrates stronger Br$\gamma$ emission, suggesting it too is
close to becoming a WNLha star. As with S131 its Br$\gamma$ profile is
strongly double peaked, while C iv 2.079$\mu$m and the emission component of
the $\sim 2.11\mu$m He i+C iii+N iii+O iii blend are unexpectedly broad.
Regarding the Br$\gamma$ line, the only comparator we are aware of is the
Quintuplet member LHO 001 which is spectroscopically variable and demonstrates
a similarly double peaked profile at some epochs. Clark et al. (2018b )
suggest that LHO 001 is a massive binary system and such an explanation is
also attractive for P23 as well; it is not obvious that a physically
justifiable combination of He-abundance, mass-loss rate, wind clumping factor
and velocity field for a single star can replicate the Br$\gamma$ line profile
observed.

The Br$\gamma$ profile of P75 also appears double peaked, although the blue
peak is less pronounced than in P23, being more comparable to S131. Other
notable features include strong, broad emission with hints of substructure in
the $\sim 2.11\mu$m He i+C iii+N iii+O iii blend, an absence of C iv emission
and, uniquely, weak N iii 2.103$\mu$m emission. The latter two observational
features are characteristic of the WN8-9ha stars rather than the mid-O
hypergiants within the Arches (Clark et al. 2018a ), although the reverse is
true for the He i 2.112$\mu$m absorption component also exhibited by P75. We
suggest this unique hybrid morphology is due to strong helium and nitrogen
enhancement (with the former yielding pronounced He i 2.161$\mu$m emission in
the blue wing of Br$\gamma$) and C depletion with respect to normal mid-O
hypergiants as the star enters the WNLha phase. As such we revise the
classification of P75 to WN9ha/O6-7 Ia+; noting that further multi-epoch
observations and quantitative analysis are required to confirm the nature of
stars such as P23, P75 and S131, which demonstrate double peaked Br$\gamma$
profiles.

Next we turn to the WNLha stars (Fig. 3). Comparison of S120 to Arches F16
suggests that it is a new WN8-9ha star; the strength of Br$\gamma$ emission
and lack of He i 2.112$\mu$m absorption distinguishing it from an O
hypergiant, while He ii 2.189$\mu$m fully in absorption suggests a late
spectral sub-type. Nevertheless there are incongruities; the Br$\gamma$ line
and the He i+C iii+N iii+O iii $\sim 2.11\mu$m emission blend both appear
anomalously narrow in comparison to other isolated examples and the cohort
within the Arches cluster (Clark et al. 2018a ). Moreover, the emission
features exhibit a significant displacement from their rest wavelengths
($\Delta RV{\gtrsim}100$kms-1); possibly indicative of binary reflex motion or
a runaway nature.

Assigned a generic O If+ classification (Muno et al. 2006a , Dong et al.
dong15 (2015)) P35 is of particular interest since it is spatially coincident
with the H ii region H2, which Dong et al. (dong17 (2017)) associates with an
apparent overdensity of bright stars. Inspection of our spectrum reveals
exceptionally strong, narrow and asymmetric He i 2.059$\mu$m and Br$\gamma$
emission, the latter with a rather broad base (Fig. 3). Such a morphology is
not characteristic of O super-/hypergiants or WNLha stars. Conversely, weak C
iv 2.079$\mu$m emission, He ii 2.189$\mu$m absorption and a strong broad pure
emission profile in the He i+C iii+N iii+O iii $\sim 2.11\mu$m blend is
reminiscent of early-mid O hypergiants and weak-lined WN8-9ha stars. We
therefore assign such a classification to P35, assuming that there is
significant contamination of the He i 2.059$\mu$m and Br$\gamma$ profiles by
nebular emission from the H2 H ii region - as suggested by the inflection in
the red flank of both lines. Finally, as discussed in Clark et al. (2019b )
P96 closely resembles the WN7-8ha Arches member F4 (Table 1); the resolution
and S/N of the published spectra of the remaining candidates (cf. Mauerhan et
al. 2010b , 2010c ) being insufficient to allow any further refinements to
current classifications.

Unlike the OB supergiants there exists sufficient quasi-homogeneous ground
based photometry (Mauerhan et al. 2010b , Dong et al. dong11 (2011)) to enable
a comparison of the properties of isolated WN7-9ha and O hypergiants to those
in the Arches cluster. Fig. 4 indicates that such stars within the Arches
exhibit a range in both $K-$band ($\sim 9.5-11$) and $(H-K)$ colour indice
($\sim 1.4-2.2)$. This is likely due to a combination of variations in both
intrinsic (luminosity, temperature/bolometric correction and continuum
emission from the stellar wind) and extrinsic properties (binarity,
differential stellar reddening); indeed Lohr et al. (lohr (2018)) find
[FGR2002] 2 to be an exceptionally luminous star with its apparent magnitude
due to extreme reddening.

Comparison to the photometric properties of the corresponding population of
isolated WN7-9ha and O hypergiants shows an encouraging co-location in the
colour/magnitude plot, although with an increased proportion of fainter,
redder examples. This is likely indicative of greater interstellar reddening
along the relevant lines of sight, although verification awaits a
parameterisation of the reddening law towards the GC. The newly identified
WN8-9ha star S120 appears an exception to this trend, being the faintest
example ($K\sim 12.5$) but with a rather moderate near-IR colour ($(H-K)\sim
1.6$), suggesting it has an intrinsically low luminosity.

Figure 2: Montage of spectra of isolated O hypergiants. Template spectra are
overplotted in red along with identifications; we also show the spectrum of
F16, the WN8-9ha star with the weakest emission lines within the Arches (Clark
et al. 2018a ), for comparison to P100 and 107. We note that a narrow emission
feature of uncertain origin at line centre of Br$\gamma$ in the spectrum of
P114 was removed, leaving an artificially flat topped profile. Figure 3:
Montage of spectra of WNLha stars. Template spectra with classifications
overplotted in red (Clark et al. 2018a ). We note that S120 appears to have an
anomalous RV redshift of $\sim+100$kms-1; the spectrum of F16 has been
artificially shifted by a comparable amount to aid in comparison. Figure 4: IR
colour magnitude diagram for WN7-9ha and mid-O hypergiants (blue symbols),
WN9-11h and early-B hypergiants (black symbols) and putative cool blue
hypergiants and supergiant B[e] stars (red symbols). Relevant members of the
Arches (WN7-9ha and mid-O hypergiants) and Quintuplet (WN9-11h and early-B
hypergiants) clusters are given by open blue and black circles respectively,
while isolated stars are represented by filled squares of the appropriate
colour. Axis scale chosen to mirror the comparable plot of WN and WC stars
(Fig. 10).

###  3.3 WN9-11h stars/early-B hypergiants

We present the spectra of WN9-11h stars and early-B hypergiants in Figs. 5 and
6. Given the large number of such stars within the Quintuplet cluster compared
to those known in the wider galaxy (particularly early-B hypergiants) we
utilise these as classification templates following the discussion in Clark et
al. (2018b ). In doing so we are able to revise the classification of P103
from generic P Cyg O-type supergiant to B0-1 Ia+/WNLh (Fig. 5). Interpreting
the spectrum of P56 - plotted against the WN11h star LHO71 in Fig. 6 - is more
difficult. Both stars clearly show prominent electron scattering wings in the
P Cygni profile of He i 2.059$\mu$m, while their Br$\gamma$ emission lines are
broadly comparable. However the He i 2.112$\mu$m doublet is in absorption in
P56 but in emission in LHO71; given this discrepancy we suggest that a
classification as either B0-1 Ia+ or WN11h would be appropriate (cf. P103).
Likewise P98 and P137 undergo less dramatic revisions to B1-2 Ia+/WNLh and
WN10h respectively, with P19 found to be a twin of the broad lined WN9h
Quintuplet member LHO 158 (cf. Clark et al. 2019b ). Of these we note that
Hankins et al. (hankins (2020)) report P137 is located within a mid-IR ring
nebula, possible indicative of a wind blown bubble or circumstellar ejecta
(cf. the Pistol star).

The combination of He i 2.059$\mu$m and Mg ii 2.138/44$\mu$m emission and
narrow Br$\gamma$ and He i 2.112 and 2.161$\mu$m absorption in the spectrum of
S132 indicates that it is a new early- B hypergiant (Fig. 5 and Table 1).
Comparison of the spectrum of SSTU J174523.11-290329.3 - which also
demonstrates Mg ii emission (Mauerhan et al. mauerhan07 (2007); their Fig. 4)
- to those of S132 and similar objects within the Quintuplet suggests that an
identification as a hypergiant, rather than the previous supergiant
classification, is more appropriate. Finally, despite the low S/N and
resolution of the spectrum of the previously unclassified 2MASS
J17461292-2849001 (Geballe et al. geballe (2019); their Fig. 6), it appears
directly comparable to the preceding two stars. As a consequence we adopt a
similar B1-3 Ia+ classification for it; further strengthened by its close
proximity to - and hence potential membership of - the Quintuplet, which hosts
a large number of such stars (Clark et al. 2018b ).

Fig. 4 illustrates the near-IR photometric properties of both isolated early-B
hypergiants and WN9-11h stars and their counterparts within the Quintuplet
cluster. The majority of Quintuplet members occupy a relatively compact region
of colour/magnitude space ($9{\lesssim}K{\lesssim}10$ and
$1.4{\lesssim}(H-K){\lesssim}1.8$)111An identical lower bound to the colour
$(H-K)$ index is also seen for the Arches cluster. This is substantially in
excess of the limit of $(H-K){\gtrsim}1.0$ adopted by Dong et al. (dong12
(2012)) for early type stars within the CMZ. However, since many of these
stars are expected to show an intrinsic IR excess due to continuum emission
from their stellar winds we caution against accepting $(H-K){\gtrsim}1.4$ as a
blanket colour cut for all massive stars in the CMZ.. Outliers include the
faint ($K\sim 10.5$) WN9h star LHO 158 and the extremely red WN10h outlier LHO
67.The former is likely the hottest of this cohort (and hence may require the
largest bolometric correction) while an understanding of the latter -
intrinsic IR excess and/or extrinsic reddening - awaits detailed quantitative
analysis.

While five of the isolated early-B hypergiants and WN9-11h stars222P56, 98 &
137, 2MASS J17461292-2849001 and WR102ka are co-located with Quintuplet
members in the colour/magnitude plot, four are outliers. Both P19 (WN9h) and
SSTU J174523.11-290329.3 (B0-2 Ia+) appear rather faint and red and likely
suffer excess interstellar reddening (cf. Arches F2; Sect 3.2). Conversely the
bright WN10h star P137 may either be seen through a window suffering reduced
extinction or is a foreground object. Finally despite being over a magnitude
brighter than any other cluster or isolated early-B hypergiant observed to
date, the $(H-K)$ colour of S132 is unexceptional, suggesting that it may be
intrinsically highly luminous.

Figure 5: Spectra of candidate isolated early-B hypergiants. Comparator
spectra of Quintuplet cluster members and appropriate classifications given in
red (Clark et al. 2018b ). Figure 6: Spectra of WN9-11h stars. Comparator
spectra of Quintuplet cluster members and appropriate classifications given in
red (Clark et al. 2018b ).

###  3.4 Candidate cool BHGs/LBVs and supergiant B[e] stars

Geballe et al. (geballe (2019); their Figs. 5 and 6) report on a cohort of ten
stars with spectra dominated by strong Br$\gamma$ and weaker emission in He i
2.059$\mu$m and various Fe ii transitions. Of these 2MASS J17452861-2856049
and J17462830-2839205 correspond to P35 (WN8-9ha +neb) and CXOGC
J174628.2-283920 (O4-6 Ia) respectively (Table 1), while 2MASS
J17455154-2900231 is source D of Cotera et al. (cotera99 (1999); Sect 3.8).
This leaves a total of seven objects of which, fortuitously, we have
observations of two - P112 and 141 (=2MASS J17453782-2857161 and
J17450929-2908164 respectively) - allowing us to verify line identifications
from spectra of greatly improved S/N and resolution. These data are presented
in Fig. 7, along with the spectrum of a third object of similar morphology -
P40 (=2MASS J17452405-2900589).

Prior to discussing spectral morphologies it is instructive to consider
photometric data. Only two of the stars - P40 and 2MASS J17470940-2849235 -
have $(H-K)<2.0$, with the latter sufficiently blue that a foreground nature
cannot be excluded (Table 1). Five of the remaining objects cluster between
$(H-K)\sim 2.4-2.6$ with the sixth, P141, an outlier with $(H-K)\sim 3.6$;
values significantly in excess of those of other early-type massive stars in
the GC (Table 1 and Fig. 4). Pre-empting the following discussion, in the
absence of a classification yielding ‘photospheric’ colours for this cohort it
is impossible to quantitatively decouple intrinsic and extrinsic contributions
to reddening, although it seems highly likely that a significant continuum
contribution from a wind or dusty circumstellar disc is present in these six
stars. Such a conclusion is supported by mid-IR observations, with a number of
stars333P40, 112, & 141, 2MASS J17444319-2937526 and J17470940-2849235.
appearing intrinsically red upon consideration of the $[4.5]-[8.0]>1.0$ colour
cut suggested by Robitaille et al. (rob (2008); we refer the reader to this
work for a detailed justification of this criterion). Unfortunately, the
parallel possibility of significant interstellar extinction affecting the
near-IR photometry precludes us from employing the colour-magnitude and
colour-colour plots of, for example, Bonanos et al. (bonanos (2009)) in order
to determine the physical nature of these stars.

Mindful of these issues we now turn to the spectra of P40, 112 and 141. All
three are dominated by strong, narrow Br$\gamma$ emission. Critically, broad
electron scattering wings are also seen in the profile of this transition in
P40 and 112; indicative of a dense stellar wind and hence a massive star
identification. Weak He i 2.059$\mu$m emission is also apparent in the spectra
of both these stars, suggesting they are hotter than P141 (where it is
absent), although no trace of the He i 2.112$\mu$m feature is present in any
of the three, nor are the spectral signatures of high excitation species such
as He ii, N iii or C iv. Instead the remaining emission features present arise
from low excitation metal transitions such as Fe ii 2.061$\mu$m (P40 & 112)
and 2.089$\mu$m (all stars), [Fe ii] 2.045$\mu$m, 2.118$\mu$m, and 2.133$\mu$m
(P141), Mg i 2.134+2.144$\mu$m (P112) and Na i 2.206+2.209$\mu$m (P40). At
longer wavelengths the CO bandheads are seen in emission in P40 (Fig. 8); they
and the Pfund series are absent from both P112 and P141. Finally, there is no
indication of H2 emission in any of the stars, disfavouring a pre-MS
classification.

The narrow emission line spectra dominated by Br$\gamma$ and low excitation
metals are reminiscent of both cool-phase LBVs (Clark et al. clark11 (2011),
2018b ) and supergiant B[e] stars (sgB[e]; Oksala et al. oksala (2013)). CO
bandhead emission is present in a substantial number of sgB[e] stars but
appears absent from most, if not all, LBVs (Morris et al. morris (1996),
Oksala et al. oksala (2013)). We present the spectra of the LBVs FMM362 and
G24.73+0.69 in Fig. 7 to illustrate the gross similarities to P40, 121 and
141, although neither star provides an exact match. As a consequence we
suggest a sgB[e] classification for P40 and, pending an evaluation of long
term variability, either a late-B hypergiant or cool LBV classification for
P112 and 141, under the assumption they are located within the CMZ; estimation
of stellar luminosities and temperatures will have to await quantitative model
atmosphere analysis.

Given that 2MASS J17444319-2937526, J17445538-2941284 and 2MASS
J17450241-2854392 are co-located with P112 in the near-IR colour/magnitude
plot (Fig. 4) and appear to show a comparable spectral morphology (subject to
the low resolution and S/N data) it is tempting to apply a similar
classification to them. While the same is true for 2MASS J17482472-2824313, we
are more cautious in this case due to a possible association with a cold,
dusty clump (Contreras et al. contreras (2013)) which could favour a pre-MS
status. Likewise the comparatively blue $(H-K)$ colour for 2MASS
J17470940-2849235 leaves open the possibility of a lower mass foreground
(post-AGB) object. Indeed, it is entirely possible that this cohort could be
rather heterogeneous - comprising stars of different luminosities and
evolutionary status but similar gross observational features. This would be
analogous to stars exhibiting the B[e] phenomenon (cf. Lamers et al. lamers
(1998)) which, as demonstrated by P40, these stars closely resemble.

Finally we note that based on its spectral morphology the LBV G0.120-0.048
would be included in this cohort had not its proximity to the Quintuplet
cluster ($\sim 7$pc distant) suggested possible cluster membership (Mauerhan
et al. 2010a ; footnote 11).

Figure 7: Montage of spectra of isolated candidate late-B hypergiants/cool
LBVs and sgB[e] stars. Spectra of the LBVs qF362 and G24.73+0.69 (Clark et al.
2018b ) shown for comparison. Figure 8: K-band spectrum of the sgB[e] star P40
plotted with an increased wavelength coverage to illustrate the presence of CO
bandhead emission.

###  3.5 WN5-7 stars

Eight isolated WN5-7 stars have been identified within the GC, of which two -
P99 and P150 - are new discoveries (Table 1). A ninth - qF353 (=P64; Steinke
et al. steinke (2016)) - is located on the periphery of the apparent wind
blown structure encircling the north and east quadrants of the Quintuplet
cluster; since it has historically been associated with this cluster, we do
not include it in this census. We note that P2 (=[MCD2010] 17) - one of the
eight stars considered here - is also proximate to the Quintuplet (Mauerhan et
al. 2010b ).

The spectra of the seven stars presented in Fig. 9 are all dominated by strong
and broad emission in He i, He ii and N iii. The line widths of P39 and 91
support a classification as broad lined systems; the low resolution spectrum
of P2 suggesting likewise (Mauerhan et al. 2010b ; their Fig. 4).

Steinke et al. (steinke (2016)) identify qF353E as a WN6 star; informed by
Rosslowe & Crowther (rosslowe (2018)) we use this as a benchmark to provide
relative classifications for the remaining stars. The close similarity of P34
to qF353E suggests an identical classification for this star. In comparison to
qF353E, the He ii 2.189$\mu$m line in P2, 39, 91, and P99 is stronger relative
to the other emission features in their spectra, suggesting a WN5-6
identification for these stars (with the lack of N v 2.10$\mu$m precluding
earlier sub-types); conversely the relative weakness of this transition in
P109 and 147 implies a WN6-7 classification. Finally the combination of
exceptionally strong He ii 2.189$\mu$m emission in P150 and the absence of the
absorption component due to He i 2.059$\mu$m that is present in all other
examples suggest that this is the hottest star observed and hence we assign a
WN5 sub-type.

We present photometry for these stars in Fig. 10, noting that the newly
discovered P99 and P150 are the faintest examples identified to date. Given
their near-IR colours P150 appears intrinsically rather faint while P99 could
suffer considerable excess extinction along its line of sight. The broad lined
WN5-6 star P39 is also noteworthy in this regard, with the most extreme value
of the cohort ($H-K\sim 3$), although it does not appear exceptionally faint
($K\sim 12.2$); if the near-IR colour is due to substantial interstellar
reddening it would appear intrinsically highly luminous.

Figure 9: Montage of spectra of isolated candidate WN5-7 Wolf-Rayets. The
spectrum of the WN6 star qF353E is shown in red for comparison. Prominent
transitions are indicated; for completeness the unlabelled He i lines at
2.059$\mu$m and 2.1126+2.1138$\mu$m are the 2p1Po \- 2s1S and the 4s3S - 3p3Po
\+ 4s1S - 3p1Po transitions respectively.

###  3.6 WC8-9 stars

Inspection of the relevant literature (e.g. Mauerhan et al. 2010b , 2010c ,
and associated errata; Geballe et al. geballe (2019)) reveals 13 isolated
stars classified as WC stars444We note that the proximity of WR102ca and CXOGC
J174617.7-285007 to the Quintuplet cluster suggest a physical association;
hence we do not consider these members of the isolated WC star cohort.. As
with the Quintuplet cohort (Clark et al. 2018b )555The inclusion of CXOGC
J174617.7-285007 brings the cluster total at the time of writing to 14. these
are all WC8 and WC9 stars; indeed only a single example of an earlier subtype
- the WC5/6 star IRS 3E (associated with Galactic Centre cluster; Paumard et
al. paumard (2006)) - has been identified within the CMZ. We obtained spectra
of six examples which are presented in Fig. 11. Despite improved S/N and
resolution, our spectra of P28, 49, 53, 101, and 151 do not necessitate re-
classification, although they do illustrate two essential features of this
cohort: that stars of the same sub-type present spectra with unexpectedly
diverse morphologies - presumably indicative of differences in both stellar
and wind properties - and that dilution/obscuration of emission features by a
substantial near-IR continuum excess due to the presence of hot dust is
common.

The spectrum of the final object, MP13 (=CXOGC J174519.1-290321; WC9d),
exemplifies the latter phenomenon, with the weak emission line spectrum
indicating the presence of hot dust. Intriguingly, the He ii lines that are
evident are suggestive of a WN7 classification (Fig. 9), with none of the
weaker C ii-iv features that characterise WCL stars being visible, despite
their presence in the spectrum of Mauerhan et al. (2010c ; their Fig. 6).
While we retain a WC9d classification for this object on the basis of the
latter work, we suspect that it may be intrinsically variable, with greater
dilution in our spectrum compared to that of Mauerhan et al. (2010c ) -
although we cannot exclude the possibility of a WN7 companion at this time.

Within the GC, excess continuum emission from hot dust was first recognised
for five exceptionally bright near-IR sources within the Quintuplet cluster.
High spatial resolution near-IR imaging revealed these to be colliding wind
binaries (Tuthill et al. tuthill (2006)), with subsequent high S/N and
resolution $JHK$ spectroscopic observations identifying strongly diluted
emission features characteristic of WCL stars (Najarro et al. paco17 (2017)).

Despite its featureless $K-$band spectrum Mauerhan et al. (2010c ) assigned a
WCLd classification to CXOGC J174645.2-281547 by analogy to the Quintuplet
cohort and by virtue of its hard X-ray emission. Geballe et al. (geballe
(2019); their Fig. 2) report five further objects with very red, essentially
featureless $K-$band spectra; however, despite the possible presence of weak
He i 2.059$\mu$m emission in two examples they refrain from classifying these
as dusty WCL stars.

We may question whether it is possible to provide a more definitive
classification of CXOGC J174645.2-281547 and the five sources from Geballe et
al. (geballe (2019)) via consideration of photometry and other observational
data. Of these 2MASS J17431001-2951460 appears associated with the methanol
maser MMB G358.931-0.030 (0.3” distant; Caswell et al. caswell77 (1977),
caswell10 (2010)) which argues for an object in a pre-MS evolutionary phase.
2MASS J17445461-2852042 (= MGM 1 1, [MKN2009] 7) is a large-amplitude
photometric variable with an unconstrained, but apparently long period
(${\Delta}H\sim 0.64$, ${\Delta}K\sim 1.12$; Monte et al. mon92 (1992),
Matsunaga et al. matsunaga (2009)); as such we exclude it from the
colour/magnitude plot noting that it is most likely a Mira variable. This
leaves three remaining candidate WCLd stars - 2MASS J17432173-2951430,
J17432988-2950074, and 2MASS J17460215-2857235 - which we plot along with the
remaining isolated WCL stars and, for context, the Quintuplet cohort in Fig.
10.

Comparison to other evolutionary groupings reveals that the WCLd stars are the
most photometrically diverse, with $K\sim 6.5-12.7$ (Figs. 4 & 10). There is a
significant scatter in $(H-K)$ colour index but an overall correlation - in
the sense that brighter sources are redder - is present. While multiple
physical causes are clearly implied - such as intrinsic differences in stellar
and wind properties and differential interstellar reddening - we suppose that
an increasing contribution from hot circumstellar dust dominates this
relationship; a conclusion supported by the fact that the brightest and
reddest sources within the Quintuplet cluster are those with essentially
featureless spectra due to dust dilution.

It is therefore encouraging that the spectroscopically confirmed, isolated
WCLd stars plotted in the colour/magnitude diagram are coincident with
examples found within the Quintuplet, while CXOGC J174645.2-281547, 2MASS
J17432173-2951430, J17432988-2950074, and 2MASS J17460215-2857235 seamlessly
extend this co-location to brighter, redder objects with featureless
spectra666The final star with a featureless spectrum from Geballe et al.
(geballe (2019)) - 2MASS J17431001-2951460 - is $\sim 2.5$ magnitudes fainter
and has a significantly bluer $(H-K)$ colour index than the preceding objects,
bolstering the conclusion that it is in a pre-MS evolutionary phase given its
association with a methanol maser.. Moving to fainter $K-$band magnitudes and,
of the three isolated WC stars with $K\sim 9-9.5$, the emission lines in the
spectra of the two stars with the largest $(H-K)$ values - 2MASS
J17444083-2926550 and J17463219-2844546 - are also very weak (Geballe et al.
geballe (2019); their Fig. 4). This is consistent with the presence of
substantial continuum veiling due to emission from hot dust, an hypothesis
strengthened by their extremely red $(J-K)$ colour indices.

Given this we conclude that, as assumed for CXOGC J174645.2-281547, the three
isolated objects from Geballe et al. (geballe (2019)) with featureless
$K-$band spectra are also bona fide dusty WCL stars. Indeed we may invert the
argument: given that the Quintuplet clearly hosts such stars one would
anticipate their presence in the isolated stellar cohort, so if these objects
are not dusty WCL stars one would be need to explain their absence. In either
eventuality we close this discussion by noting that the number of isolated
dusty WCs within the CMZ - 13 or 16, depending on the nature of these sources
- is directly comparable to the number associated with the Quintuplet cluster
(see footnote five).

Figure 10: IR colour magnitude diagram for candidate and confirmed WN5-7 (red)
and WCL stars (black). Members of the Quintuplet cluster and isolated examples
given by open circles and filled squares respectively. Stars with featureless
K-band spectra are indicated by nested symbols, the anomalously faint example
being 2MASS J17431001-2951460 (with 2MASS J17445461-2852042 excluded due to
variability; Sect. 3.8). Ground-based photometry for the Quintuplet members
given in Cutri et al. (cutri (2003)), Dong et al. (dong12 (2012)), and
Hussmann et al. (hussmann (2012)). No photometry is available for qF235N while
the location of qF76 & 309 ($K\sim 11.2,(H-K)\sim 2.0$ and $K\sim
13.4,(H-K)\sim 2.0$ respectively) have been displaced slightly to avoid
overlap with P28 and 101. Figure 11: Montage of spectra of isolated WC8-9 Wolf
Rayet stars, with the Quintuplet WC8 star LHO 47 plotted in red for comparison
(Clark et al. 2018b ). Given the remarkable diversity of morphologies, no
additional WC9 comparator spectra from the Quintuplet are available. The
emission features in MP13 are particularly weak; we therefore enhance these by
a factor of five and overplot the resultant spectrum in blue for clarity.

###  3.7 Classical Oe/Be stars

The last homogeneous cohort that may be identified in our observations are a
group of six faint ($K\sim 13-14.6$) stars with spectra dominated by strong
Br$\gamma$ emission and, in a subset, weak He i 2.059$\mu$m emission (Fig.
12). He i and He ii photospheric lines - which might enable a temperature
determination - are absent, as is emission in both high and low excitation
metallic transitions (although the Mg ii 2.138+2.144$\mu$m doublet was
identified in the spectrum of P105 presented by de Witt et al. dewitt (2013)).
The lack of low excitation atomic (Na i, Ca i) or molecular (CO bandhead)
absorption features disfavours the possibility that these are foreground
cataclysmic variables. Moreover, the absence of CO bandhead emission
distinguishes these stars from the IR excess objects within the Arches
cluster, which Stolte et al. (stolte10 (2010)) suggest are B-type stars
surrounded by remnant protostellar discs. Instead they most closely resembles
classical Oe/Be stars (Clark & Steele clark00 (2000)); rapidly rotating non-
supergiant late-O to early-A stars, characterised by gaseous, quasi-Keplerian
circumstellar decretion discs that generate line emission in H i, He i and low
excitation metallic transitions as well as a near-IR continuum excess (cf.
Porter & Rivinius PR (2003)).

We may ask whether the observational properties of this cohort are consonant
with such a classification. The equivalent widths, full-width half maxima and
line intensities of the Br$\gamma$ and - where present - He i 2.059$\mu$m
emission lines are consistent with the range expected for early-B stars
exhibiting the Be phenomenon (Clark & Steele clark00 (2000)). Likewise their
broad, asymmetric emission profiles are a natural consequence of the one-armed
density waves that commonly perturb the quasi-Keplerian discs of Be stars,
with the double peaked He i 2.059$\mu$m line profile of S124 clearly
indicative of rotation. Finally such discs are transient phenomena, leading to
significant spectral variability on the timescale of years; potentially
explaining the disappearance of Mg ii emission in the four years between the
two observations of P105.

Turning to photometric properties and assuming a distance to the GC of $\sim
8$kpc and a representative interstellar extinction of $A_{K}\sim 3$ one would
expect an O9.5V (B3V) star located there to have $K\sim 13.4$ ($K\sim 15.7$).
Furthermore adopting an indicative continuum excess of $K{\gtrsim}1$mag due to
emission from the circumstellar disc (e.g. Dougherty et al. dougherty (1994))
implies that the range of $K-$band magnitudes anticipated for Be stars within
the CMZ is consonant with the stellar cohort considered here. Likewise,
comparison of the intrinsic near-IR colours of Be stars ($(H-K)\sim 0.0-0.5$;
Lada & Adams lada (1992), Dougherty et al. dougherty (1994)) to the values
exhibited by this cohort ($(H-K)\sim 1.5-2.0$; Table 1) suggests a degree of
interstellar extinction that is fully consistent with that expected for
sightlines towards the GC.

We may also essentially invert this argument. Between $\sim 10-20$% of B0-3
stars exhibit the Be phenomenon at Galactic metallicities (Wisniewski &
Bjorkman wis (2006)) and, as demonstrated above, their properties (an IR
excess and strong line emission) favour their detection via surveys such as
that of Dong et al. (dong11 (2011)). Consequently it would be surprising if
none were to be found in the GC - one would be forced to invoke a rather
contrived star formation history that limited the formation of stars of
$\sim$10-20M⊙ over the past $\sim$25Myr (Wisniewski & Bjorkman wis (2006)) or
assume that physical conditions there preclude the formation of classical Be
stars.

Therefore, even though we may not exclude alternative classifications - such
as pre-MS stars/massive young stellar objects (YSOs; cf. Bik et al. bik05
(2005), bik06 (2006)) - at this time, we consider an identification of this
cohort as classical Be stars located within the CMZ to be the most compelling
explanation for their spectroscopic and photometric properties as reported
here.

Figure 12: Montage of spectra of possible isolated classical Be stars. We
caution that the central region of the Br$\gamma$ profile of P135 has been
artificially removed due to the presence of hot pixels.

###  3.8 Miscellaneous and uncertain classification

Figure 13: Montage of spectra of three sources dominated be narrow H i
recombination lines (Sect. 3.8); the star embedded in the UCH ii region Sgr
A-D, the MYSO/sgB[e] star P57 and MYSO P58.

Six sources do not fit into any of the above classifications, of which we have
new observations of three. Cotera et al. (cotera99 (1999)) reported that the
spectrum of the highly reddened ($(H-K)\sim 3.45$) star associated with the
ultra-compact (UC) H ii region Sgr A-D was dominated by strong He i
2.059$\mu$m and Br$\gamma$ emission lines; on this basis they suggested that
this indicated a nebular, rather than stellar origin. Despite the increased
S/N and resolution of our spectrum (Fig. 13), these remained the only
identifiable features. With FWHM$\lesssim 100$kms-1, the line profiles are
narrower than expected for an origin in either the spherical wind of a hot
star or the disc of a classical Be star777The Br$\gamma$ lines of the six Be
stars identified in Sect. 3.7 have FHWM$\gtrsim 200$kms-1.; we therefore
concur with Cotera et al. (cotera99 (1999)) that the spectrum is dominated by
emission from the UCH ii region.

Both P57 and P58 show strong, single peaked emission in Br$\gamma$ and the
Pfund series up to at least Pf-30, with FWHM$\sim 200$kms-1 in our new
observations (Fig. 13). Emission in the Pfund series is observed in massive
YSOs (Bik et al. bik05 (2005), bik06 (2006)), sgB[e] stars (Oksala et al.
oksala (2013), Kraus et al. kraus (2020)) and LBVs (Oksala et al. oksala
(2013), Najarro et al. paco15 (2015)). The line widths of both stars are
consistent with any of these possibilities; however the ratio of
(Br$\gamma$/Pf-20)$\sim 2.5$ for P57 and P58 is significantly lower than that
found for for the Pistol star, where the emission arises in a spherical
stellar wind ((Br$\gamma$/Pf-20)$\sim 10$; Najarro et al. paco15 (2015)).
Hence we conclude that neither star is likely to be an LBV and instead the
emission arises in a circumstellar disc. The presence of weak H2 2.12$\mu$m
emission in P58 marks it out as a massive YSO; with $(H-K)\sim 1.0$ it is on
the cusp of the colour cut applied by Dong et al. (dong12 (2012)) for
foreground objects888Since pre-MS objects are typically intrinsically red (cf.
2MASS J17431001-2951460 and J17470921-2846161; Table 1) it is possible that
P58 is an interloper.. No H2 emission is seen in P57, while the marginal
detection of Mg ii 2.138$\mu$m is consistent with either a massive YSO or a
sgB[e] star; as a consequence we leave both options open at this time.
However, in either eventuality the presence of pronounced He i 2.059$\mu$m
emission implies a rather hot source in order to provide the requisite UV
photons to drive the line into emission.

Of the remainder 2MASS J17431001-2951460 has already been briefly discussed in
Sect. 3.6 where, despite the featureless spectrum presented by Geballe et al.
(geballe (2019); their Fig. 2), near-IR photometry is discrepant with
expectations for a dusty WCL star; rather the presence of the methanol maser
source MMB G358.931-0.030 only 0.3” away (Caswell et al. caswell77 (1977),
caswell10 (2010)) is suggestive of a YSO classification. The presence of CO
bandhead emission associated with 2MASS J17470921-2846161 (Geballe et al.
geballe (2019); their Fig. 6) implies a dense, cool circumstellar envelope;
the addition of H2 molecular emission further points towards a (M)YSO
classification (Bik et al. bik05 (2005), bik06 (2006)). Unfortunately, given
the potential for a near-IR continuum excess from the circumstellar envelope
and uncertain interstellar extinction it is not possible to make any further
inferences as to the nature of the source from extant photometry at this time.

Finally 2MASS J17444840-2902163 has a spectrum dominated by Br$\gamma$
absorption and Na i and CO bandhead emission (Geballe et al. geballe (2019);
their Fig. 6). Comparison to the spectrum of the YHG $\rho$ Cas presented in
Yamamuro et al. (yamamuro (2007)) suggest a similar classification for this
star, although the near-IR properties (Table 1) imply a rather moderate
luminosity in comparison to e.g. the YHG cohort of Westerlund 1 (Clark et al.
clark05 (2005)).

###  3.9 Interlopers and unclassifiable stars

Given the crowded nature of the CMZ, the size of the IFUs and the selection
criteria employed and it is inevitable that interlopers will be included in
our survey. The most common contaminants were found to be cool, late-type
stars in the line of sight which mirror the expected magnitudes and colours of
early-type stars in the GC. Somewhat unexpectedly these were prevalent amongst
targets selected from the list of putative Pa$\alpha$ emitters of Dong et al.
(dong11 (2011)). Table 2 provides a list of such stars identified amongst our
primary targets, along with those with either featureless spectra or
insufficient S/N to attempt a classification. Where photometry was available,
a combination of $(H-K)$ colour index and $K>10.5$mag implies that the cool
stars are either foreground objects or descendants of rather low mass
progenitors if found within the CMZ; hence we do not discuss these further. A
large number of the IFUs contained one or more additional objects. In all
relevant fields999Those surrounding stars [DWC2011] 15, 25, 34, 40, 43, 47,
48, 49, 57, 75, 78, 99, 105, 106, 135, 136, 143, and 147; S59, 64, 117, 123,
131, 136, and 183; MP13. the additional spectra were examined and found to be
either featureless or those of cool stars.

This leaves a handful of additional stars worthy of brief comment. Dong et al.
(dong12 (2012)) classify both P38 and 140 as massive foreground objects on the
basis of their near-IR colours. Based on our new spectrum, we reclassify the
X-ray bright P38 slightly to O4 Ia. Mauerhan et al. (2010a ; their Fig. 3)
give P140 as B0-2 Ia. However the spectrum presented by Geballe et al.
(geballe (2019); their Fig. 6) shows a pattern of emission (including Fe ii
and Na i) and absorption (bluewards of Br$\gamma$) more reminiscent of a cool
LBV candidate; intriguingly, comparison of these spectra implies variability
between these observations (obtained in 2009 August and 2016 September
respectively). The third object, P102, likewise has near IR colours indicative
of a foreground object, while the double peaked Br$\gamma$ profile suggest
that it is a classical Be star.

Lastly we turn to two objects for which we cannot yet advance a
classification. The spectrum of P25 is featureless save for a single peaked
Br$\gamma$ line of moderate intensity ($I/I_{cont}\sim 1.4$). Unfortunately,
while the $K-$band measurement is consistent with the cohort of classical Be
stars in the absence of colour information we may not infer a distance to the
object and hence advance a classification. Finally P110 is of a comparable
$K-$band magnitude and $(H-K)$ colour to the candidate classical Be stars
(Sect. 3.7) and shows emission coincident with the wavelength of Br$\gamma$.
However multiple apparently spurious emission features of comparable strength
are also present in the spectrum, casting some doubt as to the astrophysical
origin for the putative Br$\gamma$ emission; hence we refrain from classifying
the star at this time.

###  3.10 Synopsis

In total we have observed 82 different primary targets, derived from the list
of Pa$\alpha$ excess sources of Dong et al. (dong11 (2011))101010For
completeness we provide a list of Pa$\alpha$ excess sources remaining to be
observed in Appendix A. and previous reported candidates from the literature.
Of these the IFUs of 26 sources contained one or more additional objects -
yielding spectra of $>100$ stars in total - although none of these additional
serendipitous sources were found to be massive stars. Of the primary targets a
total of 31 were either cool foreground interlopers, displayed apparently
featureless spectra or were of insufficient S/N to attempt classification
(Table 2). Including one literature classification, three objects were
assessed as foreground massive stars while two further were unclassifiable
from our spectra (Sect. 3.9).

This left a total of 47 objects which spectroscopy revealed to be massive
stars. Of these 17 are new identifications, while the spectra of a further 19
stars allowed improved spectral classifications. The remaining 11 stars
retained the same classification as previously reported.

Table 1 lists a total of 83 isolated massive stars - derived from our
observations and the literature - with near-IR photometry consistent with a
location in the GC and for which classification has proved possible111111As
noted in Sects. 3.3-3.7, the diffuse, extended nature of the Quintuplet
cluster makes it difficult to determine whether outlying massive stars are
physically associated with it or not. Historically the LBV G0.120-0.048 and WR
stars WR102c (=qF353E; WN6), WR102ca (WC8-9) and CXOGC J174617.7-285007
(=MP14; WC8-9) have been included as probable cluster members (cf. Clark et
al. 2018b , clark20 (2020)), despite their displacement from the central
stellar concentration. As a consequence, and in order to avoid double
counting, we do not include these in the census given here, but we highlight
that their physical association with the cluster is as yet unproven.. These
comprise:

$\bullet$ Five mid-O and four late-O/early-B supergiants. One further object,
XID #947, was assigned a generic O classification with no indication of
luminosity class by de Witt et al. (dewitt (2013)).

$\bullet$ Eight mid-to late-O hypergiants, 11 WNLha stars and one WN8-9ha/O6-7
Ia+ hybrid.

$\bullet$ Four WN9-11h stars, three early-B hypergiants and two hybrid early-B
hypergiant/WNLh stars. Of these Geballe et al. (geballe (2019)) highlight the
close proximity of 2MASS J17461292-2839001 (B1-3Ia+) to the Quintuplet; we
retain it here for completeness, although consider cluster membership highly
likely.

$\bullet$ Eight objects with spectra characterised by strong narrow Br$\gamma$
and weak, low excitation metallic emission lines. We identify one as a
supergiant B[e] star on the basis of pronounced CO bandhead emission. We
tentatively classify the remaining stars as either late-B hypergiants or cool
phase LBVs; further photometric and spectroscopic monitoring being required to
distinguish between these possibilities.

$\bullet$ One star, 2MASS J17444840-2902163, which we provisionally classify
as a low-luminosity YHG and hence may extend the preceding cohort to lower
temperatures.

$\bullet$ Eight WN5-7 stars, of which three are broad lined systems. Of these
Mauerhan et al. (2010b ) note the proximity of P2 to the Quintuplet cluster.

$\bullet$ 16 WC8-9 stars, including the featureless sources CXOGC
J174645.2-281547, 2MASS J17432173-2951430, J17432988-2950074, and 2MASS
J17460215-2857235 (Mauerhan et al. 2010c , Geballe et al. geballe (2019)).

$\bullet$ Six fainter stars with spectra dominated by strong Br$\gamma$
emission that we classify as classical Be stars.

$\bullet$ Three apparent (massive) YSOs and a further source displaying
nebular emission associated with an UCH ii region (noting that a considerably
larger population of (massive) YSOs have been identified on the basis of IR-
radio continuum and mid-IR spectroscopic observations; An et al. an (2011),
Ginsburg et al. 2018a , Yusef-Zadeh et al. yz09 (2009)).

$\bullet$ One star, P57, with a spectrum consistent with a classification as
either a massive YSO or sgB[e] star.

Given the nature of the surveys employed for target selection we emphasise
that this census is highly likely to be incomplete. We discuss this limitation
and the wider implications of this stellar population in the following
sections.

Table 1: Census of massive stars in the sightline towards the GC ID | RA | Dec | J | H | K | Aliases | Classification
---|---|---|---|---|---|---|---
[DWC2011] | (h m s) | (d m s) | (mag) | (mag) | (mag) |  | Old | New
OB supergiants |  |  |  |  |  |
50 | 17 45 02.89 | -29 08 59.8 | 13.9$\pm$0.05 | 11.4$\pm$0.02 | 9.9$\pm$0.01 | CXOGC J174502.8-290859 | O9-B0 Ia | -
95 | 17 45 59.44 | -28 52 50.7 | 14.9$\pm$0.02 | 12.1$\pm$0.01 | 10.5$\pm$0.02 |  | - | O9 Ia
| 17 44 45.02 | -29 19 30.7 | 14.37 | 10.97 | 9.07 | 2MASS J17444501-2919307 | B2-3 Ia+ | [B0-3 Ia]
| 17 45 29.89 | -28 54 28.9 | - | 12.2 | 10.8 | S152 | - | O4-5 Ia
| 17 45 30.32 | -28 52 07.0 | - | 13.4 | 11.9 | S73 | - | O4-5 Ia
| 17 45 37.30 | -28 53 53.7 | 15.75$\pm$0.02 | 12.80$\pm$0.01 | 11.23$\pm$0.01 | CXOGC J174537.3-285354 | O9-B0 Ia | Similar
| 17 46 28.31 | -28 39 20.5 | 16.99$\pm$0.07 | 13.36$\pm$0.03 | 11.49$\pm$0.04 | CXOGC J174628.2-283920 | O4-6 Ia | -
| 17 47 03.14 | -28 31 20.2 | 16.23$\pm$0.03 | 13.03$\pm$0.01 | 11.27$\pm$0.01 | CXOGC J174703.1-283119 | O4-6 Ia | -
| 17 47 25.41 | -28 25 23.0 | - | 13.37$\pm$0.01 | 11.30$\pm$0.04 | CXOGC J174725.3-282523 | O4-6 Ia | -
WN7-9ha/O hypergiants |  |  |  |  |  |
15 | 17 46 03.21 | -28 48 58.4 | - | 13.2$\pm$0.05 | 11.7$\pm$0.03 |  | - | O4-5 Ia+
22 | 17 45 53.40 | -28 49 36.9 | 15.4$\pm$0.02 | 12.5$\pm$0.02 | 11.0$\pm$0.01 | [MCD2010] 12 | WN8-9ha | -
23 | 17 46 10.01 | -28 55 32.4 | 15.0$\pm$0.02 | 12.3$\pm$0.02 | 10.8$\pm$0.02 | [MCD2010] 15 | WN8-9ha | O6-7 Ia+
35 | 17 45 28.62 | -28 56 05.0 | 14.5$\pm$0.02 | 11.5$\pm$0.02 | 9.7$\pm$0.03 | CXOGC J174528.6-285605, GCCR073 | O If+ | WN8-9ha +neb
36 | 17 45 31.50 | -28 57 16.8 | 15.1$\pm$0.02 | 12.7$\pm$0.01 | 11.4$\pm$0.02 | [MCD2010] 6, CXOGC J174531.4-285716 | O4-6 Ia | O4-5 Ia+
75 | 17 46 17.10 | -28 51 31.5 | 15.0$\pm$0.03 | 12.1$\pm$0.12 | 10.5$\pm$0.02 | CXOGC J174617.0-285131 | O6 If+ | WN8-9ha/O6-7 Ia+
77 | 17 46 17.54 | -28 53 03.5 | 15.0$\pm$0.02 | 12.1$\pm$0.01 | 10.5$\pm$0.02 | [MCD2010] 16 | WN8-9ha | -
96 | 17 45 48.57 | -28 50 05.7 | 17.6$\pm$0.10 | 13.3$\pm$0.02 | 11.0$\pm$0.02 | WR102a | WN8/Of | WN7-8ha
97 | 17 45 47.72 | -28 50 49.2 | 15.3$\pm$0.19 | 12.3$\pm$0.09 | 10.7$\pm$0.06 |  | O4-6 Ia+ | O6-7 Ia+
100 | 17 45 42.32 | -28 52 47.1 | 14.7$\pm$0.02 | 11.7$\pm$0.02 | 10.1$\pm$0.03 | [MCD2010] 10 | O4-6 Ia+ | O7-8 Ia+
107 | 17 45 39.34 | -28 53 21.1 | 14.7$\pm$0.02 | 11.8$\pm$0.01 | 10.2$\pm$0.01 |  | O4-6 Ia+ | O7-8 Ia+
111 | 17 45 36.12 | -28 56 38.7 | 15.6$\pm$0.03 | 12.3$\pm$0.01 | 10.4$\pm$0.02 | CXOGC J174536.1-285638, GCCR107 | WN8-9ha | -
114 | 17 45 32.78 | -28 56 16.6 | 14.7$\pm$0.02 | 12.1$\pm$0.01 | 10.7$\pm$0.01 | CXOGC J174532.7-285617, GCCR110 | O4-6 Ia | O4-5 Ia+
134 | 17 45 16.74 | -28 58 25.1 | 16.7$\pm$0.06 | 13.1$\pm$0.03 | 11.1$\pm$0.02 | CXOGC J174516.7-285824 | WN7-8ha | -
| 17 46 18.79 | -28 49 48.4 | - | 12.5 | 11.1 | S131 | - | O7-8 Ia+
| 17 46 20.87 | -28 46 58.8 | - | 14.1 | 12.5 | S120 | - | WN8-9ha
| 17 46 56.36 | -28 32 32.3 | - | 13.74$\pm$0.05 | 11.24$\pm$0.02 | CXOGC J174656.3-283232 | WN8-9ha | -
| 17 47 11.47 | -28 30 07.0 | 16.56$\pm 0.06$ | 12.72$\pm$0.02 | 10.54$\pm$0.01 | CXOGC J174711.4-283006 | WN8-9ha | -
| 17 47 12.25 | -28 31 21.6 | 17.06$\pm$0.07 | 13.07$\pm$0.01 | 10.78$\pm$0.02 | CXOGC J174712.2-283121 | WN7-8ha | -
| 17 47 13.03 | -28 27 08.2 | - | 14.22$\pm$0.01 | 11.86$\pm$0.01 | CXOGC J174713.0-282709 | WN7-8ha | -
WN9-11h stars/early-B hypergiants |  |  |  |  |  |
19 | 17 45 48.61 | -28 49 42.2 | - | 13.6$\pm$0.03 | 11.1$\pm$0.01 | [MCD2010] 11 | WN8-9h | WN9h(broad)
56 | 17 46 27.60 | -28 46 11.8 | 14.4$\pm$0.02 | 11.3$\pm$0.01 | 9.5$\pm$0.04 | [MCD2010] 18 | PCyg O Ia | WN11h
98 | 17 45 41.27 | -28 51 47.7 | 14.8$\pm$0.02 | 11.6$\pm$0.01 | 9.9$\pm$0.01 | [MCD2010] 9 | O9-B0 If+ | B1-2 Ia+/WNLh
103 | 17 46 01.65 | -28 55 15.3 | 13.4$\pm$0.01 | 10.7$\pm$0.01 | 9.1$\pm$0.05 | [MCD2010] 13 | PCyg OIa | B0-1 Ia+/WNLh
137 | 17 45 16.17 | -29 03 14.7 | 11.6$\pm$0.03 | 9.2$\pm$0.03 | 7.9$\pm$0.03 | CXOU J174516.1-290315, GCCR082 | Ofpe/WN9 | WN10h
| 17 45 23.11 | -29 03 29.3 | 16.7$\pm$0.03 | 12.7$\pm$0.02 | 10.3$\pm$ 0.02 | SSTU J174523.11-290329.3 | B0-2 Ia | [B0-2 Ia+]
| 17 45 54.65 | -28 47 44.9 | - | 9.5 | 7.9 | S132 | - | B1-3 Ia+
| 17 46 12.93 | -28 49 00.2 | 13.79 | 10.53 | 8.85 | 2MASS J17461292-2849001 | - | [B1-3 Ia+]
| 17 46 18.12 | -29 01 36.6 | 12.97$\pm$0.02 | 10.27$\pm$0.03 | 8.84$\pm$0.03 | WR102ka | WN10h | -
Cool BHGs/sgB[e] stars |  |  |  |  |  |
40 | 17 45 24.06 | -29 00 58.9 | 13.2$\pm$0.03 | 10.4$\pm$0.04 | 8.8$\pm$0.11 | [MCD2010] 5, 2MASS J17452405-2900589 | B0-2 Ia | sgB[e]
112 | 17 45 37.81 | -28 57 16.2 | - | 13.1$\pm$0.02 | 10.7$\pm$0.02 | 2MASS J17453782-2857161 | B[e] | late BHG/LBV?
141 | 17 45 09.29 | -29 08 16.2 | - | 15.3$\pm$0.05 | 11.7$\pm$0.02 | 2MASS J17450929-2908164 | - | late BHG/LBV?
| 17 44 43.20 | -29 37 52.6 | 16.00 | 12.78 | 10.14 | 2MASS J17444319-2937526 | uncl. | [late BHG/LBV?]
| 17 44 55.38 | -29 41 28.5 | 16.72 | 12.63 | 10.14 | 2MASS J17445538-2941284 | uncl. | [late BHG/LBV?]
| 17 45 02.41 | -28 54 39.2 | 15.81 | 12.56 | 10.00 | 2MASS J17450241-2854392 | uncl. | [late BHG/LBV?]
| 17 47 09.40 | -28 49 23.6 | 12.66 | 10.8 | 9.46 | 2MASS J17470940-2849235 | uncl. | [late BHG/LBV?]
| 17 48 24.73 | -28 24 31.3 | 15.86 | 12.12 | 9.54 | 2MASS J17482472-2824313 | uncl. | [late BHG/LBV?]
WN5-7 stars |  |  |  |  |  |
2 | 17 46 23.81 | -28 48 10.8 | 16.3$\pm$0.05 | 13.3$\pm$0.01 | 11.4$\pm$0.02 | [MCD2010] 17 | WN5b | [WN5-6b]
34 | 17 45 50.55 | -28 57 26.1 | 15.8$\pm$0.03 | 12.9$\pm$0.03 | 11.3$\pm$0.01 | [MCD2010] 19 | WN5b | WN6
39 | 17 45 22.70 | -28 58 44.1 | - | 15.2$\pm$0.06 | 12.2$\pm$0.03 | CXOGC J174522.6-285844 | WN5-6b | Similar
91 | 17 45 55.38 | -28 51 26.0 | 15.4$\pm$0.19 | 12.5$\pm$0.04 | 11.0$\pm$0.07 | CXOGC J174555.3-285126 | WN5-6b | Similar
99 | 17 45 38.68 | -28 52 10.5 | - | 16.4$\pm$0.06 | 13.9$\pm$0.02 |  | - | WN5-6
109 | 17 45 50.61 | -28 59 19.6 | 15.4$\pm$0.02 | 12.5$\pm$0.01 | 10.9$\pm$0.01 | WR102b, CXOGC J174550.6-285919, GCCR085 | WN7 | WN6-7
147 | 17 45 08.98 | -29 12 17.9 | 15.1$\pm$0.03 | 12.6$\pm$0.02 | 11.1$\pm$0.04 | CXOGC J174508.9-291218 | WN7 | WN6-7
150 | 17 45 15.05 | -29 14 36.2 | - | 14.5$\pm$0.02 | 12.7$\pm$0.03 |  | - | WN5
WC8-9 stars |  |  |  |  |  |
28 | 17 45 57.76 | -28 54 45.8 | 16.3$\pm$0.03 | 13.4$\pm$0.01 | 11.4$\pm$0.02 | WR 101q | WC8-9 | Similar
42 | 17 45 32.50 | -29 04 58.0 | - | 14.6$\pm$0.02 | 12.0$\pm$0.01 | [MCD2010] 8 | WC9 | -
49 | 17 45 21.89 | -29 11 59.6 | - | 14.7$\pm$0.03 | 12.7$\pm$0.03 | [MCD2010] 3 | WC9 | Similar
53 | 17 45 07.07 | -29 12 00.6 | 16.8$\pm$0.07 | 13.5$\pm$0.02 | 11.1$\pm$0.03 | [MCD2010] 2 | WC9?d | Similar
94 | 17 46 02.59 | -28 54 14.0 | 16.4$\pm$0.04 | 13.5$\pm$0.02 | 11.6$\pm$0.02 | [MCD2010] 14 | WC9 | -
101 | 17 45 42.47 | -28 52 53.3 | 16.3$\pm$0.04 | 13.2$\pm$0.01 | 11.2$\pm$0.02 | WR101p | WC8-9 | Similar
151 | 17 45 09.80 | -29 14 13.1 | 15.6$\pm$0.03 | 13.0$\pm$0.01 | 11.1$\pm$0.02 | [MCD2010] 4 | WC9?d | Similar
| 17 43 21.73 | -29 51 43.0 | 14.03 | 9.37 | 6.48 | 2MASS J17432173-2951430 | uncl. | [WCLd]
| 17 43 29.88 | -29 50 07.4 | 17.54 | 12.56 | 8.82 | 2MASS J17432988-2950074 | uncl. | [WCLd]
| 17 44 37.35 | -29 27 55.7 | 17.56 | 13.91 | 10.29 | 2MASS J17443734-2927557 | WCLd | -
| 17 44 40.83 | -29 26 55.1 | 16.76 | 12.81 | 9.40 | 2MASS J17444083-2926550 | WCLd | -
| 17 45 04.84 | -29 11 46.5 | 15.02 | 11.63 | 9.04 | 2MASS J17450483-2911464 | WCLd | -
| 17 45 19.17 | -29 03 22.0 | 17.10 | 13.30$\pm$0.05 | 10.40$\pm$0.05 | CXOGC J174519.1-290321, MP13 | WC9 | Similar
| 17 46 02.16 | -28 57 23.5 | 14.87 | 11.43 | 8.08 | 2MASS J17460215-2857235 | uncl. | [WCLd]
| 17 46 32.20 | -28 44 54.6 | 17.14 | 12.86 | 9.21 | 2MASS J17463219-2844546 | WCLd | -
| 17 46 45.25 | -28 15 47.7 | 15.39 | 9.96$\pm$0.03 | 7.18 | CXOGC J174645.2-281547 | WCLd | -
Classical Be stars |  |  |  |  |  |
105 | 17 45 52.97 | -28 55 36.9 | 17.69$\pm$0.16 | 14.75$\pm$0.05 | 13.08$\pm$0.05 | CXOGC J174552.9-285537, XID #3275 | Be star | Similar
106 | 17 45 52.26 | -28 56 46.1 | - | 16.1$\pm$0.08 | 14.5$\pm$0.13 |  | - | Be star
108 | 17 45 47.04 | -28 56 46.0 | - | 15.5$\pm$0.07 | 14.0$\pm$0.09 |  | - | Be star
135 | 17 45 17.76 | -28 58 19.9 | - | 16.1$\pm$0.10 | 14.1$\pm$00.06 |  | - | Be star
Classical Be stars (cont.) |  |  |  |  |  |
| 17 45 52.16 | -28 55 01.5 | - | 17.4 | 14.6 | S69 | - | Be star
| 17 46 12.95 | -28 47 16.3 | - | 14.6 | 13.1 | S124 | - | Be star
Miscellaneous and uncertain classification |  |  |  |  |
57 | 17 46 29.90 | -28 46 39.9 | - | 14.7$\pm$0.02 | 12.6$\pm$0.02 |  | - | MYSO/sgB[e]
58 | 17 46 31.85 | -28 46 47.1 | 14.3$\pm$0.02 | 12.6$\pm$0.01 | 11.6$\pm$0.01 |  | - | MYSO
| 17 43 10.02 | -29 51 46.1 | 17.01 | 13.91 | 11.58 | 2MASS J17431001-2951460 | uncl. | [MYSO]
| 17 44 48.40 | -29 02 16.4 | 13.81 | 12.08 | 10.34 | 2MASS J17444840-2902163 | uncl. | [YHG]
| 17 47 09.22 | -28 46 16.2 | 14.21 | 12.58 | 10.18 | 2MASS J17470921-2846161 | uncl. | [MYSO]
| 17 45 28.88 | -28 57 26.4 | 16.23$\pm$0.06 | 13.29$\pm$0.03 | 11.61$\pm$0.03 | XID #947, GCCR072 | O star? | -
| 17 45 51.54 | -29 00 23.2 | - | 14.88 | 11.43 | [HJB85] Sgr A-D | LBV/ucH ii | ucH ii
Foreground (($H-K)\leq 1$) and unclassifiable |  |  |  |  |
25 | 17 45 58.31 | -28 52 20.0 | - | - | 13.4$\pm$0.06 |  | - | Uncl.
38 | 17 45 37.99 | -29 01 34.5 | 11.1$\pm$0.05 | 9.7$\pm$0.05 | 8.9$\pm$0.13 | [MCD2010] 7, CXOGC J174537.9-290134 | O4-6 Ia | O4 Ia
102 | 17 45 44.04 | -28 53 16.8 | 15.0$\pm$0.02 | 13.3$\pm$0.03 | 12.4$\pm$0.04 |  | - | Be star
110 | 17 45 48.94 | -28 59 01.8 | - | 15.7$\pm$0.04 | 14.3$\pm$0.03 |  | - | Uncl.
140 | 17 44 59.46 | -29 05 25.9 | 8.8$\pm$0.03 | 7.7$\pm$0.04 | 7.0$\pm$0.03 | [MCD2010] 1, 2MASS J17445945-2905258 | B0-2 Ia | [cool LBV]
Table 1: continued.

Summary of known and candidate isolated massive stars in the sightline to the
GC. Columns 1 and 7 identify the source and provide relevant aliases, columns
2 and 3 provide co-ordinates and 4-6 provide broadband near-IR photometry.
Finally column 8 provides, where available, historical classifications for
these stars and column 9 our new or revised classifications from this work. We
highlight that a subset of such new classifications are given in square
brackets - these derive from our reappraisal of published data. Where
available we adopt the naming convention of Dong et al. (dong11 (2011);
[DWC2011] xxx) for their primary list of Pa$\alpha$ excess sources (column 1),
noting that S69, 73, 120, 124, 131, 132, and 152 derive from their secondary
list. Additional identifications derive from Mauerhan et al. (2010b ; CXOGC
sources), Mauerhan et al. (2010c ; [MCD2010]xx) de Witt et al. (dewitt (2013)
XID xxx), Geballe et al. (geballe (2019), 2MASS sources) and Zhao et al.
(zhao20 (2020); GCCRxxx). In all cases photometry (and associated errors where
available) and classifications were obtained from these works, supplemented by
Homeier et al. (homeier (2003)), Mauerhan et al. (mauerhan07 (2007)), Clark et
al. (2009b ), Oskinova et al. (oskinova (2013)), de Witt et al. (dewitt
(2013)) and Dong et al. (dong15 (2015)) where required.

Table 2: Non massive star interlopers projected onto the CMZ

[DWC2011] | RA (J2000) | Dec (J2000) | Appearance
---|---|---|---
3 | 17 46 03.60 | -28 47 09.8 | Cool
16 | 17 45 54.85 | -28 47 13.2 | Cool
29 | 17 46 09.60 | -28 57 13.5 | Cool
30 | 17 46 08.10 | -28 58 24.2 | Cool
31 | 17 46 06.27 | -28 59 14.9 | Cool
43 | 17 45 13.95 | -29 04 38.2 | Cool
47 | 17 45 15.05 | -29 09 08.3 | Featureless
48 | 17 45 22.68 | -29 10 56.5 | Featureless
78 | 17 45 48.22 | -28 47 26.0 | Cool
104 | 17 46 07.30 | -28 57 17.5 | Cool
113 | 17 45 33.69 | -28 57 49.7 | Cool
133 | 17 45 36.86 | -29 01 17.5 | Featureless
136 | 17 45 28.22 | -29 03 27.1 | Cool
142 | 17 45 19.45 | -29 10 33.9 | Low S/N
143 | 17 45 23.91 | -29 10 24.7 | Low S/N
144 | 17 45 14.22 | -29 11 41.7 | Low S/N
(S47) | 17 46 31.07 | -28 46 14.2 | Cool
(S52) | 17 46 24.30 | -28 46 53.1 | Cool
(S59) | 17 46 01.64 | -28 49 00.0 | Cool
(S64) | 17 46 05.02 | -28 52 06.7 | Cool
(S78) | 17 45 53.36 | -29 00 49.1 | Low S/N
(S113) | 17 44 55.32 | -29 11 54.8 | Cool
(S117) | 17 46 26.56 | -28 46 26.4 | Cool
(S119) | 17 46 32.72 | -28 46 08.1 | Cool
(S123) | 17 46 29.95 | -28 49 32.2 | Low S/N
(S136) | 17 46 01.64 | -28 51 10.3 | Featureless
(S139) | 17 46 02.44 | -28 53 13.7 | Cool
(S141) | 17 46 06.14 | -28 54 41.4 | Low S/N
(S146) | 17 46 07.43 | -28 59 18.5 | Low S/N
(S153) | 17 45 35.44 | -28 55 12.3 | Low S/N
(S183) | 17 45 04.24 | -29 09 26.7 | Cool

Stars given in parentheses with an ‘Sxxx’ designation are from the secondary
list of Pa$\alpha$ emitters in Dong et al. (dong11 (2011)).

##  4 Discussion

The results described in the preceding section reveal that a large number of
isolated and potentially very massive stars of diverse nature appear
distributed through the CMZ. These include very rare phases - such as LBVs and
sgB[e] stars - that are thought to play an important role in the lifecycle of
single and binary stars. As such characterising this population, including
observational biases, will be critical to further understanding the evolution
of such extreme objects as well as the wider ecology and star formation
history of the circumnuclear region of the Galaxy.

###  4.1 Survey completeness and biases

Centred on Sgr A∗, the HST Pa$\alpha$ excess survey has the smallest
(asymmetric) footprint of those utilised in our study ($39\times 15$ arcmin or
$93.6\times 36$ pc at a distance of 8kpc; Wang et al. wang (2010), Dong et al.
dong11 (2011)) and is expected to detect early-type stars via emission from
their ionised wind. As such detection probability will be a function of both
stellar mass and evolutionary phase, with mass loss rates greater for more
massive stars and wind densities increasing through the stellar
lifecycle121212Subject to the caveat that stars in a cool yellow
hypergiant/red supergiant phase will be undetectable unless their winds are
subject to external ionsiation (cf. Westerlund 1; Dougherty et al. dougherty10
(2010), Fenech et al. fenech (2018)). Since all three young massive clusters
are within the survey footprint we may utilise their well defined stellar
populations to empirically determine its sensitivity to spectral type.

All 13 WNLha stars and 5/7 of the O hypergiants within the Arches are detected
via their Pa$\alpha$ excess, but only 3/30 of the mid-O supergiants and none
of the $>50$ O5-9 stars of luminosity class III to V, although source
blending/confusion may compromise identification given the compact nature of
the cluster (Dong et al. dong11 (2011), Clark et al. 2018a , 2019b ). In the
Quintuplet - which is less compact and so presumably less prone to blending -
all three LBVs and 8/10 of the early-B HGs/WNLh are detected (with the two
missing examples having the weakest He i 2.059 emission and no trace of BrG
emission), along with the sole WN6, 7/16 of the WC stars and all 5 of the
candidate ‘blue stragglers’ of spectral types WN8-9ha and O7-8Ia+. However, as
with the Arches only 1/23 of the O7-B0 Ia supergiants are detected. Although
apparently of lower luminosity (Martins et al. martins07 (2007)), the same
pattern is repeated for stars within the Galactic Centre cluster, with 25/33
of the Wolf-Rayet cohort detected but only one of the 26 OB supergiants
present.

Consistent with our findings for the isolated stellar cohort (Tables 1 and 3),
the cluster detection demographics imply that even stars as extreme as O4
supergiants may be routinely undetectable via their excess Pa$\alpha$
emission. This is all the more striking since such objects likely derive from
very massive stars (M${}_{\rm init}{\gtrsim}40M_{\odot}$; Groh et al. groh14
(2014), Martins & Palacios martins17 (2017), Clark et al. 2018a ).

The Chandra GC survey of Muno et al. (muno09 (2009)) covers a larger field
than the preceding study ($2^{\rm o}\times 0.8^{\rm o}$ or $\sim 280\times
112$ pc at a distance of 8kpc). In total Mauerhan (mauerhan09 (2009)) suggest
$\sim 100-300$ X-ray sources are coincident with near-IR sources with
$K<15.6$; consistent with both late-type giants and massive stars with
luminosities equal to, or greater than, early-B dwarfs. Due to the high column
density towards the GC we would not anticipate detecting the comparatively
soft X-ray emission expected from shocks embedded in the winds of single
stars, suggesting the latter cohort comprise colliding wind binaries (CWB)
and, if present, accreting high-mass X-ray binaries.

Unfortunately, the X-ray emission from CWBs appears a sensitive function of a
number of physical properties (component masses, wind velocities and mass loss
rates, orbital separation and eccentricity) to the extent that such systems
can show no enhancement in X-ray emission over that expected from a single
star, enhanced but soft X-ray emission or excess hard X-ray emission (cf.
Westerlund 1; Clark et al. 2019c ). Given the first two scenarios would not in
general lead to detectable sources we assume this survey is likely incomplete
even for massive CWBs. Empirically, Mauerhan et al. (2010c ) identified 18
massive stars from a sample of 52 near-IR bright ($K<12$) matches to hard
X-ray sources. However a number of targets were selected on the basis of their
proximity to mid-IR structures indicative of the presence of massive stars
(i.e. wind-blown bubbles and bow shocks), precluding a statistical analysis of
the success rate deriving from the application of such observational criteria.
Nevertheless, the distribution of the spectral types of the X-ray bright
massive stars essentially mirrors that derived from the Pa$\alpha$ survey
(Table 1 and Mauerhan et al. 2010c ); such a detection bias is unsurprising
since both surveys are expected to be sensitive to stars supporting dense,
high velocity winds.

The photometrically selected survey of Geballe et al. (geballe (2019)) has a
footprint intermediate between the preceding studies ($2.4^{\rm o}\times
0.6^{\rm o}$ or $\sim 336\times 84$pc at a distance of 8kpc). Designed to
select candidate for follow-up studies of warm diffuse gas it employed a mid-
IR cut ([3.6]$<8$) that is not optimised for identifying massive stars. This
is evident in the detection rate, with $\sim 32$ candidate massive stars from
over 500 spectroscopically surveyed; these being strongly biased towards
sources potentially associated with circumstellar dust such as cool
LBVs/sgB[e] and WCLd stars131313Intriguingly the sample of WCLd stars returned
shows no overlap with that selected via Pa$\alpha$ excess; presumably emission
from the warm dust that allows their photometric detection dilutes emission
features in the spectrum to the point at which they cannot meet the detection
threshold for the survey of Dong et al. (dong11 (2011)).. No WN5-7 or Be stars
were detected, and the three O hypergiants and WNLha stars identified were
coincident with extended mid-IR nebulae, suggesting they were still associated
with their natal material or bow shocks (P35 and 114 respectively; Dong et al.
dong17 (2017)).

In conclusion the combination of different methodologies (and in some cases
subjective criteria) means that we are unable to provide robust quantitative
estimates for the detection thresholds and hence level of incompleteness of
the surveys informing our target selection. However it seem likely that all
three are insensitive to a large number of stars over a wide range of initial
masses and evolutionary phases. Specifically, one would only expect to detect
comparatively low mass objects ($M_{init}\sim 8-25M_{\odot}$; Groh et al.
groh13 (2013)) at the end their lives, via dust emission in an RSG phase or
via a Pa$\alpha$ excess in a Be star episode. Stars above this threshold are
expected to loop back to higher temperatures and hence one might also
anticipate detecting them in a LBV/BHG or WR phase. The same is expected at
still higher masses ($M_{init}\gtrsim 40M_{\odot}$) - a regime in which stars
remain at high temperatures throughout their lives. However, evolutionary
simulations suggest that such stars will only support winds of sufficient
density to permit detection after a considerable proportion of their H-burning
lifetime has elapsed. For example the simulations of Groh et al. (groh14
(2014)) show that a M${}_{\rm init}{\sim}60M_{\odot}$ star has a lifetime of
$\sim 4.0$Myr, but only reaches the early-B hypergiant phase - which
observations of the Quintuplet suggest are detectable via their Pa$\alpha$
excess - after $\sim 3.3$Myr, potentially leaving it undetectable by such a
survey for ${\gtrsim}80$% of its life.

Informed by the stellar population of Westerlund 1, one might expect binarity
to aid detection via all survey methodologies. Mass loss during the active
interaction potentially leads to the formation of sgB[e] stars (cf. Wd1-9;
Clark et al. clark13 (2013); Kastner et al. kastner (2006)) which should be
readily detectable as Pa$\alpha$ excess sources (cf. P40 and 57). Likewise
stripped primaries entering a (proto-)WR phase (cf. Wd1-5; Clark et al.
clark14 (2014)) and mass-gainers/merger products (cf. Wd1-27 and 30a; Clark et
al. 2019a ) both support pronounced emission line spectra which are absent in
their late-O supergiant progenitors. Moreover, dust production in CWBs
containing WC stars enhances the likelihood of their detection at mid-IR
wavelengths, while X-ray emission via wind collision or, more rarely,
accretion onto a compact companion may render identifiable otherwise
undetectable stars.

###  4.2 The origin and spatial distribution of isolated massive stars

Various authors have suggested that massive stars may form in comparative
‘isolation’ in regions of low molecular and, subsequently, stellar density (de
Wit dewit04 (2004), dewit05 (2005), Parker & Goodwin parker (2007)). Indeed,
observations of the Cyg OB2 association (Wright et al. wright14 (2014),
wright16 (2016)) the 30 Dor star forming region (Bressert et al. bressert
(2012), Schneider et al. schneider18 (2018)) and the Small Magellanic Cloud
(Lamb et al. lamb (2016)) are consistent with such an hypothesis. However,
determining the origin of isolated stars is notoriously difficult; the WN5h
star VFTS 682 - which closely resembles the WNLha stars distributed through
the CMZ - being a case in point. Bestenlehner et al. (best (2011)) highlight
its location in the outskirts of 30 Dor and the lack of an associated stellar
aggregate as consistent with its formation in isolation, but are unable to
exclude an origin in - and subsequent ejection from - the young massive
cluster R136, some 29pc distant (which is itself an analogue of the Arches;
Crowther et al. crowther16 (2016)). Therefore before addressing the
distribution of isolated stars through the CMZ it is instructive to consider
the physical mechanisms that may redistribute stars from cluster to field.

####  4.2.1 Stellar redistribution across the CMZ

Two physical mechanisms are thought to give rise to the majority of runaway
stars in the Galactic disc; ejection via SN explosions in binaries (Blaauw
blaauw (1961)) or dynamical interaction (Poveda et al. poveda (1967), Banerjee
et al. banerjee (2012), Fujii & Portegies Zwart fujii (2011)). Considerable
effort has been invested in determining the magnitudes of SN kicks and their
effects on the survivability and motion of binary systems (e.g Renzo et al.
renzo (2019) and refs. therein). With $v\sim 90$kms-1, the high mass X-ray
binary Vela X-1 (B0 Ib + neutron star) suggests that a considerable velocity
may be imparted in at least some cases (Kaper et al. kaper97 (1997)). While
this channel appears inapplicable for the Arches given its age (2-3Myr; Clark
et al. 2018a ), it is likely viable for both the Quintuplet and Galactic
Centre clusters ($\sim 3-3.6$Myr and $\sim 4-8$Myr respectively; Clark et al.
2018b , Paumard et al. paumard (2006)). Indeed the presence of the magnetar
SGR J1745-29 within $\sim 3$” of Sgr A* (Kennea et al. kennea (2013), Mori et
al. mori (2013)) and the young pulsar J1746-2850I within 2’ of the Quintuplet
(Deneva et al. deneva (2009)) are suggestive of ongoing SNe activity in these
regions.

Turning to dynamical ejection and one example of a very massive and high
velocity runaway is VFTS 16 ($M\sim 100M_{\odot}$, $v\sim 112$kms-1), which
Lennon et al. (lennon (2018)) demonstrate to have a proper motion consistent
with an origin in the LMC cluster R136; an aggregate that is too young to host
SNe at this time (Crowther et al. crowther16 (2016)). This is of particular
interest since R136 appears similar to the Arches in terms of its youth,
stellar density and masses of constituent stars (Clark et al. 2018a , 2019b );
suggesting that similar high velocity runaways might be expected to originate
from the latter cluster.

On larger physical scales, the expulsion of residual gas from compact clusters
via stellar feedback has long been posited as a mechanism for driving their
expansion and, in some cases, destruction - with the now supervirial
velocities of the constituent stars dispersing them into the wider field (e.g.
Goodwin & Bastian gb (2006); see also Park et al. park18 (2018) for a
discussion of this effect in the context of the GC). Moreover, the tidal
stripping and the eventual disruption of clusters may also distribute massive
stars through the GC. Simulations of this phenomenon for both the Arches and
Quintuplet suggest that tidal arms of several tens of parsecs may result from
this process after only a few Myr, although the extent of such structures is a
sensitive function of both cluster age and distance from Sgr A∗ (Habibi et al.
habibi (2014), Park et al. park (2020)). By comparison, assuming that both
dynamical interactions and SNe kicks may generate runaway velocities of up to
$\sim 10^{2}$kms-1 such stars may be displaced from their natal clusters by up
to $\sim 100$pc ($\sim 0.7^{\rm o}$ at 8kpc) within $10^{6}$yr - comfortably
less than the age of either the Arches or Quintuplet. Combined with the bulk
orbital motion of both aggregates ($232\pm 30$kms-1 and $167\pm 15$kms-1
respectively; Stolte et al. stolte08 (2008), stolte14 (2014)) one may
anticipate a combination of these three processes potentially stripping
massive stars from their natal clusters and distributing them across a
significant fraction of the CMZ.

####  4.2.2 The distribution of massive stars across the CMZ

Figure 14: Location of different classes of massive stars superimposed on a
greyscale representation of Spitzer 8$\mu$m continuum data. The location of
the Galactic Centre, Arches and Quintuplet clusters are given by green
crosses. Given their displacement from the nominal location of the Quintuplet
cluster we have also plotted the positions of the LBV G0.120-0.048 and the WN6
star qF353E even though they are not included in the census on Table 1. Each
panel covers $\sim 208\times 70$pc at a distance of 8kpc.

We show the locations of the isolated massive stars across the GC in Fig. 14.
In constructing this plot we omitted both OB dwarfs which, to date, have
solely been identified within the Arches and Galactic Centre clusters (Clark
et al. 2019b , Paumard et al. paumard (2006)) and red supergiants, which are
present (e.g. Wollman et al. wollman (1982), Cunha et al. cunha (2007),
Liermann et al. liermann12 (2012)) but difficult to distinguish from cool
interlopers with the data to hand.

It is immediately apparent that there is a large overdensity of stars in the
inner regions of the CMZ; we consider it likely that this is in part due to
the limited footprint of the Pa$\alpha$ survey (running from
$\sim+0^{\circ}14^{\prime}$ to $\sim 359^{\circ}38^{\prime}$; Wang et al. wang
(2010)) which drove placement of the KMOS IFUs. Given such observational
biases and the potential redistribution of stars from their birth sites, we
are limited in the conclusions that may be drawn from these data, especially
for individual objects.

Nevertheless, thanks to the sample size we are able to plot massive, post-main
sequence stars as a function of spectral type which, informed by cluster
demographics, we may use as a proxy for stellar age. Comparison to the Arches
cluster suggests that the mid-O super-/hypergiants and WNLha stars plotted in
the uppermost panel of Fig. 14 are likely to be the youngest and most massive
cohort present ($<3$Myr and $M_{\rm init}\gtrsim 40M_{\odot}$; cf. Clark et
al. 2018a ). A cursory examination reveals this cohort is observed at low
Galactic latitudes, as might be expected for young objects. More intriguingly,
the entire population appears to be distributed at positive galactic
longitudes, with no examples significantly westward of Sgr A∗ (the right half
of the panel in Fig. 14). Given that the entire region is covered by both the
Pa$\alpha$ and X-ray surveys, which are sensitive to such stars, it appears
difficult to attribute this to observational bias.

Historically it had been thought that the bulk of the dense molecular gas
within the CMZ was located at positive galactic longitudes (so eastwards of
Sgr A∗ in Fig. 14) while the majority of young stars identified by mid-IR
observations were instead found at negative longitudes (Morris & Serabyn MS
(1996), Longmore & Kruijssen longmore (2018)). However, the distribution of
mid-O super-/hypergiants and WNLha stars shows the situation is less clear
cut, with a rich population of young, very massive stars coincident with
active star forming regions. Habibi et al. (habibi (2014)) suggested that a
noteworthy association of six stars141414CXOGC J174656.3-283232 (WN8-9ha),
CXOGC J174703.1-283119 (O4-6 Ia), CXOGC J174711.4-283006 (WN8-9ha), CXOGC
J174712.2-283121 (WN7-8ha), CXOGC J174713.0-282709 (WN7-8ha) and CXOGC
J174725.3-282523 (O4-6 Ia). co-located with the Sgr B complex (RA $\sim
0^{\circ}27^{\prime}\rightarrow 0^{\circ}42^{\prime}$, $\delta\sim
0^{\circ}0^{\prime}\rightarrow-0^{\circ}8^{\prime}$) could originate in a
tidal arm associated with the Quintuplet. However the cluster age and
membership profile precludes such an origin for these objects - and indeed for
any of the mid-O super-/hypergiants and WNLha stars within the GC distributed
via any dispersal mechanism (cf. Clark et al. 2018b ). The only cluster young
enough to serve as a viable birthsite is the Arches (Clark et al. 2018a );
however predictions for the geometry of a putative tidal tail suggest that it
would not intersect with Sgr B (Habibi et al. habibi (2014)). Therefore, given
their apparent co-evality and proximity to one another we consider it likely
that the sextet form a distinct, physically related group; we discuss this
putative aggregate further in Sect. 4.2.3.

Conversely, as discussed in Clark et al. (2019b ) the mid-O supergiants and
WNLha stars adjacent to the Arches cluster could have originated there and
been ejected via dynamical interactions or tidal stripping. A further grouping
of four WNLha stars and mid-O hypergiants with small angular separations from
one another are located north east of Sgr A∗ (RA $\sim
359^{\circ}59^{\prime}$, $\delta\sim 0^{\circ}0^{\prime}$); we revisit this
possible assemblage in Sect. 4.2.3.

We plot the locations of the late-O to late-B super-/hypergiants, LBVs and
WN9-11h stars - so called ‘transitional’ objects - in panel two and the
H-depleted WN5-7 and WC8-9 stars in panel three of Fig. 14. Comparison to the
populations of the Quintuplet and GC suggest they derive from an older stellar
population ($\sim 3-8$Myr) than the mid-O hypergiants and WNLha stars and one
that is likely to be more heterogeneous in terms of initial mass
($M_{init}\gtrsim 30M_{\odot}$; cf. Paumard et al. paumard (2006), Martins et
al. martins07 (2007), Clark et al. 2018b ). While both cohorts are again
centrally concentrated there appears less evidence for segregation as a
function of galactic longitude in comparison to the younger stars. We note,
however, that there is an absence of older stars coincident with Sgr B; on
this basis one might speculate that active star formation in this region has
proceded for $\lesssim 3$Myr.

Mindful of observation biases and a comparatively small sample size, a greater
number of the transitional stars and H-depleted WRs appear to be seen in
relative isolation at more extreme galactic latitudes and longitudes than
found for the younger cohort of mid-O hypergiants and WNLha stars. If these
trends are borne out by future observations one might anticipate that their
greater ages have allowed runaways to move further from their birthsites
(noting that some stars, such as the WN10h star WR102ka, appear to have formed
in isolation; Oskinova et al. oskinova (2013)). Indeed, there are a large
number of stars seen in close proximity to both the Quintuplet and Galactic
Centre clusters that could plausibly have originated in those aggregates;
follow up proper motion observations of these would be of considerable
interest (cf. Sect. 4.2.3).

Finally, in this regard we note that a substantial number of isolated stars
appear to demonstrate multiwavelength properties suggestive of a binary
nature. Specifically, given the high column density towards the Galactic
centre, the 24 X-ray detections must by necessity be hard, luminous sources
indicative of emission from a wind collision zone (Mauerhan et al. 2010b ,
Table 1). Similarly, such structures are also thought to mediate the
production of hot dust, yielding a further six candidate binaries amongst the
WCL stars (Table 1)151515The remaining dusty source, CXOGC J174645.2-281547
has already been counted as an X-ray source.. While such systems could arise
from in situ formation, the tidal disruption of a cluster or dynamical
ejection from such a site (Oh & Kroupa oh (2016)) formation via a SN kick
would appear disfavoured; future studies to compare the binary properties of
isolated stars and those within clusters would be of particular interest in
order to elucidate their formation pathways.

####  4.2.3 Clustered star formation within the CMZ

Simple visual inspection of the CMZ fails to reveal any comparably massive
clusters to the Arches or Quintuplet. This absence is unlikely to be driven by
stellar evolution; although the most massive members will be lost to SNe the
remnant population of an ageing cluster will increasingly become dominated by
IR-bright RSGs. For example, if placed in the GC, and assuming a
representative extinction of $A_{K}{\sim}3$, the ten cool super-/hypergiants
within Westerlund 1 would span $K\sim 5-8$ and lie within a nominal cluster
diameter of $\sim 1.25$arcmin (cf. Borgman et al. borgman (1970)); the
respective parameters for RSGC1-4 being broadly equivalent (Figer et al.
figer06 (2006), Davies et al. davies07 (2007), davies08 (2008), Clark et al.
2009a , Negueruela et al. negueruela10 (2009)). Such apparent magnitudes are
comparable to those of the WCLd stars that dominate the appearance of the
Quintuplet - which is also of similar angular extent - suggesting that if
older stellar aggregates were present within the GC they should be readily
identifiable in existing surveys. Since the timescale for cluster disruption
due to tidal forces is expected to be short (e.g. 5-10Myr; Portegies-Zwart et
al. pz (2002)) we might anticipate that their absence is due to dissolution;
although individually luminous, if their constituent stars were distributed
through the CMZ they would be difficult to distinguish from foreground
objects.

We may ask whether there is any evidence for such a process occurring within
the GC. Located within 6” of Sgr A∗, GC IRS 13E is a compact (d$\sim 0.5$
arcsec) group of three co-moving massive stars (Fritz et al. fritz (2010))
that has been suggested to be the tidally stripped remnant of a young massive
cluster and hence provides a possible template for the appearance of such
physical systems. Although the angular resolution of our KMOS observations is
lower than those used to characterise IRS 13E, the field of view of each IFU
($2.8\times 2.8$ arcsec) is ideally suited to the detection of such compact
stellar aggregates. Despite the emplacement of 82 IFUs across the GC and the
presence of multiple stars within $\sim 32$% of these, no further candidates
were found in our observations (Sect. 3.9).

At larger angular scales the spatial coincidence of a number of very massive,
apparently co-eval stars with the Sgr B star forming complex is of
considerable interest (Fig. 14 and Sect. 4.2.2). Sgr B comprises three
distinct regions that appear to be embedded within the same molecular cloud;
Sgr B1, B2 and G0.6-0.0, which bridges the gap between the two former zones.
Observations from mid-IR to radio wavelengths suggest that Sgr B2 is actively
forming stars at this time (e.g. Ginsburg et al. 2018a ), while Sgr B1 appears
older, with bright rimmed and shell-like structures suggestive of the action
of stellar winds (Mehringer et al. mehringer (1992), Hankins et al. hankins
(2020)). Simpson et al. (simpson (2018)) infer $T_{\rm eff}\sim 32-35$kK for
the ionising sources of Sgr B1; directly comparable to the temperatures
expected for the WNLha and mid-O super-/hypergiants co-located with it
(Footnote 14; Martins et al. martins08 (2008), Lohr et al. lohr (2018)).
Moreover, such stars generate the powerful stellar winds and ionising
radiation fields required to sculpt and disperse the molecular material
associated with Sgr B1; further strengthening the possibility of a physical
association between the two.

Simpson et al. (simpson (2018)) suggest that the exciting sources of Sgr B1
did not originate there, but instead are only now impinging on the molecular
cloud by virtue of their orbital motion. While proper motion studies will be
required to assess whether they were born in situ or not, either eventuality
implies that the WNHLha and mid-O super-/hypergiants form a genuine, co-eval
physical association. This is particularly interesting since, lying outside of
the Pa$\alpha$ survey footprint, all six were detected via their hard X-ray
emission and hence are likely massive binaries. Despite over 50 stars of
comparable spectral types being found within the Arches (Clark et al. 2018a ,
2019b ) only four members are known X-ray emitters (Wang et al. wang06
(2006)). While this low detection rate may in part be a function of source
confusion in the crowded confines of the cluster, it raises the possibility
that the ‘Sgr B1’ cohort represents the ‘tip of the iceberg’ in terms of a
similarly rich massive stellar population in this region.

Following on from the potential ‘Sgr B1 association’ and, as might be
anticipated given the source density in the inner reaches of the GC (Fig. 14),
there are multiple further examples of massive stars with comparatively small
angular separations. Examples include (but not limited to): P100 (O4-6 Ia+)
and P101 (WC8-9); P134 (WN7-8ha) and P135 (Be star); and P35 (WN8-9ha), P36
(O4-5 Ia+), P111 (WN8-9ha), P112 (late-B HG/LBV) and P114 (O4-5 Ia+). The
spectral classifications - and hence relative ages - of the components in the
first two pairs disfavours a physical association, although this remains a
possibility for the third group once P112 is excluded due to its evolutionary
state. Of the remaining four stars only P35 and P114 have been subject to
further examination; the radial velocity of the former suggests that it is a
runaway, while the latter appears to have formed formed in situ (and is
possibly associated with a stellar overdensity; Dong et al. dong17 (2017)).
This implies that P35 and 114 are not part of a bound physical system;
although in this regard it is interesting that P35 appears to be a member of a
small group of co-moving stars, possibly the remnant of a disrupted cluster
(Shahzamanian et al. shah (2019)). Given the limitations of the current
analysis no further conclusions may be drawn as to the relationship of either
P36 or P111 to these stars.

Finally, two isolated WCL stars - 2MASS J17443734-2927557 and
J17444083-2926550 - are seen in close proximity to one another and along the
line of sight to the Sgr C H ii region (RA $\sim
359^{\circ}24^{\prime}\rightarrow 359^{\circ}28^{\prime}$, $\delta\sim
0^{\circ}4^{\prime}\rightarrow-0^{\circ}7^{\prime}$; Fig. 14). The mid-IR
morphology of Sgr C suggests ongoing star formation (Hankins et al. hankins
(2020)); if these stars are physically associated with one other and Sgr C it
would extend the duration of such activity to at least $\sim 3$Myr given the
time required for massive stars to progress to the WC phase (e.g. Clark et al.
2018b ). However kinematic data will be required to address this potential
connection.

Indeed, it seems apparent that due to the rich sightlines towards the GC,
unambiguous identification of sparsely populated clusters will require deep
photometric and spectroscopic observations to identify overdensities of co-
eval stars (cf. Sgr B), with radial velocity and proper motion follow up
needed to confirm the reality of putative aggregates (cf. the OB stars
observed in the vicinity of qF353E; Steinke et al. steinke (2016)). This will
be the subject of a future paper, in which we combine radial velocity
measurements from our KMOS observations with proper motion data derived from a
multi-epoch HST survey of the GC (Libralato et al. libralato (2021)).

Table 3: Summary of the population of massive, evolved stars within the GC
broken down by location and spectral type.

| Isolated | Arches | Quint. | Gal Cen. |
---|---|---|---|---|---
Sub-type |  |  |  |  | Total
O4-6 Ia | 5 | 35 | 0 | 0 | 40
O7-8 Ia | 0 | 0 | 19 | 0 | 19
O9-B0 Ia | 3 | 0 | 9 | 0 | 12
OB Ia | 1 | 0 | 10 | 26 | 37
O4-5 Ia+ | 3 | 3 | 0 | 0 | 6
O5-6 Ia+ | 0 | 2 | 0 | 1 | 3
O6-7 Ia+ | 2 | 2 | 2 | 0 | 6
O7-8 Ia+ | 3 | 1 | 0 | 0 | 4
WN7-9ha | 12 | 13 | 2 | 0 | 27
WN9-11h/ | 9 | 0 | 10 | 8 | 27
early-B HG |  |  |  |  |
sgB[e]/LBV/ | 8 | 0 | 3 | 0 | 11
late-B HG |  |  |  |  |
WN5-7 | 8 | 0 | 1 | 4 | 13
WN8 | 0 | 0 | 0 | 5 | 5
WN/WC | 0 | 0 | 0 | 2 | 2
WC5-6 | 0 | 0 | 0 | 1 | 1
WC8-9 | 16 | 0 | 14 | 13 | 43

Numbers derive from this work, Clark et al. (2018a , 2018b , 2019b ) and
Paumard et al. (paumard (2006)). We expect the census for the Arches to be
largely complete for WNLha stars and O hypergiants (Sect. 4.3); however
following from Bartko et al. (bartko (2010)) the numbers for the GC cluster
are lower limits, while it is almost certain that the same is true for both
the Quintuplet cluster members and isolated stars (Sect. 4.3). Given the
uncertainty in spectral types for supergiants within the GC cluster we assign
them a generic OB Ia classification; see text for details. For brevity we
assign the isolated B0-3 Ia star 2MASS J17444501-2919307 to this cohort and
the hybrid star P75 to the WN7-9ha total. See footnotes 5 and 11 regarding the
totals derived for the Quintuplet; the generic OB Ia stars listed for this
cluster are the faint cohort identified by Clark et al. (2018b ); $m_{\rm
F205W}>13$). Finally the 35 O4-6Ia stars within the Arches include five
objects with luminosity class I-III for completeness.

###  4.3 Towards a census of hot, massive stars within the GC

We are now in a position to construct, and interpret, a census of evolved
massive stars in the GC from extant data, encompassing both cluster members
and isolated stars (Table 3). Given the sample size we are able to break down
the supergiants, hypergiants and WRs by spectral sub-type in order to
distinguish stars of differing ages and initial masses. However, we explicitly
exclude cool evolved stars such as 2MASS J17444840-2902163 (Table 1), IRS 7
(Wollman et al. wollman (1982)) and a number of lower luminosity stars of
spectral type K-M along the line of sight to the Quintuplet ($\sim
9-15M_{\odot}$; Liermann et al. liermann12 (2012)) since it is difficult to
reliably distinguish these from the foreground population of cool dwarfs and
giants and hence deliver accurate population statistics.

Observational biases associated with the census of isolated stars have already
been discussed (Sect. 4.1) but it is worth briefly addressing the limitations
of current cluster surveys. Of these we expect the population of WNLha stars
and O super-/hypergiants within the Arches to be essentially complete (Table
3; Clark et al. 2018a , 2019b ); conversely the cohort of 42 O giants and
dwarfs ($M_{\rm init}\gtrsim 16M_{\odot}$) is likely to be increasingly
incomplete as one moves to lower luminosities. As a consequence we omit the
latter from Table 3, emphasising that our observations of the Quintuplet and
the isolated stellar cohort are not sufficiently sensitive to identify such
objects and hence place the respective populations in context161616For this
reason we also exclude the cohort of isolated classical Be stars from the
census at this time (Sect. 3.7), as well as those stars of uncertain
classification (Sect. 3.8).

With current spectral classifications only available for objects within the
central $\sim 40$”$\times 40$” region of the more diffuse Quintuplet cluster -
and being of insufficient depth to reach stars of luminosity class III-V - the
expectation is that the current census will be highly incomplete, even for
very luminous sources (Table 3; Clark et al. 2019c ). This appears borne out
by the low resolution and S/N spectra presented by Figer et al. (figer99
(1999)) which imply that several outlying members appear to be additional
late-O/early B hypergiants of extreme intrinsic luminosity.

Interpretation of the GC cluster is more complex for a number of reasons.
Firstly, Bartko et al. (bartko (2010)) report a further 28 WR/O stars and 34
B-dwarfs (their notation) over those described by Paumard et al. (paumard
(2006)), for a total of 177 early-type stars. Unfortunately no break down of
these revised totals by spectral sub-type was presented; hence we are forced
to utilise the incomplete census of the latter work in the construction of
Table 3. Secondly, since there is considerable observational uncertainty in
the spectral subtypes assigned to supergiants - ranging from $\sim$O7-B3
(Paumard et al. paumard (2006)) - we simply adopt a generic OB supergiant
classification for the 26 stars of luminosity class I-II pending higher S/N
data. While more precise classifications are available for the WN and WC
stars, modelling suggests that they are intrinsically fainter than comparable
members of the Quintuplet, implying systematic differences in initial masses
(Martins et al. martins07 (2007), Clark et al. 2018b ); complicating any
direct juxtaposition of the two samples. Finally, Paumard et al. (paumard
(2006)) identify a cohort of lower luminosity and less evolved objects within
the Galactic centre cluster, reporting three O7-9 giants and 18 dwarfs stars
with spectral types of O9 and later. Following the same line of reasoning
applied to the comparable Arches cohort, we exclude these from the breakdown
of stars by spectral type presented in Table 3, as we do for the large number
of additional objects of uncertain spectral type and/or luminosity class
identified by these authors.

Despite these limitations, Table 3 lists a total of 259 spectroscopically
classified massive, evolved stars located within the GC. It is immediately
apparent that isolated stars contribute significantly, comprising $\sim 26$%
of the currently identified population; therefore any discussion of the role
of such stars in the wider ecology of the GC must account for their
contribution. This is particularly important for the cohort comprising O
super-/hypergiants and WN7-9ha stars which, given their stellar properties,
are likely to dominate mechanical and radiative feedback (Martins et al.
martins08 (2008), Doran et al. doran (2013), Lohr et al. lohr (2018)).

As such, it is striking that the number of WN7-9ha and O hypergiants within
the Arches cluster ($M_{\rm total}\gtrsim 10^{4}M_{\odot}$) is directly
comparable to the isolated population of such objects (Table 3). While the
current count of mid-O supergiants differs between the two settings, with only
five isolated examples identified compared to 35 within the Arches, following
the discussion in Sect. 4.1 we attribute this discrepancy to observational
incompleteness in the field population. Likewise, comparison of the numbers of
isolated early-B hypergiants, WN9-11h and WCL stars to those found within the
comparably massive Quintuplet show that both populations are essentially
identical in size, with the dearth of O7-8 and O9-B0 supergiants (three versus
28; Table 3) again attributable to incompleteness.

The presence of a large number of isolated WN5-7 stars points to an additional
population that is under-represented in both the Arches and Quintuplet (Table
3). Quantitative analysis of two examples - qF353 (WN6; Steinke et al. steinke
(2016)) and IRS 16SE2 (WN5/6; Martins et al. martins07 (2007)) - reveal
extreme temperatures ($T_{\rm eff}>50$kK) for both stars. This suggests that,
alongside the highly luminous WNLha and O-hypergiants, they may be an
important source of ionising radiation in the CMZ. We also highlight that the
population of early-B hypergiants (and WN9-11h stars) within the GC appears
unexpectedly rich when compared to the disc population (cf. Clark et al.
clark12 (2012)). Whether this is due to the particular history of the GC, with
bursts of star formation occurring at times that favour the production of such
stars, observational biases disfavouring the identification of such stars in
the wider galactic disc, or a more exotic explanation, such as the
modification of stellar evolution due to the potentially high metallicity
environment of the CMZ, is currently uncertain. Unfortunately, the cohort of
isolated sgB[e] stars, cool-phase LBVs and/or late-B hypergiants is expected
to be rather heterogeneous in terms of initial mass, thus preventing direct
comparison to the relevant subset of Quintuplet members.

Based on these number counts we suggest that if the isolated stellar
population is drawn from the same (initial) mass function as that of the
Arches and Quintuplet clusters, then it should rival the combined stellar
content of these aggregates. However, allowing for incompleteness and
following suggestions that the quiescent star formation within the GC has
proceeded at an essentially constant rate for the past 5-10Myr ($\sim
0.1M_{\odot}$ yr-1; Barnes et al. barnes (2017)) it is likely that population
of isolated massive stars dominates that of the currently identified stellar
clusters. Even in the case that the isolated stars are drawn from an
unexpectedly top heavy mass function - unlikely given the presence of isolated
classical Be stars and low luminosity red and blue supergiants - the large
number of O super-/hypergiants, WN7-9ha and WN5-7 stars identified to date
confirms that this cohort will play an important role in the ecology of the GC
via radiative feedback.

Nevertheless, upon consideration of all the above studies we arrive at a total
of 437 spectroscopically classified early-type/massive stars within the CMZ.
These comprise 177 stars within the central cluster (Bartko et al. bartko
(2010)), 105 within the Arches (Clark et al. 2019b ), 72 within the Quintuplet
(Clark et al. 2018b  and footnote 11) and 83 isolated examples (Sect. 3.10).

In the absence of reliable mass functions for both clusters and isolated
stars, it is premature to employ these data to estimate the global star
formation rate for the CMZ, which will be dominated by the low stellar mass
component. However we may attempt to place useful lower limits to the SN rate
from these number counts. With masses of $\gtrsim 16M_{\odot}$ expected for
the O9.5 V stars within the Arches (Clark et al. 2018a , 2019b ) and $\sim
8-14M_{\odot}$ for both the isolated classical Be stars and the B0-3 V stars
within the Galactic Centre cluster (Habibi et al. habibi17 (2017)) one would
expect a minimum of $\sim 322$ ($\gtrsim 74$%) of these stars to undergo core
collapse within the next 20Myr (Groh et al. groh13 (2013))171717Exceptions,
which are limited to the GC cluster, include five B4-9 V stars as well as the
large number of early-type stars of uncertain classification., implying a
time-averaged rate of $\sim 2\times 10^{-5}$yr-1. More realistic assumptions
lead to higher rates; for example supposing the 65 H-depleted WRs undergo core
collapse in the next $\sim 0.5$Myr yields a rate of $\sim 1\times
10^{-4}$yr-1, with a similar number returned if we also include the mid-O
super-/hypergiants, early-B hypergiants, WNLha, and WNLh stars and allow for a
greater time until supernova (under the assumption that such stars derive from
$M_{init}\sim 40M_{\odot}$ and will undergo core-collapse within $\sim 2$Myr;
Groh et al. groh13 (2013), Clark et al. 2018a , 2018b ).

By way of comparison, based on the number of pulsars in the GC, Deneva et al.
(deneva (2009)) suggest a birth rate of $\gtrsim 2\times 10^{-4}$yr-1, which
in turn is consistent with a SN rate of $\sim 10^{-3}$yr-1 derived from,
variously, radio observations (Lazio & Cordes lazio (2008)), SN remnant counts
(Ponti et al. ponti (2015)) and simulations of mass and energy flows through
the innermost $\sim 200$pc of the Galaxy (Crocker crocker (2012)). While all
values are comparable, we suspect the incomplete nature of the current stellar
census, particularly for lower mass objects (cf. Sect. 4.1), helps to explain
the order of magnitude discrepancy between the lower limit derived here and
the alternative direct and indirect estimates.

##  5 Conclusions and future prospects

We have presented the results of an extensive $K-$band spectroscopic survey
designed to characterise the population of isolated massive stars within the
GC. The resultant dataset enabled the identification of 17 new objects and the
reclassification of an additional 19 known examples; a further 11 stars
retained extant classifications while a large number of candidates were found
to be cool, foreground sources. Including previous identifications yields a
total of 83 isolated massive stars, of which the vast majority are evolved
objects.

Given the nature of the surveys utilised to construct the target list, the
census is heavily biased towards objects with strong stellar winds - such as
OB hypergiants and both H-rich and H-free WRs - to the extent that even stars
as extreme mid-O supergiants are significantly under-represented. Cool
supergiants do not feature due to the difficulty in distinguishing them from
foreground objects. Amongst the WRs, a large number of H-free WN5-7 stars were
identified, of particular interest since they are almost entirely absent from
both the Arches and Quintuplet clusters. We also report the detection of
classical Be stars; given that the Be phenomenon is limited to spectral type
$\sim$O9 III-V and later, these are likely to be the least massive stars
($M_{\rm init}\lesssim 16M_{\odot}$) in our census. Rare transitional objects
such as (candidate) sgB[e] and LBVs were also recognised; as a consequence
this population will be invaluable for constraining massive stellar evolution
- even for very rapid phases - a possibility exemplified by P75, which appears
to be the first example of an O hypergiant transitioning to a WNLha stage.
Intriguingly, a relatively large number of stars appear to be candidate
binaries by virtue of their multiwavelength properties (Sect. 4.2.2), although
the target selection criteria employed (i.e. X-ray or IR luminosity; Sect.
4.1) will preferentially identify such systems.

Isolated massive stars are found throughout the GC, although the distribution
of mid-O super-/hypergiants and WNLha stars - so the youngest and most massive
cohort sampled - is significantly asymmetric as a function of galactic
longitude. In particular there is an overdensity of such objects spatially
coincident with the young H ii complex Sgr B1; if physically associated they
would imply star formation has proceeded in this region for at least 2Myr.
Intriguingly all these stars were identified via their hard X-ray emission; by
comparison only four from fifty stars of similar spectral types within the
Arches cluster are X-ray bright; on this basis one might therefore anticipate
that this region hosts an exceptionally rich OB association. No further
clusters comparable to the Arches or Quintuplet were identifiable, despite an
expectation that they should be visually prominent for at least the first
$\sim 20$Myr of their lives; if such aggregates formed in the past the most
likely explanation for their current absence is tidal disruption. The compact
stellar grouping IRS13E is likely to represent the endpoint of such a process;
however, despite our survey being sensitive to such objects, no further
examples were identified.

The origin of the remaining isolated massive stars is currently uncertain. A
subset appear likely to have originally formed within a clustered environment
before being ejected; prominent candidates include the WNLha and O-hypergiants
P22, 96, and 97, which are seen in close proximity to the Arches and possess
identical spectral classifications to cluster members. Nevertheless, the
overall population appears too large for them to have all originated in known
clusters via such physical processes (S. Goodwin, priv. comm. 2020); hence a
number were likely born in relative isolation, possibly as the most massive
component of sparse, low density aggregates.

Comparison to extant surveys of the Arches, Quintuplet and Galactic Centre
clusters allow us to place the isolated population into a wider context.
Specifically, the number of isolated WNLha and O-hypergiants is directly
comparable to the Arches cohort, a finding replicated for lone WN9-11h/early-B
hypergiants and WCL stars and those within the Quintuplet. It is currently
uncertain whether the mode(s) of star formation that generate the high mass
clusters and apparently isolated massive stars (or sparse, low mass
aggregates) yield a single, universal IMF; if this is the case it would imply
an isolated stellar population that is, at a minimum, of comparable size to
that of both clusters combined. In total $\sim 437$ massive cluster members
and isolated stars have been identified spectroscopically within the GC; an
unprecedented number in comparison to other resolved star forming regions. Of
these $\gtrsim 74$% will undergo core-collapse, implying a lower limit on the
SN rate of $\gtrsim 1\times 10^{-4}$yr-1; in reasonable agreement with other
direct and indirect estimates but likely a significant underestimate given the
incompleteness of the current surveys.

Moving forward, our homogeneous, high S/N and resolution spectroscopic dataset
will permit tailored quantitative analysis for individual stars in order to
derive their underlying physical parameters (cf. Clark et al. 2018a , 2018b );
a significant by-product of which will be the determination of (differential)
interstellar reddening along multiple sightlines to the GC (cf. Geballe et al.
geballe (2019)). Such a methodology will allow us to populate HR diagrams for
both cluster and isolated cohorts. This is an essential first step in
exploiting the potential of these data to improve our understanding of massive
stellar evolution as well as determining the bulk properties of the
populations. Such an analysis will enable recent star formation across the GC
to be quantitatively constrained (cf. 30 Dor; Schneider et al. schneider18
(2018)) and, critically, will permit calibration of cluster luminosity
functions derived from extant HST photometry in order to construct (initial)
mass functions. In parallel abundance determinations - particularly of
$\alpha$-elements - will help determine whether historical activity proceeded
via a mode characterised by the production of a normal or top-heavy mass
function (cf. Najarro et al. paco09 (2009)). Combining the spectroscopic
dataset with proper motions derived from HST observations will allow us to
identify co-moving stellar groups as well as identify high velocity massive
runaways and, in turn, the relative proportions of massive stars that formed
in true isolation.

Nevertheless, additional observations are required in order to elucidate the
nature and yield of recent star formation activity in the GC. The list of
candidates returned by both the Paschen $\alpha$ and X-ray surveys remain to
be fully exploited (cf. Appendix A), while an extension of the former to
include regions such as Sgr B1 would be invaluable. Fortuitously, despite the
incomplete nature of the current census(es), observations of the 30 Dor star
forming complex (Doran et al. doran (2013)) suggest that the objects
identified - and identifiable via such an approach - are expected to dominate
radiative feedback within the CMZ due to their extreme luminosities (e.g.
WNLha stars and O super-/hypergiants) and/or temperatures (e.g. WN5-7 stars).
However, we are almost entirely insensitive to the expected population of
moderately massive ($\sim 8-20M_{\odot}$) stars that, for any reasonable IMF,
will comprise the majority of core collapse candidates.

Photometric pre-selection followed by a proper motion cut will be necessary to