# Census of R Coronae Borealis stars I: Infrared light curves from Palomar
Gattini IR

Viraj R. Karambelkar<EMAIL_ADDRESS>Cahill Center for Astrophysics,
California Institute of Technology, Pasadena, CA 91125, USA  Mansi M. Kasliwal
Cahill Center for Astrophysics, California Institute of Technology, Pasadena,
CA 91125, USA  Patrick Tisserand  Sorbonne Universites, UPMC Univ. Paris 6 et
CNRS, UMR 7095, Institut d’Astrophysique de Paris, IAP, 75014 Paris, France
Kishalay De  Cahill Center for Astrophysics, California Institute of
Technology, Pasadena, CA 91125, USA  Shreya Anand Cahill Center for
Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
Michael C. B. Ashley  School of Physics, University of New South Wales,
Sydney, NSW 2052, Australia  Alex Delacroix  Caltech Optical Observatories,
California Institute of Technology, Pasadena, CA 91125, USA  Matthew Hankins
Arkansas Tech University, 203 W O St, Russellville, AR 72801  Jacob E. Jencson
University of Arizona, Steward Observatory, 933 N. Cherry Avenue, Tucson, AZ
85721, USA  Ryan M. Lau  Institute of Space and Astronautical Science, Japan
Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara,
Kanagawa 252-5210, Japan  Dan McKenna  Caltech Optical Observatories,
California Institute of Technology, Pasadena, CA 91125, USA  Anna Moore
Research School of Astronomy and Astrophysics, Australian National University,
Canberra, ACT 2611, Australia  Eran O. Ofek  Department of Particle Physics
and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel  Roger
M. Smith Caltech Optical Observatories, California Institute of Technology,
Pasadena, CA 91125, USA  Roberto Soria College of Astronomy and Space
Sciences, University of the Chinese Academy of Sciences, Beijing 100049, China
Sydney Institute for Astronomy, School of Physics A28, The University of
Sydney, Sydney, NSW 2006, Australia  Jamie Soon  Research School of Astronomy
and Astrophysics, Australian National University, Canberra, ACT 2611,
Australia  Samaporn Tinyanont University of California Santa Cruz, 1156 High
St, Santa Cruz, CA 95064  Tony Travouillon  Research School of Astronomy and
Astrophysics, Australian National University, Canberra, ACT 2611, Australia
Yuhan Yao Cahill Center for Astrophysics, California Institute of Technology,
Pasadena, CA 91125, USA

###### Abstract

We are undertaking the first systematic infrared (IR) census of R Coronae
Borealis (RCB) stars in the Milky Way, beginning with IR light curves from the
Palomar Gattini IR (PGIR) survey. PGIR is a 30 cm $J$-band telescope with a 25
deg2 camera that is surveying 18000 deg2 of the northern sky
($\delta>-28^{o}$) at a cadence of 2 days. We present PGIR light curves for
922 RCB candidates selected from a mid-IR color-based catalog (Tisserand et
al., 2020). Of these 922, 149 are promising RCB candidates as they show
pulsations or declines similar to RCB stars. Majority of the candidates that
are not RCB stars are either long period variables (LPVs) or RV-Tauri stars.
We identify IR color-based criteria to better distinguish between RCB stars
and LPVs. As part of a pilot spectroscopic run, we obtained NIR spectra for 26
out of the 149 promising candidates and spectroscopically confirm 11 new RCB
stars. We detect strong He I $\lambda 10830$ features in spectra of all RCB
stars, likely originating within high velocity (200-400 km-s-1) winds in their
atmospheres. 9 of these RCB stars show 12C16O and 12C18O molecular absorption
features, suggesting that they are formed through a white dwarf merger. We
detect quasi-periodic pulsations in the light curves of 5 RCB stars. The
periods range between 30-125 days and likely originate from the strange-mode
instability in these stars. Our pilot run results motivate a dedicated IR
spectroscopic campaign to classify all RCB candidates.

##  1 Introduction

R Coronae Borealis (RCB) stars form a distinct class of variable stars. These
stars are notable for their extreme photometric variations and characteristic
chemical compositions. They are characterised by deep, rapid declines in their
brightness ($\approx 9$ mag in $V$ band, Clayton 1996), that can last for
hundreds of days before they emerge from the low state back to initial
brightness. RCB stars also belong to the class of hydrogen-deficient stars,
with helium being the most abundant element in their atmospheres, followed by
carbon and nitrogen (Asplund et al., 2000). It is debated whether these
objects originate in a white dwarf (WD) merger (Webbink, 1984) or from a final
helium shell flash in an evolved low mass star (Iben et al., 1996). RCB stars
are thus potential low mass counterparts of type Ia supernovae in the double-
degenerate (DD) scenario (Fryer & Diehl, 2008).

Photometric and spectroscopic properties of RCB stars have been studied
extensively. While at maximum light, they are known to pulsate with periods
between 40-100 days and amplitudes of few tenths of a magnitude (Lawson &
Kilkenny, 1996). The stars can then undergo mass loss episodes that eject
“puffs" of dust around them (Feast et al., 1997). If the dust is ejected along
the line of sight to a star, its brightness decreases rapidly. The dust is
eventually blown away by radiation pressure and the star rises back to its
initial brightness. Spectra of RCB stars taken at maximum light suggest that
most of them are F-G type supergiants (Iben et al., 1996) (although a few
“hot" RCB stars with T$>10000$K exist). RCB stars with effective temperatures
$T_{\mathrm{eff}}<6800$K show spectra with absorption features of molecules
such as CO, CN and C2 (Morgan et al., 2003; Tisserand et al., 2020). Most RCB
stars do not show any Hydrogen features in their spectra. However, a few
exceptions exist. The stars V854Cen, VCrA, U Aquarii and DY Cen are relatively
more hydrogen-rich and their spectra show H-Balmer lines and CH bands
(Kilkenny & Marang, 1989; Lawson & Cottrell, 1989). During a photometric
decline, the dust enshrouded RCB star shows a mostly featureless spectrum with
few emission lines.

There are two contending theories to explain the formation of RCB stars -
double degenerate (DD) and the final flash (FF). In the DD scenario, RCB stars
are proposed to be remnants of the merger of a He-core WD and a CO-core WD. In
the FF scenario, they are proposed to be the product of a final helium flash
in the central star of a planetary nebula. The DD scenario is supported by the
discovery of large abundances of 18O in atmospheres of some RCB stars (Clayton
et al., 2007; García-Hernández et al., 2009). Using the CO bandhead in the
near-infrared (NIR) spectra, these studies measured an 16O/18O ratio of order
unity in several RCB stars. For comparison, this ratio is $\approx$500 in the
solar system (Geiss et al., 2002). Such high quantities of 18O can be produced
during a white dwarf merger (Clayton et al., 2007; Jeffery et al., 2011). In
contrast, there is no such model in the FF scenario that can explain the 18O
overabundance (García-Hernández et al., 2009). However, the FF scenario is
supported by the detection of Li and 13C in few RCB stars (Asplund et al.,
2000; Rao & Lambert, 2008). Thus, the DD scenario could account for most of
the RCB stars while a small fraction, those without an 18O overbundance, may
be formed through the FF channel.

There is a big discrepancy between the number of known and predicted Galactic
RCB stars. 117 Galactic RCB stars are currently known (Tisserand et al.,
2020), while the total number is expected to be much larger. Assuming a DD
origin with a WD merger rate of $\approx 10^{-2}$ yr-1, Clayton (2012)
estimate that there are $\approx 5400$ RCB stars in the Milky Way. This number
is consistent with that extrapolated from the RCB population of the Large
Magellanic Cloud (Alcock et al., 2001). From a campaign to identify Galactic
RCB stars using mid-IR colors, Tisserand et al. (2020) estimate the total
number to be $\approx 380-550$. This suggests a lower WD merger rate of $\sim
10^{-3}$ yr-1, which is consistent with theoretical estimates made from
population synthesis via the DD channel (Ruiter et al., 2009; Karakas et al.,
2015). An accurate estimate of the number of RCB stars is important to shed
further light on their progenitors. In the DD scenario, this number will
provide an independent probe of the rate of mergers of white dwarf binaries.
This will be particularly useful as close white-dwarf binaries will be
important gravitational wave sources for LISA (Amaro-Seoane et al., 2017;
Burdge et al., 2020).

Modern time domain surveys provide an attractive avenue for resolving this
discrepancy. The high cadence, long baseline photometric observations are
ideal to flag RCB stars (from their declines). These can be further followed
up spectroscopically to study their chemical compositions. Such studies have
been carried out in the past with data from optical surveys such as the
Massive Compact Halo Object project (MACHO, Alcock et al. 2001; Zaniewski et
al. 2005), EROS-2 (Tisserand et al., 2004, 2008, 2009), the Catalina Survey
(Lee, 2015), All Sky Automated Survey for Supernovae (ASASSN, Tisserand et al.
2013; Shields et al. 2019; Otero et al. 2014) and the Zwicky Transient
Facility (ZTF, Lee et al. 2020). In this paper, we use data from Palomar
Gattini IR (PGIR), an infrared time domain facility to conduct the first near-
infrared (NIR) search for RCB stars.

Palomar Gattini IR (PGIR, De et al. 2020; Moore & Kasliwal 2019) is a 25 sq.
degree J-band camera on a 30 cm telescope located atop Mt. Palomar. PGIR was
commissioned in September 2018 and commenced survey operations in July 2019.
PGIR surveys the entire northern sky (18000 sq. degrees, $\delta>-28^{o}$) to
a depth of $J\approx 16$ mag (AB) and a cadence of $\approx 2$ days. In the
Galactic Plane, the limiting magnitude drops to $J\approx 14$ mag (AB) due to
confusion noise. RCB stars are inherently brighter in the J band compared to
the optical. 95$\%$ of all known Galactic RCB stars have 2MASS J band
magnitudes brighter than 14 mag. Even during a photometric decline, the J band
brightness decreases by $\approx 3$ mag (Feast et al., 1997) as opposed to 9
mag in the optical. Of the known Galactic RCB stars, 66$\%$ are brighter than
11 mag in the J band. This puts a majority of the Galactic RCB stars above the
sensitivity of PGIR. Additionally, a large number of RCB stars are expected to
be located towards the Galactic Center, in regions of high dust-extinction
(Tisserand et al., 2020). The extinction due to dust is significantly lower in
the IR compared to optical wavelengths. This makes PGIR an ideal instrument to
conduct a systematic search for Galactic RCB stars. In the first part of this
search, we focus on PGIR $J-$ band light curves for objects in a pre-existing
catalog of candidate Galactic RCB stars (Tisserand et al., 2020).

The remainder of this paper is structured as follows$-$ in Section 2, we
describe our source catalog and present PGIR $J-$band light curves of 922
objects from this catalog. We use the light curves to identify promising RCB
candidates from contaminants such as long period variables (LPVs) and RV-Tauri
stars. In Section 3, we use our lightcurve-based classifications to identify
IR color-criteria to distinguish between RCB stars and LPVs to subsequently
prioritise our spectroscopic followup. In Section 4 we present results from a
pilot NIR spectroscopic campaign to identify new RCB stars from our list. In
Section 5 we analyse the NIR spectra and light curves of several RCB stars to
derive their radial velocities, photospheric temperatures, 16O/18O ratios,
pulsation periods and discuss the implications of these quantities on their
formation channels. We conclude with a summary of our results and future
prospects in Section 6.

##  2 IR lightcurve-based prioritization

Our source catalog comes from the Tisserand et al. (2020) list of 2194
Galactic RCB candidates selected based on their WISE colors. We performed
forced aperture photometry using a 3 pixel ($\approx 13\arcsec$) aperture on
all J-band PGIR images since November 2018 at the locations of these
candidates to generate light curves for each of them. The average cadence
between observations is $\approx$ 2 days. The baseline of observations is
$\approx$ 500 days, which corresponds to two observing seasons of the Galactic
plane. Further details of the imaging and photometric pipeline can be found in
De et al. (2020). We note that the photometry for sources brighter than
J$\approx 8.5$ mag may not be accurate due to non-linearity effects in the
detector.

Of the 2194 targets of interest, 1209 sources lie in the on-sky area covered
by PGIR ($\delta\gtrapprox-28^{o}$). Of these, 287 are fainter than the
detection limit of PGIR. The remaining 922 sources have J-band light curves.
In addition to RCB stars, this list also contains several contaminants. These
mainly include Miras and dust enshrouded RV-Tauri stars and TTauri stars (cf.
Section 2.4 in Tisserand et al. 2020). Miras are long-period variables (LPVs)
with periods longer than 150 days. RV-Tauri and TTauri stars show pulsations
on timescales of a few weeks (Grankin et al., 2007). In this aspect, their
light curves are similar to RCB stars at maximum light. However, unlike RCB
stars, some RV Tauri stars show subsequent deep and shallow minima. Some also
exhibit a long term periodic trend in addition to the shorter periodic
variations 111http://ogle.astrouw.edu.pl/atlas/RV_Tau.html. To weed out these
periodic impostors, we fit sinusoids to the J band light curves of all
candidates using the GATSPY (VanderPlas & Ivezić, 2015) implementation of the
Lomb Scargle method (Lomb, 1976; Scargle, 1982). We visually examine the fits
to determine the reliability of the derived period.

With this information in hand, we find that the 1209 candidates can be divided
into the following priority groups based on their PGIR light curves $-$

  * •

Priority A : The lightcurve shows sharp, deep declines or rises, or pulsations
at timescales of few weeks, and resembles that of an RCB star. We identify 149
candidates in this category. We further divide this category into 3 sub-
categories based on the trend shown by their light curves as of July 2020 $-$
48 candidates that are _declining_ , 53 candidates that are _rising_ and 48
candidates that are _pulsating_ (with periods $\lessapprox 100$ days).

  * •

Priority B : The lightcurve is ambiguous, not sufficient for any firm
classification. There are 279 candidates in this category. Of these 279, 58
candidates show an overall rise in their brightness and 44 show an overall
decline. 29 candidates have too large photometric errors to identify any
trend. 148 sources show erratic photometric variations.

  * •

Priority C : The lightcurve shows no significant photometric evolution
($<\approx 0.1$ mag) for the last 500 days. There are 199 candidates in this
category.

  * •

Priority D : There are no detections in PGIR data. 287 candidates belong to
this category.

  * •

Priority E : The lightcurve is consistent with that of an RV Tauri or a TTauri
star. 23 candidates belong to this category.

  * •

Priority F : The lightcurve is consistent with an LPV. There are 272
candidates in this category.

The groups are listed in order of priority for spectroscopic followup. Figure
1 show examples of lightcurves in each priority group. The Priority A group is
expected to be rich in RCB stars. Some objects in this group could be LPVs
that show non-sinusoidal variations. Priority B, C and D groups are also
expected to contain RCB stars. Additional PGIR observations will help identify
candidates with RCB-like declines or pulsations in these groups. Table 1 lists
the lightcurve based priorities and sub-priorities for each of the 1209
candidates.

Table 1: Light curve (LC) based priorities, sub-classifications and IR color
based priorities for spectroscopic followup of candidates in Tisserand et al.
(2020) catalog. We assign color-based priorities to all 2194 candidates in the
catalog, and lightcurve-based priorities to 1209 candidates that have PGIR
coverage.

WISE | R.A | Dec | LC-based | LC-based | IR-color based
---|---|---|---|---|---
ToI | (deg) | (deg) | Priority | sub-class | priority
1 | 2.1755 | 63.0093 | A | pulsating | 1-b
2 | 4.7942 | 52.0343 | F |  | 1-a
3 | 7.3382 | 64.8141 | C |  | 1-a
4 | 9.7554 | 59.4681 | B | erratic | 1-a
5 | 11.6178 | 58.9057 | C |  | 1-a
6 | 12.0930 | 74.2992 | A | declining | 1-b
… |  |  |  |  |

  * •

This table is published online in its entirety in the machine-readable format.

Figure 1: Examples of PGIR J-band lightcurves for candidates in each of the
lc-based priority groups described in Section 2. The solid dots represent
detections and triangles represent 5-$\sigma$ upper limits. Of the 1209
candidates from Tisserand et al. (2020) catalog that have PGIR coverage, we
assign 149 candidates Priority A (RCB-like), 279 candidates Priority B
(ambiguous light curves), 199 candidates Priority C (flat light curves), 287
candidates Priority D (no detections), 23 candidates Priority E (RV Tauri-
like) and 272 candidates Priority F (LPV-like). The Priority A group is
expected to be rich in RCB stars, but the groups B, C and D could also contain
RCB stars.

##  3 IR color-based prioritization

Figure 2: The WISE [3.4]$-$[4.6] vs [12]$-$[22] and 2MASS $K$ vs $J-K$ color-
color diagrams for all candidates in the Tisserand et al. (2020) catalog. The
left column shows the plots for candidates in Group 1, middle column shows
candidates in Group 2 and the right column shows candidates in Groups 3. All
candidates in each group are plotted as gray background points. On each color-
color diagram, we highlight candidates that have light curves similar to RCB
stars (Priority A, blue circles), RV Tauri stars (Priority E, red circles) and
LPVs (Priority F, orange circles) present in the respective group. In
addition, we also plot all known RCB stars as black crosses. We assign a new
color based priority to each candidate based on its position in the K vs J$-$K
diagram, as indicated. In the WISE color-color plots, we also indicate the
“disc-box" where RV-Tauri stars surrounded by dust discs are expected to lie
(Gezer et al., 2015). In Group 1, J-K$=4$ line separates LPVs from the known
RCB stars and lc-Priority A candidates. We assign all candidates in Group 1
that have $J-K<4$ mag and lie outside the RV-Tauri disc box to “Group-1-a" and
those inside the disc box to “Group-1-b". For Groups 2 and 3, we cannot
identify a similar distinction between LPVs and RCB candidates. Instead, we
use the positions of known RCB stars and our light curve based priority A
candidates to prioritise these groups for spectroscopic followup (see text).
We prioritise candidates in Group-1-a, Group-2-a and Group-3-a for
spectroscopic followup. Figure 3: Schematic of light curve-based priorities
and IR color based priorities for RCB candidates. We assign light curve based
priorities (A$-$F) to 1209 of the 2194 candidates in Tisserand et al. (2020)
catalog. We also divide the full catalog into 7 groups based on the IR colors
of the candidates. In this figure, we indicate the overlap between each
lightcurve based and IR color based group. For our campaign, we aim to observe
all 149 candidates with lightcurve-priority A. We will also observe 241
additional candidates that have lc-Priorities B, C, D _and_ belong to IR-color
Groups 1-a, 2-a and 3-a. We highlight these 390 candidates in green.

In Section 2, we prioritized 1209 of the 2194 candidates from Tisserand et al.
(2020)’s catalog based on their PGIR light curves. Importantly, we identified
272 LPVs and 23 RV-Tauri stars from their light curves. Here, we explore the
positions of these contaminants in the WISE and 2MASS color diagrams. We use
this to identify color criteria that distinguish between RCB stars, LPVs and
RV Tauri stars. We then assign a color-based priority to each of the 2194
candidates to optimise further NIR spectroscopic follow-up. This priority will
be particularly useful for the 985 candidates that do not have PGIR coverage.

The WISE and 2MASS color-color diagrams were used previously by Tisserand et
al. (2020) to divide their catalog into different groups (their Figure 4).
Group 1 occupies the bottom-right region in the WISE [3.4]$-$[4.6] vs
[12]$-$[22] diagram. This group is expected to contain the maximum number of
RCB stars. Group 2 occupies the top-left region in the diagram and is expected
to contain rare RCB stars that are surrounded by thin or cold dust shells
(T$<500$ K). Finally, Group 3 comprises of objects that are reported with
upper limits in at least one WISE band and objects that have prior
classifications. We analyse the candidates in each of these groups separately
222Tisserand et al. (2020) further divide Group 1 and 3 into two subgroups
each.

Figure 2 shows the WISE [3.4]$-$[4.6] vs [12]$-$[22] and 2MASS $K$ vs $J-K$
color-color diagrams for all 2194 candidates in the source catalog. We plot
these diagrams separately for Groups 1, 2 and 3. All candidates in a group are
plotted as gray background points. On each plot we highlight the candidates
with lightcurve-based priority A (RCB-like, blue dots), priority E (RV-Tauri,
red dots) and priority F (LPVs, orange dots). We also indicate the position of
known RCB stars in these diagrams (black crosses). In the [3.4]$-$[4.6] vs
[12]$-$[22] diagram, we also mark the “disk-box" region, which is expected to
contain RV Tauri stars with dust disks around them (Gezer et al., 2015). We
note that our lightcurve-Priority E (RV-Tauri) candidates lie in this region.

Based on these diagrams, we assign a color-based classification to each of the
2194 candidates as follows -

  * •

Group 1 : This group has 785 candidates, and are expected to contain the
maximum number of RCB stars. The left column in Fig. 2 shows the color-color
diagrams for candidates in this group. These candidates lie in the bottom-
right part of the [3.4]$-$[4.6] vs [12]$-$[22] diagram. We first note that the
K vs J$-$K diagram can be used to separate most LPVs from RCB stars in this
group. If we apply a cut of J$-$K $<4$ mag, we eliminate 90$\%$ LPVs, while
retaining 90$\%$ of known RCB stars and 73$\%$ priority A candidates. We
assign all 422 candidates with J$-$K $>4$ mag the priority “Group-1-c". For
the few RCBs stars lying within this group, the 2MASS observations were likely
obtained while they were in a deep decline phase, being, consequently, highly
reddened. Of the remaining 363 candidates, 111 candidates lie in the RV Tauri
disk box. We assign a priority “Group-1-b" to these stars. We give the highest
priority for spectroscopic follow-up to the remaining 252 candidates –
“Group-1-a".

  * •

Group 2 : This group alone has 987 candidates, and is expected to have the
highest contamination. The middle column in Fig. 2 shows the color-color
diagrams for candidates in Group 2. These candidates lie in the top left part
of the [3.4]$-$[4.6] vs [12]$-$[22] diagram. Although this group contains the
largest number of candidates, very few known RCB stars (7 out of 117) lie in
this region of the diagram. The disk box is not a particularly useful metric
for prioritization as almost all of these candidates lie in the disk box.
Unlike Group 1, no clear distinction can be made between the LPVs, known RCB
stars and photometric RCB candidates in the $K$ vs $J-K$ diagram. If we apply
a cut of $K<9$ mag, we retain 70$\%$ of known RCB stars and 75$\%$ of lc-
priority A candidates. In absence of any other criteria, we prioritise the
sources with $K<9$ mag over the remaining candidates. We assign 548 candidates
with K$<$ 9 mag to the higher priority “Group-2-a" and the remaining 439
candidates to the lower priority “Group-2-b".

  * •

Group 3 : This group has 422 candidates. The last column in Fig. 2 shows the
color-color diagrams for candidates in Groups 3. Based on the K vs J-K
diagram, we prioritise sources that lie inside the box defined by $4<K<10$ and
$J-K<4$ mag. This box contains 85$\%$ of known RCB stars, 88$\%$ of priority A
candidates and only $50\%$ of LPVs. We assign 150 candidates that lie in this
box priority “Group-3-a" and the remaining 272 candidates “Group-3-b".

We list these candidates with their color-based priorities in Table 1. For
spectroscopic followup, we prioritise candidates in Group-1-a, Group-2-a and
Group-3-a in decreasing order. Group-1-b is also expected to contain RCB stars
but is likely contaminated by RV-Tauri stars. The IR colors thus provide an
additional metric to prioritise the candidates for spectroscopic followup.

Our spectroscopic campaign focuses on candidates in the northern hemisphere
that lie in the area covered by PGIR. First, we aim to spectroscopically
follow-up all 149 candidates that have light curve-based Priority A. From the
765 candidates that have light curve-Priority B, C or D, we will follow-up
only 241 candidates that belong to our top three IR-color based groups
(Groups-1-a, 2-a and 3-a). This leaves us with a sample of 390 candidates for
spectroscopic followup. The distribution of the candidates among the different
priority groups is illustrated in Figure 3.

##  4 Pilot NIR Spectroscopic Followup

In Sections 2 and 3, we reprioritised candidates in the Tisserand et al.
(2020) catalog based on their IR light curves and colors. We aim to
spectroscopically followup the candidates in the top priorities to confirm
their true nature. Traditionally, large scale spectroscopic campaigns for
identifying RCB stars have made use of optical spectra. As noted in Section 1,
RCB stars are significantly brighter in the NIR compared to the optical. Thus,
NIR wavebands are better suited to efficiently observe large number of
candidate RCB stars. In addition, NIR spectral features such as the 12C16O and
12C18O bandhead can provide useful diagnostics about the progenitors of these
stars. Thus, a NIR spectroscopic campaign promises to be highly productive in
identifying and characterising RCB stars. No such campaign has been conducted
in the past. As a pilot for such a campaign, we obtained NIR spectra for eight
known RCB stars and 31 of our top lightcurve-based priority candidates. Of the
31 candidates, 26 have lightcurve-priority A and five have lightcurve-priority
B. Here, we present results from this pilot run.

We used Triplespec (Herter et al., 2008), a medium resolution (R$\approx
2700$) _JHK_ band spectrograph on the 200-inch Hale telescope at Mt. Palomar.
We observed eight known RCB stars and 21 candidates on 2019 October 23, and
three additional candidates on 2020 February 3. We further observed seven more
candidates on 2020 June 28 with the Spex spectrograph (R$\approx 1500$) on the
NASA Infrared Telescope Facility (IRTF,Rayner et al. 2003) at Mauna Kea. The
IRTF observations were acquired as part of program 2020A111 (PI: K. De). All
spectra were extracted using the IDL package spextool (Cushing et al., 2004).
The extracted spectra were flux calibrated and corrected for telluric
absorption with standard star observations using xtellcor (Vacca et al.,
2003).

###  4.1 NIR spectral features of known RCB stars

We obtained NIR spectra of eight known RCB stars (as listed on
SIMBAD333https://simbad.u-strasbg.fr) $-$ A0 Her, V391 Sct, NSV 11154, WISE
J184158.40-054819.2 (WISE J18+), WISE J175749.76-075314.9 (WISE J17+), ASAS-
RCB-21, WISE J194218.38-203247.5 (WISE J19+) and ASAS-RCB-20.

Figure 4 shows the _J_ band light curves of these stars. Seven of the known
RCB stars have been reported as cold ($T_{\mathrm{eff}}<6800$K) $-$ AO Her
(Tisserand P., private communication), NSV11154 (Kijbunchoo et al., 2011),
ASAS-RCB-20, ASAS-RCB-21 (Tisserand et al., 2013), WISE-J17+, WISE-J18+ and
WISE-J19+ (Tisserand et al., 2020). The star V391 Sct has been reported as a
warm RCB by Tisserand et al. (2013). Four out of the eight RCB stars $-$
V391Sct, NSV11154, AOHer and WISE-J18+ were at the maximum light phase when
the spectra were obtained. All four stars exhibit quasi-periodic pulsations
with periods between 40$-$140 days. The RCB stars WISE-J19+ and ASAS-RCB-20
were in the photometric minimum phase. WISE-J17+ and ASAS-RCB-21 were rising
out of a photometric minimum towards maximum light. Figure 5 shows the NIR
spectra of these stars, grouped by their photometric phase. We note the
following characteristic features in these spectra $-$

  * •

All RCB stars show He I ($\lambda 10830$) emission or absorption features. The
two stars inside a photometric minimum show helium emission. The emission is
strong for WISE-J19+ (just entering the minimum) and very weak for ASAS-RCB-20
(deep inside the minimum, featureless spectrum). The pulsating and rising
stars show either a fully developed P-cygni profile or strong blueshifted
absorption. We discuss the He I line profiles in more detail in Section 5.3.

  * •

The RCB stars inside a minimum show a mostly featureless reddened spectrum
with He I emission. If the star has entered the minimum recently, other
emission lines such as Si I and C2 are detected. These features are absent if
the star is deep inside the minimum.

  * •

The rising and pulsating RCB stars show a variety of atomic absorption lines.
These include C I (most prominently at 1.0686 and 1.0688$\upmu$m) along with
Fe I, Si I and K I.

  * •

Of the rising and pulsating RCB stars, all but V391 Sct show the CN (1.0875,
1.0929, 1.0966 and 1.0999$\upmu$m) and 12C16O (2.2935, 2.3227, 2.3525, 2.3829,
2.4141$\upmu$m) absorption bandheads. The presence of these molecular
bandheads suggests that these stars have cold photospheres
($T_{\mathrm{eff}}<6800$ K). Importantly, we also detect 12C18O absorption
features in the K band of all these stars. We discuss the CO bandhead in more
detail in Section 5.1.2.

  * •

None of the 8 RCB stars show any Hydrogen features in their NIR spectra.

Figure 4: PGIR _J_ Band light curves for 8 known RCB stars. The black vertical
lines at 0 days mark the epoch at which the NIR spectra were acquired
(indicated in each plot). We note that photometry brighter than 8.5 mag may
not be reliable due to non-linearity effects in the detector. The J-band
magnitudes are in Vega system. Figure 5: NIR spectra of 8 known RCB stars on
2019 November 23. The stars are grouped by their photometric phase - 4
pulsating, 2 rising and 2 fading. We also show a zoom in of the J-band and
K-band spectra of the RCB star WISE-J18+. In the J-band inset plot, we mark
the positions of the CN absorption bandhead in blue and He I absorption in
gray. In the K-band inset, we mark the position of 12C16O and 12C18O
absorption bandheads in blue and black respectively.

###  4.2 New RCB stars

Figure 6: PGIR J band light curves of new RCB stars identified in this paper.
The black vertical lines at 0 days mark the epochs at which the NIR spectra
were acquired. For each RCB star, we also indicate the date when the spectrum
was obtained. We note that photometry brighter than 8.5 mag may not be
reliable due to non-linearity effects in the detector. All J-band magnitudes
are in Vega system. Figure 7: NIR spectra of new RCB stars identified in this
paper. All RCB stars show He I $\lambda 10830$ features. 5 RCB stars that were
in a deep decline show strong emission, while others show either fully
developed p-cygni profiles or strong blueshifted absorption. 4 RCB stars show
the CO absorption bandhead, suggesting that they are cold. Table 2: Properties
of newly identified RCB stars

WISE | WISE | RA | Dec | Light-curve | Temperature | A${}_{\mathrm{V}}^{b}$ | Comments
---|---|---|---|---|---|---|---
Name | ToI | (deg) | (deg) | Phasea |  | mag |
J004822.34+741757.4 | 6 | 12.0931 | 74.2993 | Declining | cold | 4.3 |
J005128.08+645651.7 | 8 | 12.8670 | 64.9477 | Pulsating | hot | 1.2 | pulsating with period $\approx 30$ days
J181836.38-181732.8 | 222 | 274.6516 | -18.2925 | Declining |  | 8.7 |
J182010.96-193453.4 | 223 | 275.0457 | -19.5815 | Rising | cold | 4.9 |
J182235.25-033213.2 | 225 | 275.6469 | -3.5370 | Declining |  | 6.5 |
J190813.12+042154.1 | 268 | 287.0547 | 4.3650 | Declining | cold | 6.8 |
J191243.06+055313.1 | 274 | 288.1795 | 5.8870 | Rising | cold | 7.7 | sharp rise out of a decline
J192348.98+161433.7 | 281 | 290.9541 | 16.2427 | Declining | cold | 22 |
J170552.81-163416.5 | 1213 | 256.4701 | -16.5713 | Declining |  | 1.3 |
J173737.07-072828.1 | 1225 | 264.4045 | -7.4745 | Declining |  | 2.8 |
J185726.40+134909.4 | 1309 | 284.3600 | 13.8193 | Rising | cold | 2.4 | rising from 600 day long minimum

  * •

a : The photometric phase when the spectrum was obtained.

  * •

b : Line of sight extinction, values taken from Schlafly & Finkbeiner (2011)

We obtained NIR spectra for 31 RCB candidates $-$ 26 with lightcurve-priority
A and five with lightcurve-priority B. None of the five priority B candidates
show spectra resembling RCB stars. Of the 26 priority A candidates, we
identify 11 new RCB stars using the spectral features described in Section
4.1. These stars are $-$ WISE-ToI-6, 8, 222, 223, 225, 268, 274, 281, 1213,
1225 and 1309. Table 2 summarises the properties of these stars. Figure 6
shows the light curves of these stars and Figure 7 shows their spectra. Here,
we discuss the properties of these newly identified RCB stars. We group the
stars by their photometric phase at the time the spectra were obtained.

####  4.2.1 Declining

Seven of the 11 new RCB stars $-$ WISE-ToI-6, 222, 225, 268, 281, 1213 and
1225 were in a photometric minimum or were undergoing a photometric decline
when the spectra were obtained. The light curves of WISE-ToI-6, 222, 225 and
1225 indicate that these stars were in a photometric minimum for more than a
hundred days before the spectra were taken. These stars showed photometric
declines of $\approx 3$ mag in the J band and present featureless spectra with
strong He I emission. WISE-ToI-268 and 281 show shallower declines ($\approx
1.5$ mag) and also show brief recoveries from the declines. Their spectra show
strong blueshifted He I absorption and 12C16O and 12C18O absorption bands,
suggesting that they are cold RCB stars. The lightcurve of WISE-ToI-1213 shows
a complex photometric evolution where the star underwent shallow declines and
recoveries of 1 mag _and_ a deep decline of $>3$ mag. The spectrum obtained
during this decline shows He I with a P-cygni profile. We also detect the
12C16O absorption bandhead with traces of 12C18O, suggesting that this is a
cold RCB star.

We note that WISE-ToI-225, 222 and 281 lie in highly extincted regions of the
Galaxy. The integrated line of sight extinction in their direction ($A_{V}$)
is 6.5, 8.7 and 22 mag respectively (Schlafly & Finkbeiner, 2011). Their 2MASS
$J$ band magnitudes are 10.8, 9.0, 10.9 mag and PanSTARRS $g$ band magnitudes
are 19.9, 20.2 and 22.4 mag respectively. These stars thus have $J-g\gtrapprox
10$ mag and illustrate the advantage that NIR searches offer over even very
deep optical searches for RCB stars.

####  4.2.2 Rising

Three stars WISE-ToI-223, 274 and 1309 were rising out of a photometric
decline when the spectra were obtained. ToI 223 was at constant minimum
brightness for 200 days before it rose out of the minimum, brightening by 2.5
mag in J band. ToI 274 previously underwent a decline of 2 mag but recovered
sharply without flattening at the minimum brightness. This variation is
similar to the one observed with DYPer type stars (Tisserand et al., 2009).
The J band lightcurve of ToI 1309 suggests that this star was in a minimum for
at least 600 days before our spectrum was obtained. The spectra of these stars
show strong blueshifted He I absorption, CN, 12C16O and 12C18O absorption
features. ToI 274 also shows a small redshifted He I emission component.

ToI 223 and 274 have been listed as strong RCB candidates by Tisserand et al.
(2020). They note that these stars are the fifth and sixth brightest RCB stars
in the 12$\upmu$m band. They have WISE [12] = 1.0 and 1.2 mag respectively,
indicative of a very bright circumstellar dust shell. The integrated Galactic
extinction in their direction ($A_{V}$) is 4.9 and 7.7 mag respectively,
suggesting that they lie in regions of high dust extinction.

####  4.2.3 Pulsating

The lightcurve of WISE-ToI-8 was showing quasi-periodic variations when the
spectrum was obtained. We identify a tentative period of $\approx 30$ days for
this star. This star was photometrically identified as an RCB candidate on
AAVSO and by the ASAS-SN survey (Jayasinghe et al., 2018), as it underwent
photometric declines in 1999 and 2012. This star has not shown any obvious
photometric declines at least for the last 500 days. The spectrum shows
blueshifted helium absorption and no Hydrogen absorption lines. No CO or CN
features are present, suggesting that this is not a cold RCB star. The
spectrum shows several emission lines, resembling spectra of hot
(T${}_{\mathrm{eff}}>10000$ K) RCB stars. We identify CI emission at
1.68$\upmu$m, CII emission at 1.784$\upmu$m, FeII emission at 1.766 and
1.778$\upmu$m and possibly Ne I emission at 1.7961$\upmu$m. Similar to other
hot RCB stars, the emission lines are double-peaked, which could indicate the
presence of an equatorial outflow around the star (De Marco et al., 2002).

Our pilot campaign has demonstrated the utility of NIR spectra in searches for
RCB stars. We will continue this spectroscopic followup campaign in the
future, aiming for completeness in the top priority classes.

##  5 Preliminary Implications for progenitors

In this paper, we have presented NIR spectra for 19 RCB stars (eight
previously known, 11 newly identified). We have also presented long baseline
($\approx 500$ days) high cadence ($\approx 2$ day) light curves for these
stars. In this section, we analyse the NIR spectra to derive diagnostic
quantities about the nature of RCB stars. In particular, we focus on
photospheric temperatures, oxygen isotope ratios and helium line profiles. We
also derive maximum-light pulsation periods for 5 RCB stars. We discuss the
implications of these quantities on the progenitors and nature of RCB stars.
Although our sample size is not sufficient for any profound insights, the
results are indicative of the potential of the NIR campaign. Future
observations will increase our sample size and shed more light on the open
questions about RCB stars.

###  5.1 Photospheric Temperatures and Isotope ratios

####  5.1.1 SED modeling

We searched archival data from optical, NIR and mid-IR surveys for maximum-
light photometric measurements of RCB stars analysed in this paper. For
optical wavebands we used data from AAVSO, ZTF and ASAS-SN sky surveys. For
NIR wavebands, we compare the 2MASS J-band magnitudes to PGIR lightcurves to
identify RCB stars for which the 2MASS measurements correspond to maximum
light. For mid-IR wavebands, we use the WISE magnitudes directly as the mid-IR
fluxes of RCB stars do not vary significantly (Feast et al., 1997). We
corrected the WISE 4.6$\upmu$m magnitudes for saturation effects based on the
prescription in Tisserand (2012). As the distances to these stars are not
known accurately ($\approx 30\%$ uncertainty from GAIA-DR2, Bailer-Jones et
al. 2018), we cannot estimate the line-of-sight extinction in their direction.
We restrict our analysis to only those stars that have integrated $E_{B-V}$
values smaller than 1.5 mag. We assume 10% uncertainties on the photometric
measurements to account for variations in the light curve and uncertainties in
extinction. We construct maximum light spectral energy distributions (SEDs)
for six RCB stars analysed in this paper. This includes five previously known
RCB stars $-$ AOHer, NSV11154, ASAS-RCB-21, WISE-J17+, WISE-J19+ and one new
RCB WISE-ToI-1213.

We model the SEDs with the radiative transfer code DUSTY (Ivezic & Elitzur,
1997; Ivezic et al., 1999; Elitzur & Ivezić, 2001). In our models, DUSTY is
embedded in a Markov Chain Monte Carlo (MCMC) wrapper using the python package
emcee (Foreman-Mackey et al., 2013). We modeled the star as a blackbody
corrected for molecular absorption. We used synthetic spectra of RCB stars
(see Sec. 5.1.2) to calculate the molecular absorption fractions. The dust is
treated as a shell with density profile $\rho\propto r^{-2}$. We fix the outer
radius of the shell to be ten times the inner radius (Clayton et al., 2011).
We examine optical depth values in the visual band within the range 0
$<\tau_{V}<$ 10 and dust temperatures in the range 300 K
$<T_{\mathrm{dust}}<1200$ K. We assume amorphous carbonaceous dust with a
standard MRN size distribution (Mathis et al., 1977). One limitation of this
analysis is the assumption of spherical symmetry. Therefore, the estimated
dust optical depth could be an over or underestimation depending on the
viewing angle.

The SEDs and best fit models from the dust analysis are shown in Figure 8. The
model parameters and their 1-$\sigma$ uncertainties are listed in Table 3. The
stars are consistent with temperatures between 4500 and 6000 K, surrounded by
dust shells of temperatures between $700-900$ K. With improved distance
estimates from the next GAIA data release and additional monitoring from
optical and NIR surveys, we will be able to estimate the temperatures for
several more RCB stars. Accurate distance measurements will also enable us to
derive luminosities of these stars and place them on the Hertzsprung-Russell
(HR) diagram.

Figure 8: Maximum light spectral energy distributions (SEDs) for six RCB stars
from our sample. We plot the best-fit DUSTY models in blue. For each star, we
also indicate the assumed values of EB-V. The derived model parameters are
listed in Table 3

####  5.1.2 NIR spectral modeling

The NIR spectra of cold RCB stars provide a host of information about their
compositions. Here, we model the CO bandhead in the K band to constrain the
temperatures and oxygen isotope ratios of these stars. However, the
circumstellar dust shells of RCB stars can contribute significantly to the
observed K-band fluxes (up to 80$\%$ of the total flux, Tisserand 2012). The
observed absorption depth of the CO bands depends largely on the assumed dust
contribution. We defer a detailed characterisation of dust shells around these
stars to a future paper. Instead, we leave the dust contribution as a free
parameter in our model, and report a range of effective temperatures and
oxygen isotope ratios for these stars. Secondly, given the medium resolution
of our spectra, it is possible that 12C16O bandheads of some RCB stars are
saturated (García-Hernández et al., 2009). It is thus possible that the values
of the oxygen isotope ratios are lower limits for some stars.

We use a grid of Hydrogen deficient spherically symmetric MARCS (Model
Atmospheres in Radiative and Convective Scheme) atmospheric models with input
compositions characteristic of RCB stars (log $\epsilon$(H) = 7.5, C/He ratio
of 1%, Gustafsson et al. 1975, 2008; Bell et al. 1976; Plez 2008). We
generated the synthetic spectra using the package TURBOSPECTRUM (Alvarez &
Plez, 1998). We set log $g$ = 1.0 and varied the effective temperatures from
4000 to 7500 K in intervals of 250 K. We chose values of 0.5, 1, 2, 5, 10, 20,
50, 500 and infinity for the 16O/18O ratio. We also introduce an additional
parameter
f${}_{\mathrm{dust}}=\frac{F_{\mathrm{shell,K}}}{F_{\mathrm{total,K}}}$ to
quantify the dust shell contribution to the K-band flux, and vary it between 0
to 0.8 in steps of 0.1.

We fit the synthetic spectra to the continuum normalised NIR spectra in the
region 2.26-2.4$\upmu$m. For each star, we visually selected the spectra that
best fit the data for each value of fdust. We used these fits to derive upper
limits on the photospheric temperature and a range of O16/O18 ratios,
irrespective of the dust contribution of the star. Figure 9 shows the best-fit
synthetic spectra together with the continuum normalised NIR spectra. The five
RCB stars WISE-J17+, WISE-J18+, WISE-ToI-223, ASAS-RCB-21 and WISE-ToI-268
show the strongest 12C18O absorption features. The first three are consistent
with having 16O/18O in the range $0.5-10$ while ASAS-RCB-21 and WISE-ToI-268
are consistent with 16O/18O in $2-50$ and $5-50$ respectively (higher value
for higher assumed dust contribution). Using the temperature range for
NSV11154 derived in Sec. 5.1.1, we constrain the 16O/18O ratio in the range
10-50. For the remaining stars, the 12C18O absorption is weaker. Given the
uncertainty in the dust contribution, we cannot put similar constraints on
16O/18O of these stars using our medium resolution spectra. For these stars,
we only indicate a lower limit on the oxygen isotope ratios from our fits.
Table 3 lists the derived effective temperature constraints and oxygen isotope
ratio ranges. In Section 5.1.1, we modeled the SEDs of four of the nine stars
analysed here (AOHer, NSV11154, WISE-J17+ and ASAS-RCB-21). We note that the
temperatures derived from SED modeling are consistent with constraints derived
from NIR spectra.

Figure 9: Best-fits to the K band spectra of RCB stars. The continuum
normalised spectra are plotted as black solid lines and the best-fit synthetic
spectra are plotted as red dashed lines. We mark the positions of 12C16O
(black lines) and 12C18O (blue lines) absorption bandheads. The derived ranges
of best-fit temperatures and oxygen isotope ratios are listed in Table 3.

The high 18O abundances in RCB stars could be key to identifying their
formation mechanism. The reaction 14N($\alpha$,$\gamma$)
F18($\beta^{+}\nu$)18O is an important channel for synthesizing 18O. The cores
of intermediate mass stars just before He burning commences can reach the
temperatures required for this reaction (Warner, 1967). However, if a star
undergoes a final helium flash, the temperature becomes high enough for all
18O to be destroyed. The FF scenario is thus unlikely to explain the high 18O
abundances (García-Hernández et al., 2009). On the other hand, dynamically
accreting material in mergers of CO-core and He-core white dwarfs can be sites
for synthesis of 18O (Clayton et al., 2007). Jeffery et al. (2011) suggest
that even a cold white dwarf merger (i.e., without any nucleosynthesis) could
explain large quantities of 18O in the remnant. These merger models
successfully predict the high 12C/13C ratio observed for most RCB stars
(Asplund et al., 2000). Most RCB stars are thus thought to be formed from the
DD channel. García-Hernández et al. (2010) suggest that the DD scenario could
link RCB stars with Hydrogen-deficient carbon (HdC) stars. This is supported
by the high 18O abundances measured in HdC stars (Clayton et al., 2005).
García-Hernández et al. (2010) note that HdC stars have 16O/18O $<1$ while RCB
stars have 16O/18O $>1$.

There are a few challenges to the DD picture. Lithium has been detected in the
spectra of one HdC (Rao & Lambert, 1996) and four RCB stars (Asplund et al.,
2000). The RCB star VCrA has a high 13C abundance (Rao & Lambert, 2008). The
presence of Li and 13C in these stars are inconsistent with DD models, and
instead favour an FF origin for them (Asplund et al., 2000; Clayton, 2012).
None of these anomalous RCB stars have an 18O abundance measurement, as most
of them are warm.

Oxygen isotope ratios have been previously measured for six known Galactic RCB
stars and three HdC stars (Clayton et al., 2007; García-Hernández et al.,
2009, 2010; Bhowmick et al., 2018). We have reported constraints on the ratios
of six more RCB stars and provided lower limits for three RCB stars. Figure 10
shows a histogram of 16O/18O ratios for all of these RCB stars. We also
indicate ratios for the two HdC stars, and mark the solar 16O/18O value. The
oxygen isotope ratios in the six RCB stars are at least an order of magnitude
lower than the solar value. These stars also do not show any 13C16O features,
further indicating that they originate in a WD merger. Additional NIR
spectroscopic observations that populate this diagram will provide clues the
evolutionary channels of RCB stars. In particular, measuring the isotope
ratios for anomalous cold RCB stars will help illuminate the multiple possible
channels for RCB-formation.

Figure 10: Distribution of 16O/18O ratios of RCB and HdC stars. In red, we
plot all 6 previously available measurements of the isotope ratios in RCB
stars derived from high resolution spectra García-Hernández et al. (2009). In
this paper, we report preliminary constraints on 16O/18O for 4 new RCB stars.
We indicate these new measurements in green. We note that 5 additional RCB
stars reported in this paper show weak 12C18O absorption features, and are
consistent with 16O/18O > 10\. In blue, we plot the isotope ratios measured
for HdC stars, taken from García-Hernández et al. (2009). The high 18O content
of RCB stars compared to sun-like stars favours a white dwarf merger origin
for them. Further NIR spectroscopic observations that populate this diagram
will provide additional clues about the formation channels of RCB stars.

###  5.2 Pulsation Periods and effective temperatures

At maximum light, some RCB stars are known to pulsate with periods between
40-100 days and amplitudes of a few magnitudes (Lawson & Cottrell, 1997).
These pulsations can be fairly irregular $-$ the star can exhibit multiple
pulsation modes or undergo changes in the dominant period. RCrB has shown
pulsations with periods of 33, 44, 52 and 60 days (Lawson & Kilkenny, 1996).
Saio (2008) suggest that these semi-regular or irregular pulsations can be
explained by the onset of the strange instability in the remnant of a WD
merger. They also find that longer period (P$>100$ days) non-radial modes
could be excited in these stars. Using WD merger models, they find that
pulsation periods together with effective temperatures of RCB stars can be
indicative of their mass. These values have been measured previously for only
three RCB stars $-$ RCrB, RY SGr and UW Cen.

The high cadence PGIR light curves are ideal for identifying maximum-light
pulsations in RCB stars. Of the 19 RCB stars analysed in this paper, we can
measure pulsations for five $-$ AO Her, NSV11154 , V391 Sct, WISE-J18+ and
WISE-ToI-8. Four of these are previously known RCB stars but their pulsation
periods and temperatures have not been reported. WISE-J18+ has the most well-
defined period of 41 days with a Lomb-Scargle (LS) score of 0.4. AO Her shows
a period of 56 days (LS score = 0.2), but with significant scatter that could
be attributed partly to non-linearilty effects in the PGIR detector. For
NSV11154, we detect two distinct pulsation modes. The lightcurve shows one
full cycle with a period of 96 days (LS score 0.5) followed by another cycle
with a period of 48 days (LS score 0.5) and slightly smaller amplitude. It is
possible that the first period is an overtone of the second. V391 Sct shows a
full cycle with a period of 124 days (LS score = 0.4). WISE-ToI-8 shows a
period of 30 days at low significance (LS score = 0.13).

Using our NIR spectra, we have put constraints on effective temperatures of 3
of these stars $-$ AO Her, NSV11154 and WISE-J18+ (Table 3). WISE-ToI-8 shows
several strong emission lines, suggesting that it has
$T_{\mathrm{eff}}>10000$K. Tisserand et al. (2013) suggest that V391 Sct has
$T_{\mathrm{eff}}>7500$ K based on the presence of metal absorption lines in
its spectrum. In Figure 11, we plot the periods of these stars against their
effective temperatures.

Figure 11: Pulsation periods against effective temperatures for RCB stars. The
hollow circles indicate the 3 previously known positions of RCB stars (RCrB,
RY Sgr, UW Cen)in this diagram. The solid circles indicate RCB stars analysed
in this paper. The positions of RCB stars in this diagram can be compared to
white dwarf merger models to infer the masses of the remnants (cf. Saio 2008).

###  5.3 Dust driven winds in RCB stars

Clayton et al. (2003) showed that most (if not all) RCB stars have winds.
These winds are thought to be driven by the dust surrounding the star. RCB
stars undergo frequent dust formation episodes (a photometric decline results
if dust condenses along the line of sight to the star). Radiation pressure
from the star can accelerate these dust particles to supersonic velocities.
The dust particles drag the gas along with them, giving rise to a wind.
Consequently, HdC stars $-$ close relatives of RCB stars that do not undergo
major dust formation episodes are not expected to show such winds.

The helium $\lambda$10830 line is an important tracer of winds in RCB stars
for the following reasons $-$ 1) helium is the most abundant elements in RCB
stars, 2) The upper level for this line is 20eV above the ground state, and is
thus unlikely to be populated by photospheric radiation of F-G type RCB stars.
However, this level can be populated by collisional excitation in high
velocity winds around these stars. Clayton et al. (2003) first observed this
line and noted its ubiquitous presence in RCB stars in their sample. While
some stars show a clear p-cygni profile, most show only a strong blueshifted
absorption component, as the emission component is affected by other
photospheric absorption. They model these profiles to find wind velocities of
200-350 km-s-1, with column densities of $10^{12}$cm-2. Clayton et al. (2013)
observed a larger sample and noted that winds are usually strongest when the
RCB is undergoing a decline.

Our NIR spectroscopic campaign is ideal to monitor the HeI $\lambda$10833 line
in a number of RCB stars in various evolutionary phases. Similar to Clayton et
al. (2003) and Clayton et al. (2013), we detect strong HeI features in spectra
of all RCB stars analysed in this paper. Of the 19 total RCB stars analysed, 7
are in a deep decline and show the helium line only in emission. As these
stars do not have any other spectral features, we cannot measure their radial
velocities. We are thus unable to measure wind velocities for these 7 stars.
Figure 12 shows the line profiles for the remaining 12 RCB stars. It is
remarkable that some stars show terminal velocities as high as 400-500 km s-1.
We will present a detailed analysis of the He I profiles in a future paper.

Table 3: Properties derived from the spectra of RCB stars. The three stars
V391 Sct, WISE-ToI-6 and WISE-ToI-1309 do not show strong CO absorption
features.

RCB | RV a | $T_{\mathrm{eff}}^{b}$ | $T_{\mathrm{dust}}^{b}$ | $\tau_{V}^{b}$ | $T_{\mathrm{eff}}^{c}$ | O16/O18 c
---|---|---|---|---|---|---
Name | km-s-1 | K | K |  | K |
AO Her | 500 | 4730${}^{+240}_{-200}$ | 830${}^{+35}_{-45}$ | 1.25${}^{+0.14}_{-0.15}$ | $<5750$ |  $>$10
NSV11154 | 300 | 5480${}^{+140}_{-170}$ | 770${}^{+33}_{-35}$ | 0.38${}^{+0.05}_{-0.04}$ | 4750$-$5750 | 10$-$50
WISE J17+ | 0 | 4850${}^{+160}_{-130}$ | 830${}^{+45}_{-60}$ | 0.36${}^{+0.08}_{-0.05}$ | $<5750$ | 1$-$5
ASAS-RCB-21 | -50 | 5790${}^{+300}_{-390}$ | 800${}^{+25}_{-35}$ | 2.10${}^{+0.16}_{-0.22}$ | $<6250$ | 2$-$50
WISE J19+ | -50 | 5440${}^{+350}_{-350}$ | 920${}^{+30}_{-50}$ | 1.67${}^{+0.16}_{-0.21}$ |  |
WISE-ToI-1213 | -50 | 4980${}^{+360}_{-400}$ | 790${}^{+40}_{-60}$ | 2.21 ${}^{+0.25}_{-0.46}$ |  |
WISE J18+ | -150 |  |  |  | $<6250$ | 0.5$-$2
WISE-ToI-223 | -100 |  |  |  | 4750$-$5750 | 2$-$10
WISE-ToI-268 | -150 |  |  |  | $<6000$ | 5$-$50
WISE-ToI-274 | 0 |  |  |  | $<6000$ |  $>$10
WISE-ToI-281 | 0 |  |  |  | $<6250$ |  $>$5
WISE-ToI-1309 | 0 | .. | .. |  |  |
WISE-ToI-6 | 0 | .. | .. |  |  |
V391 Sct | 0 | .. | .. |  |  |

  * •

$a$ : Radial velocity of the RCB star, rounded to nearest 50 km-s-1.

  * •

$b$ : Quantities derived from modeling of maximum-light SEDs (Sec. 5.1.1).

  * •

$c$ : Quantities derived from NIR spectral modeling (Sec. 5.1.2).

Figure 12: He I $\lambda 10830$ line profiles of RCB stars analysed in this
paper. The helium line is an almost ubiquitous feature of RCB stars. This line
originates in high velocity winds in the atmospheres of these stars. Some RCB
stars show a fully developed p-cygni profile, while some stars show only the
blueshifted component. In the latter case, other photospheric absorption
reduces the emission component. Here, we have excluded the 7 RCB stars that
are in a dust enshrouded dip, as we cannot derive radial velocities for them.
These stars show the helium line in emission. The absorption component is
absent as almost all the continuum is extincted due to dust.

###  5.4 Radial Velocities of RCB stars

The distribution of RCB stars and their velocities will shed further light on
their formation channels. Of the 19 RCB stars (11 new + 8 previously known)
analysed in this paper, we can measure radial velocities for 11 using C I, CN
and CO absorption features in the NIR spectra. Figure 13 shows a histogram of
the measured velocities. In this figure, we also indicate velocity
measurements of other known RCB stars from GAIA DR2. We note that the stars AO
Her and NSV11154 have particularly high radial velocities ($\gtrapprox 300$
km-s-1). Such high velocities could be indicative that these stars could be
part of a halo population. Additional measurements that probe the velocity
distribution of RCB stars will provide better insights about their formation
mechanisms.

Figure 13: Distribution of the radial velocities of RCB stars. Previously
known velocity measurements from GAIA DR2 are plotted in green and
measurements from this study are plotted in red. We note that two RCB stars
have very high radial velocities ($\gtrapprox 300$ km/s), which could be
indicative of a halo population for these objects.

##  6 Summary and way forward

The population of RCB stars in the Milky Way is largely unexplored $-$ only
about a third of Galactic RCB stars are currently known. A systematic census
of RCB stars is necessary to shed light on the nature of these mysterious
objects. Major advances in this direction were made by Tisserand et al. (2020)
with a catalog of 2194 WISE sources expected to contain $\approx 85\%$ of the
RCB stars in the Milky Way. In this paper, we used data from Palomar Gattini
IR, a J band time domain survey, to prioritise this catalog for spectroscopic
followup. 1209 candidates have PGIR coverage, of which 922 are above the
sensitivity of PGIR. Based on their light curves, we assigned a “lightcurve-
based priority" to each of the 922 candidates. The top priority comprises of
149 candidates that show light curves similar to RCB stars. The lowest
priority consists of 272 candidates that we classified as long period
variables and 23 as RV Tauri stars. Using our lightcurve-based classification,
we identified IR color-based criteria that help separate the RCB stars from
the LPVs. We then assigned a second “color-based priority" to each of the 2194
candidates in the catalog. These priorities will aid the process of
spectroscopic followup and maximise the yield of RCB stars.

We then obtained NIR spectra for eight known RCB stars and 26 of our top
lightcurve-based priority RCB candidates. This was a pilot run for the first
NIR spectroscopic campaign to identify RCB stars. We spectroscopically
confirmed 11 new RCB stars and analyzed NIR spectral features of these stars.
In particular, we report constraints on the effective temperatures and oxygen
isotope ratios for these stars. These quantities serve as useful diagnostics
about the compositions and formation mechanisms of RCB stars. We derive
maximum-light pulsation periods for 5 RCB stars, and plot them in the period-
effective temperature space. The position of an RCB star in this phase space
can be used to derive the mass of the RCB star. Although the sample analysed
in this pilot run is small, it is indicative of the potential of a full NIR
spectroscopic campaign for classifying RCB stars.

It is evident that NIR searches promise to be productive in identifying and
characterising RCB stars. NIR time domain surveys are particularly well suited
to discover highly dust enshrouded RCB stars and those lying in highly
extincted regions of the Galactic Bulge and plane. The lightcurve and color-
based priorities identified in this paper will help identify the remaining RCB
stars in the Tisserand et al. (2020) catalog. We aim to complete the
spectroscopic followup of top candidates in these priority classes. Further
observations from PGIR will help improve the lightcurve-based classifications
for the (922) bright candidates visible in the northern hemisphere. The
successor to PGIR in the northern hemisphere is Wide-field Infrared Transient
Explorer (WINTER, Simcoe et al. 2019), a more sensitive J-band time domain
survey (lim. J$\approx$ 20 mag). WINTER will start operating at Mt. Palomar by
December 2021, and will help characterise the (287) fainter candidates in the
northern hemisphere. The Dynamic Red All-sky Monitoring Survey (DREAMS) survey
will be an analog of WINTER in the southern hemisphere at the Siding Springs
Observatory. DREAMS will help characterise the 985 candidates that are
accessible only from the south. Figure 14 shows a schematic of this strategy
for our NIR census of RCB stars.

Moreover, systematic blind searches using data from these telescopes will help
discover RCB stars outside this catalog. This includes the more exquisite RCB
stars with double dust shells around them and DY Per stars$-$ the colder
cousins of RCB stars. The NIR census thus promises to uncover the Galactic RCB
population.

## Acknowledgements

Palomar Gattini-IR (PGIR) is generously funded by Caltech, Australian National
University, the Mt Cuba Foundation, the Heising Simons Foundation, the Bi-
national Science Foundation. PGIR is a collaborative project among Caltech,
Australian National University, University of New South Wales, Columbia
University and the Weizmann Institute of Science. MMK acknowledges generous
support from the David and Lucille Packard Foundation. J. Soon acknowledges
the support of an Australian Government Research Training Program (RTP)
scholarship. Some of the data presented here were obtained with Visiting
Astronomer facility at the Infrared Telescope Facility, which is operated by
the University of Hawaii under contract 80HQTR19D0030 with the National
Aeronautics and Space Administration.

Figure 14: A schematic of our strategy for an NIR survey of Galactic RCB stars
focusing on the Tisserand et al. (2020) catalog. 1209 candidates from this
catalog are accessible from the northern hemisphere. Of these, 922 candidates
are bright enough to be characterised by PGIR. The remaining 287 fainter
candidates will be characterised by WINTER $-$ a deeper (J$\approx$20) NIR
time domain survey starting December 2021. The candidates in the southern
hemisphere will be characterised by DREAMS, an analog of WINTER in Australia.

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