11institutetext:  European Southern Observatory, Alonso de Cordova 3107,
Vitacura, Santiago, Chile 22institutetext: UK Astronomy Technology Centre,
Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 33institutetext: Max-
Planck-Institut fur Astronomie, Königstuhl 17. 69117 Heidelberg, Germany
44institutetext: Leiden Observatory, Leiden University, PO Box 9513, 2300 RA
Leiden, The Netherlands 55institutetext: Instituto de Astrofísica de Canarias,
C/ Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain 66institutetext:
Departamento de Astrofísica, Universidad de La Laguna, E-38200 La Laguna,
Tenerife, Spain 77institutetext: GEPI, Observatoire de Paris, PSL Research
University, CNRS, Place Jules Janssen, 92190 Meudon, France 88institutetext:
European Space Agency, ESA Office, Space Telescope Science Institute, 3700 San
Martin Drive, Baltimore, MD 21218, USA 99institutetext: Departamento de
Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. Torregjon a Ajalvir km.
4, 28850-Madrid, Spain 1010institutetext: School of Astronomy, Institute for
Research in Fundamental Sciences, 19395-5531 Tehran, Iran 1111institutetext:
ACRI-ST, Centre d’Etudes et de Recherche de Grasse (CERGA), 10 Av. Nicolas
Copernic, 06130 Grasse, France 1212institutetext: Nicolaus Copernicus
Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw,
Poland 1313institutetext: AAAS Science International, Clarendon House,
Clarendon Road, Cambridge CB2 8FH, UK 1414institutetext: European Southern
Observatory, Karl-Schwarzschild-Str. 2 85748 Garching bei München, Germany
1515institutetext: NASA Goddard Space Flight Center, 8800 Greenbelt Road,
Greenbelt, MD 20771, USA 1616institutetext: Department of Physics and
Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
1717institutetext: Institute for Earth and Space Exploration, The University
of Western Ontario, London, ON N6A 3K7, Canada 1818institutetext: SETI
Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94043, USA

# A high resolution study of near-IR diffuse interstellar bands, search for
small scale structure, time variability and stellar features

J. V. Smoker 1122 A. Müller 33 A. Monreal Ibero 445566 M. Elyajouri 77 C. J.
Evans 2288 F. Najarro 99 A. Farhang 1010 N. L. J. Cox 1111 J. Minniti 1212 K.
T. Smith 1313 J. Pritchard 1414 R. Lallement 77 A. Smette 11 H. M. J. Boffin
1414 M. Cordiner 1515 J. Cami 1616 17 17 18 18

###### Abstract

Context. The diffuse interstellar bands (DIBs) are a set of hundreds of
unidentified absorption features that appear near ubiquitously throughout the
interstellar medium. Most DIBs appear at optical wavelengths, but some are in
the near-infrared.

Aims. We aim to characterise near-infrared DIBs at high spectral resolving
power towards multiple targets.

Methods. We observed 76 early-type stars at a resolving power of 50,000
(velocity resolution $\sim$6 kms-1) and signal to noise ratios of several
hundreds using the CRyogenic high-resolution InfraRed Echelle Spectrograph
(CRIRES). These data allow us to investigate the DIBs around 1318.1, 1527.4,
1561.1, 1565.1, 1567.0, 1574.4 and/or 1624.2 nm. We detect a total of six DIB
features and 17 likely stellar features assisted by comparisons with a model
spectrum computed with cmfgen. Additionally, we measured equivalent widths of
the DIBs at 1318.1 and 1527.4 nm using observations with X-shooter towards ten
very highly reddened (3.2 $<E(B-V)<$ 6.5) Cepheid variable stars, towards four
stars observed at low values of precipitable water vapour and using other
archive data.

Results. We measure correlations (correlation coefficient $r\sim$0.73–0.96,
depending on the sub-sample used) between DIB equivalent width and reddening
for the DIBs at 1318.1, 1561.1, 1565.1 and 1567.0 nm. Comparing the near-
infrared DIBs with 50 of the strongest optical DIBs, we find correlations $r$
¿ 0.8 between the 1318, 1527, 1561, 1565 or 1567 nm and optical DIBs 5705,
5780, 6203, 6283 and 6269 Å. The 5797 Å DIB is less well correlated with the
near-infrared DIBs. The DIB at 9632.1 Å, (likely C${}_{60}^{+}$), is not well
correlated with the 1318.1 nm DIB. Partial correlation coefficients using
$E(B-V)$ as the covariate were also determined. For stars earlier than B2, the
1318.1 nm DIB is affected by an emission line on its blue wing that is likely
stellar in nature, although we cannot rule out an interstellar or
circumstellar origin for this line caused by for example a DIB in emission.
The 1318.1 nm DIB also has an extended red wing. The line is reasonably well
fitted by two gaussian components although neither the component ratios nor
separation between them are obviously correlated with indicators of such as
$\lambda\lambda$5780/5797 and reddening. EW(1318 nm) correlates with H i with
EW(1318 nm)/$E(B-V)$ decreasing with f(H2).

Five pairs of stars within one arcmin of each other show very similar 1318.1
nm DIB profiles. Possible variation in the 1318.1 nm feature is seen between
HD 145501 and HD 145502 (separated by 41 arcsec, equivalent to 7200 au) and HD
168607 and HD 168625 (separated by 67 arcsec, equivalent to 0.52 pc on the
plane of the sky). Seventeen sightlines have repeat CRIRES observations
separated by 6–14 months and two sightlines have repeat X-shooter observations
separated by 9.9 years. No time-variability is detected in the 1318.1 nm DIB
in the CRIRES data or the 5780.5, 5797.1 1318.1 and 1527.4 nm DIBs. Tentative
time variation is observed in the ”C${}_{60}^{+}$” DIBs at 9577 and 9632 Å
towards HD 183143 although very close to the noise level and with confirmation
required.

Conclusions. The NIRDIBs observed occur more in more UV-irradiated regions
than the 5797 Å DIB allowing the study of heavily reddened sightlines. Future
searches for time variability in DIBs require higher quality data and/or
larger intervals between epochs.

###### Key Words.:

ISM: clouds – ISM: lines and bands – ISM: molecules – ISM: dust, extinction –
stars: early type – stars; emission line

##  1 Introduction

The diffuse interstellar bands (DIBs) were discovered around a century ago as
unidentified absorption lines in the spectra of reddened stars (e.g. Heger,
1922). The lines are generally several angstroms wide, so cannot be atomic,
and were initially hypothesised to arise from dust grains or large molecules
(see Cox 2011 for a review). There is a correlation between reddening and DIB
strength (although with large scatter at high reddening and with
$E(B-V)\sim$0.2–0.3; Wu et al. 1981, Herbig 1993, Vos et al. 2011), which
initially favoured the dust hypothesis. However, a dust carrier was expected
to produce emission in the wings of the DIB profiles (van de Hulst, 1946)
which has not been detected, wavelength shifts and profile variations due to
changes in the composition, size and structure of the dust grains that are not
observed, and polarisation in the absorption lines which has also been ruled
out to a level of less than 0.01 percent for 45 DIBs (Cox et al., 2011).

The most supported hypothesis is that the DIB carriers are large gas-phase
molecules. Evidence for this includes the complex component structure of
several DIBs which are reminiscent of molecular rotational band profiles (Kerr
et al., 1996). Potential carriers include the polycyclic aromatic hydrocarbon
(PAH) molecules, which are known to be abundant in space from their mid-
infrared bands (Allamandola et al., 1989; Salama et al., 1996; Meeus et al.,
2001). Several hundred DIBs have been identified in the interstellar medium
(ISM), including in the Milky Way (e.g. Hobbs et al., 2009) and nearby
galaxies such as the Andromeda Galaxy (Cordiner et al., 2011), Messier 33
(Cordiner et al., 2008) the Large Magellanic Cloud (Cox et al., 2006) and the
Antennae Galaxies (Monreal-Ibero et al., 2018).

Identifying the carriers via laboratory experiments is impractical as there
are more than 1.2 million different species of PAHs with 100 or less carbon
atoms which would need to be tested (Cami et al., 2018). The only DIBs to have
been convincingly assigned to a carrier are two (possibly four) DIBs due to
ionised buckminsterfullerene C${}_{60}^{+}$ (Campbell et al. 2015, 2016; Kuhn
et al. 2016; Cordiner et al. 2017, 2019; although see discussion by
Galazutdinov et al. 2017b; Lallement et al. 2018).

Most studies of DIBs have concentrated on optical wavelengths (e.g. Merrill &
Wilson, 1938; Raimond et al., 2012; Cox et al., 2017)111In this paper we use
Angstroms for DIBs bluewards of 1000 nm and nm for redder wavelengths..
However, laboratory measurements show that larger PAHs (more carbon atoms)
have their strongest absorption lines in the near-infrared (NIR) (Mattioda et
al., 2005; Ruiterkamp et al., 2005). Indeed, several DIBs have been observed
in this region (Joblin et al. 1990; Geballe et al. 2011; Rawlings et al. 2011;
Cox et al. 2014; Elyajouri et al. 2017). These include twenty NIR DIBs that
have been identified in observations of nine stars at a spectral resolving
power ($R$) of $\sim$10,000 between 950 and 2500 nm (Cox et al., 2014). No
DIBs were found redwards of $\sim$1800 nm. The strongest correlations were
found between the equivalent widths of some NIR DIBs (e.g. that at 1318.1 nm)
and the optical DIB at 5780 Å (correlation coefficient $r$=0.98).
Additionally, many NIRDIBs were also found to correlate with reddening with
$r\sim$0.9.

Other studies include those investigating the 1527 nm DIB on a statistical
basis towards $\sim$60,000 mainly late-type stars at $R$ $\sim$ 22,000 using
data from the APOGEE survey (Zasowski et al., 2015b). They found that this DIB
persists in the ISM for long timescales and correlates with dust, but has a
scale-height above the Galactic plane of $\sim$100 pc, twice that of dust.
Elyajouri & Lallement (2019) also investigated this DIB using a larger sample
from the APOGEE survey, providing measurements for 124 064 sightlines and
demonstrating that the carrier is concentrated in the outskirts of
interstellar clouds. Hamano et al. (2015, 2016) presented DIB observations
from 0.91–1.32 $\mu$m at $R$=28,000. They concluded that the 1078.0, 1079.2
and 1179.7 nm features likely belong to related carriers and correlate with
the 5780 Å DIB. Galazutdinov et al. (2017a) studied fourteen DIBs in the 1.45
to 2.45 $\mu$m range and measured basic correlations between the observed
bands. Elyajouri et al. (2017) confirmed several weak DIBs and found that the
1527 nm DIB is most abundant in diffuse interstellar clouds with low density
and high ultraviolet radiation fields (known as $\sigma$-type clouds, with
$\zeta$-type clouds having opposite properties). Hamano et al. (2022) looked
at DIBs in the Y and J bands to postulate that DIBs at longer wavelength are
caused by larger molecules. Finally, Ebenbichler et al. (2022) observed four
stars at $R$=100,000 with CRIRES to confirm 17 new DIBs and identify a
possible 12 additional DIBs.

We seek to extend previous work by observing multiple NIR DIBs at high
spectral resolving power ($R\sim 50,000$) and to better constrain the known
correlations using data of higher signal to noise ratios (S/N). Some optical
DIBs are known to vary on small scales in the ISM (see Section 4.5 for
references), so we also searched for such small-scale structure in the NIR
DIBs. No optical DIBs are known to vary (convincingly) with time, but even so
we also searched for time variability in the NIR DIBs using twin epoch
observations of a number of sightlines.

This paper is organised as follows. Section 2 describes our sample of targets,
observations, data reduction and telluric line removal. Section 3 outlines the
analysis of the data, including profile fitting. The discussion is presented
in Section 4, including discrimination between stellar and interstellar or
circumstellar features, the shape of the DIB profiles, a comparison of NIR DIB
line strengths with optical DIBs, atomic lines and reddening, and finally a
search for small-scale and time-variability in the NIR DIBs studied. In
Section 5 we summarise our results and make suggestions for future work.

##  2 Sample, observations and data reduction

###  2.1 The Sample – CRIRES observations and data reduction

Most of the sample was observed using CRIRES (Kaeufl et al., 2004), mounted on
the 8.2-m Unit Telescope One (Antu) at the Very Large Telescope (VLT) under
programme IDs 091.C-0655 (epoch 1) and 093.C-0480 (epoch 2). In epoch 1 the
central wavelengths on detector 3 (of the four CRIRES detectors) were 1318.1
nm and/or 1574.4 nm and 1624.2 nm. In epoch 2 we observed many fewer stars but
also including 1527.0 nm and 1568.8 nm settings for a few sightlines. The
wavelength coverage in each setting was small, ranging from 31 nm for the
1318.1 nm setting to 37 nm for the 1624.2 nm setting, with gaps of around 2 nm
between the four detectors on the mosaic. The vacuum wavelengths observed for
each star are listed in Table 9. Observations were scheduled during poor
weather conditions, so we used the widest slit possible with CRIRES of 0.4″
and no adaptive optics for the majority of the stars, yielding a resolving
power of $R\equiv\lambda/\Delta\lambda\sim 50,000$, as measured from the
telluric lines.

The CRIRES sample includes 67 O- and B-type stars (median magnitude in the
$H$-band of 5.9) for observation between April and September 2013, and follow
up of 17 stars with additional observations between March and June 2014. The
latter group are either repeat observations of stars observed in the first
epoch (to search for time-varying DIB strength), or companion stars in double
or triple systems intended to search for small-scale structure in the ISM. The
total sample comprised 76 stars. We favoured early-type stars as they
typically have few stellar lines at the wavelengths of the NIR DIBs and are
often fast rotators (e.g. Stütz et al., 2006), making it easier to distinguish
between stellar and interstellar features. The CRIRES-observed sightlines have
interstellar reddening ranging from 0 $\simeq E(B-V)\simeq$ 1.5 mag (median
0.6 mag), estimated from their observed $(B-V)$ colours and the intrinsic
colour of the relevant spectral type taken from Fitzgerald (1970) and Wegner
(1994).

Total integration times ranged from 60 s to 1200 s depending on target
brightness, with a median of 600 s. An ABBA nodding pattern was used to
facilitate sky subtraction. For a handful of targets which had close
companions, adaptive optics was used to separate the components. Data were
reduced using the original CRIRES data reduction pipeline
222https://www.eso.org/sci/software/pipelines/crires/crires-pipe-recipes.html
using instrument calibration data taken the morning after the observations and
including a correction for detector non-linear response. The initial
wavelength calibration was performed using exposures of a ThAr lamp, then
refined during telluric correction (see below). The median S/N ratios of the
reduced spectra in the 1318.1 nm setup were $\sim$300 per pixel ($\sim$550 per
resolution element) with similar values for the other wavelength settings.

###  2.2 X-shooter observations and archival data

To increase the range of $E(B-V)$ sampled in this study, X-shooter (Vernet et
al., 2011) spectra of ten stars with $m_{J}<16.0$ mag. were taken from Minniti
et al. (2020), who studied Cepheid variables towards the Galactic Centre. The
S/N ratios were between 10 and 50 per pixel at 1318 nm and higher at 1527 nm,
with $R\sim 8,000$. These targets have extinction in the Ks band ranging from
1.13 to 2.25 mag, corresponding to $E(B-V)$ values between 3.13 and 6.51
($A_{V}$ from $\sim$9.7 to 20.2), assuming the extinction law of Cardelli et
al. (1989). Their distances range from 8.0 to 31.5 kpc.

Additionally, four stars were observed with X-shooter under dry weather
conditions (precipitable water vapour PWV of less than 0.5 mm), with the aim
of searching for time variation in the C${}_{60}^{+}$ and NIR DIBs. Finally,
we downloaded reduced spectra for a further nineteen X-shooter targets from
the ESO archive to investigate the dependence on NIRDIB strength with
fractional H2 content.

Figure 1 shows the distribution of the sample on the sky for the CRIRES and
X-shooter spectra with Table 9 listing basic information for all the observed
stars.

###  2.3 Telluric correction

Correction for telluric lines was performed using the molecfit
package333http://www.eso.org/sci/software/pipelines/skytools/molecfit (Smette
et al., 2015). The precipitable water vapour was fitted by the code. Example
spectra before and after telluric line correction are shown in Fig. 2 in two
wavelength settings, each at low (0.2 mm) and high (6.0 mm) precipitable water
vapour.

##  3 Data analysis

The data analysis was straightforward and consisted in normalising the spectra
with polynomials of order 3–4 either side of the DIB features and measurement
of the equivalent widths (EWs) and full-width half-maximum (FWHM) velocity-
widths by fitting Gaussians to the detected components. Errors were calculated
according to equation (10) of Ebbets (1995). Data were transformed to the
Heliocentric restframe using
rv444http://www.starlink.ac.uk/docs/sun78.htx/sun78.html. Where the expected
DIBs were not detected, five-sigma upper limits to the EWs were estimated
according to:

$EW_{\rm upperlim}=5.0\times\frac{\sqrt{N_{s}}\times\Delta\lambda}{S/N},$

where S/N is the signal to noise ratio per pixel, $\Delta\lambda$ is the width
of each intensity sample in Angstroms and $N_{s}$ is the number of samples.
The derived errors were compared with those estimated in twelve cases where
the same star was observed on different nights. The standard deviation on the
measurements is 8 percent ($N$=12) for EWs $\gtrsim$ 15 mÅ, with the largest
difference in measurements at different epochs of 23 percent being towards HD
145482 with a (small) EW of $\sim$4 mÅ for the 1318.1 nm line.

Excluding the handful of sightlines contaminated by stellar lines, the
detection rates for the DIBs approached 90 per cent for the 1318, 1561.1,
1565.1 and 1567.0 nm features. Finally, no clear DIBs were detected in the
CRIRES setting around 1624.2 nm. Table 2 shows the equivalent width results
for the current sample as well as the 5780 and 5797 Å DIBs from optical
spectra discussed in Sect. 4.3.

Figure 1: The distribution of our sample in Galactic coordinates, colour coded
by reddening. Most targets are close to the Galactic plane, with the highest
reddenings near the Galactic Centre.

Figure 2: Examples for two wavelength regions of the use of molecfit to remove
telluric lines from spectra when the precipitable water vapour (PWV) was
$\sim$6.0 mm (plots (a) and (b)) and $\sim$0.2 mm (plots (c) and (d)), for HD
92740 and HD 116119. Spectra are normalised. The median night-time PWV for
Paranal is 2.4 mm (Kerber et al., 2014). The lower spectra in each panel are
the reduced data, with the upper spectra the same objects after telluric
correction (offset in the ordinate for clarity). Vertical lines above the
corrected spectra indicate the DIBs at 1318.1, 1561.1, 1565.1 and 1567.0 nm.
Vacuum wavelengths are shown. The dashed vertical line displayed for HD 116119
at a wavelength of 1316.8 nm indicates a likely stellar feature caused by an O
i triplet.

##  4 Discussion

We detected a total of 23 non-telluric features, of which 17 are likely to be
stellar and six interstellar (or possibly circumstellar) in nature. Figure 3
shows examples of each, with all spectra being shown in Fig. A1. These are
discussed in turn. Table 1 shows all of the detected lines.

Figure 3: Normalised CRIRES spectra showing examples of all stellar and
interstellar features observed. The ordinate is the normalised flux plus an
offset with the abscissa being the wavelength in nm. Stellar lines: Red stars:
1315.4 nm, Green: 1316.8 nm, Cyan: 1316.5 nm (EmL), Purple: 1317.3 nm (EmL,
uncertain), Light Green: 1529.9 nm, Yellow: 1566.1 nm (EmL), Gold: 1562.4,
1563.0, 1563.5 nm), Navy: 1633.8; Black: 1610.9 nm; Brown: 1555.7, 1616.7,
1625.3, 1626.1, 1634.2 nm (EmL). DIBs: Blue squares: 1318.1 nm, Green: 1527.1
nm, Cyan: 1561.1 nm, Purple: 1565.1 nm, Pink: 1567.0 nm. Not shown: broad and
weak feature around 1564.4 nm. Table 1: Rest wavelengths in vacuum of lines
detected in the CRIRES spectra. We also show lines taken from the NIST
database 666$https://physics.nist.gov/PhysRefData/ASD/lines_{f}orm.html$
within $\pm$0.2 nm in vacuum wavelength.

$\lambda$ | Stellar | Abs. or | Notes | Lines from the NIST database within $\pm$0.2 nm
---|---|---|---|---
(nm) | or IS | EmL |  |
1315.4 | S | Abs. | (1) | Zn i (1315.42 nm), Al i (1315.43 nm), Th i (1315.45 nm), Fe i (1315.52 nm)
|  |  |  | Fe i (1315.52 nm), Cs ii (1315.53 nm)
1316.5 | S | EmL | (2) | C i (1316.42 nm), Fe ii (1316.60 nm), Fe ii (1316.65 nm), Cs ii (1316.68 nm)
1316.8 | S? | Abs. |  | O i (1316.75, 1316.85, 1316.87 nm), Fe ii (1316.82 nm)
1317.3 | S | EmL | (3) | Th i (1317.14, 1317.19 nm), Fe i (1317.39, 1317.42 nm), V i (1317.43 nm),
|  |  |  | Th ii (1317.52 nm)
1318.1 | IS | Abs. | DIB | Th i (1317.97, 1318.20 nm), Si i (1318.05 nm), Fe i (1318.06 nm)
|  |  |  | Kr i (1318.10 nm), Ti i 1318.20 nm), Th ii (1318.29 nm), V i 1318.29 nm)
1527.0 | IS | Abs. | DIB | Cr ii (1526.97 nm), Fe i (1526.84, 1527.12 nm), Kr i 1527.01 nm), Fe ii (1527.10 nm)
1529.9 | S | Abs. | (4) | Th ii (1529.72 nm), Fe ii (1529.73, 1529.87, 1529.90, 1529.98 nm), Cs ii (1529.80 nm),
|  |  |  | V i (1529.82 nm), Th i (1529.93, 1529.98, 1530.04 nm), Fe ii (1529.98 nm),
|  |  |  | Hg i (1530.00 nm), Fe i (1530.00, 1530.03, 1530.06, 1530.10 nm)
1555.7 | S | Abs. & EmL | Br line | Fe i (1555.64, 1555.78, 1555.88 nm), Th i (1555.59, 1555.74 nm), Ti ii (1555.63 nm),
|  |  |  | Fe ii (1555.81 nm)
1561.1 | IS | Abs. | DIB | Ti i (1560.91, 1560.94 nm), Fe ii (1560.96 nm), Cr ii (1561.14 nm), Al ii (1561.14 nm)
|  |  |  | Th i (1561.24 nm)
1562.4 | S | Abs. | (4) | Se i (1562.27 nm), Ti ii (1562.38 nm), Th i (1562.44 nm), Fe ii (1562.54, 1562.59 nm)
1563.0 | S | Abs. | (4) | Fe ii (1562.84 nm), Th ii (1562.89 nm), B i (1562.90 nm)
1563.5 | S | Abs. | (4) | B i (1563.33 nm), Fe i (1563.36, 1563.39, 1563.53 nm, Th ii (1563.58 nm)
1564.4 | ??? | Abs. | Weak | Fe i (1564.32, 1564.38 nm), Th ii (1564.47, 1564.56, 1564.61 nm), V i (1564.47 nm),
|  |  |  | Fe ii (1564.52 nm)
1565.1 | IS | Abs. | DIB | Fe i (1564.92, 1565.28 nm) Fe ii (1564.96, 1565.21 nm) Ar ii (1565.21 nm)
1566.1 | S | EmL | (5) | V i (1566.1 nm), Fe i (1566.2 nm)
1567.0 | IS | Abs. | DIB | Ti i (1566.05 nm), V i (1566.07 nm)
1610.9 | S | Abs. & EmL | Br. line | Ti i (1619.73, 1610.99, 1611.03 nm), Th i (1610.84, 1610.98 nm), Ar ii (1610.94 nm)
1616.4 | S | Abs. |  | Ti i (1616.21, 1616.47, 1616.56), Be i (1616.21)
|  |  |  | Fe i (1616.21, 1616.34 nm), Fe ii (1616.31, 1616.39 nm)
1616.7 | S | Abs. & EmL | (6) | Ti i (1616.56, 1616.84 nm), Fe i (1616.72 nm), Th ii (1616.78 nm), Si i (1616.81 nm)
1625.3 | S | EmL |  | Al ii (1625.11, 1625.23 nm), Fe i (1625.14, 1625.33 nm)
1626.1 | S | EmL | – | Fe i (1625.91 nm), P i (1625.92 nm), Fe i (1625.34 nm), Ti i (1626.37 nm)
1633.8 | S? | Abs. |  | Fe ii (1633.74 nm), Fe i (1633.76 nm), C i (1633.84 nm)
1634.2 | S | EmL | – | Fe i (1634.16 nm)

Notes: (1) Only seen towards magnetic stars. (2) Only seen towards blue
supergiants. (3) Seen in B2 and earlier spectral types. Contaminates 1318.1 nm
DIB. (4) Only seen towards one magnetically-peculiar star. (5) Only seen
towards blue supergiant and emission-line stars. (6) Only seen towards blue
supergiant and Wolf–Rayet stars.

###  4.1 Likely stellar features

A number of likely stellar features are listed in the spectra in Table 1 with
examples shown in Fig. 3. We classified lines as being stellar based on their
width compared with other metallic stellar lines. For these lines we also
searched the NIST database (www.nist.gov) for close matches in wavelength.

####  4.1.1 1315.4 nm (absorption)

This line is seen towards only two stars in our sample, HD 145501 B and HD
147932 (B5V).

The estimated vsin($i$) for HD 147932 is 153 km s-1 (Brown & Verschueren,
1997), although this was perhaps affected by a secondary component that makes
the spectrum difficult to analyse. HD 145501 A is a magnetic chemically-
peculiar star of type CP2 (Kochukhov & Bagnulo, 2006; Wraight et al., 2012,
and refs. therein) with HD 147932 also being a magnetic (and rotationally
variable) star (Alecian et al., 2014).

####  4.1.2 1316.5 nm (emission)

A weak emission line is seen in a handful of the early-type stellar spectra,
for example towards the blue supergiants HD 115363 (B1Ia), HD 148688 (B1Ia).
HD 152235 (B0.5 Ia) and HD 159455 (B0.5Ia). The FWHMs from single-component
Gaussian fits to the observed lines are 59 km s-1 for HD 115363, 61 km s-1 for
HD 148688, 92 km s-1 for HD 152235 and 66 km s-1 for HD 159455 although the
feature does not appear Gaussian in this case. The lines observed (if stellar)
are likely to be formed at the base of the wind, close to the photosphere, and
the broadening they display is basically caused by rotation and
macroturbulence and do not measure the wind terminal speed. The FWHM of the
lines are broadly consistent with this picture and indicate that the lines do
not form in the stellar wind as if they were they would be much broader (Vink
& Sander, 2021). We also note that in the case of HD 152235 the H$\alpha$ line
at 6556 Å is approximately twice as wide (in km s-1) as this IR emission line.
Finally, the difference in the wavelength of the peak emission of the 1316.5
nm line and 1317.3 nm line is not the same for the HD 152235 and HD 159455
sightlines, although the measurement is difficult.

####  4.1.3 1316.8 nm (absorption)

This line is seen towards several stars which are typically late B-types: HD
105071 (B8Ia), HD 106068 (B8Ia), HD 115088 (B9.5), HD 116119 (A0Ia), HD 111774
(B8V), HD 125007 (B9V), HD 145501 (B8), HD 157038 (B3 Iap), HD 164865 (B9Ia),
HD 183143 (B6Ia) and HD 318014 (B8Ia). The measured FWHMs range from 44 km s-1
for HD 157038, with a FWHM of 50 km s-1 from Si iii (4567Å), to 71 km s-1 for
HD 105071, which has somewhat lower FWHMs in the optical of 55 km s-1 (Si ii
4128 and 4130Å) and 48 km s-1 (N ii 3995Å) from archival UVES spectra (Dekker
et al., 2000; Smoker et al., 2009). Malkan et al. (2002) noted the presence of
a blend of Al i at 1312.6 nm and Si i at 1318.0 nm in cooler stars, although
only detectable in their data for stars of type A4 or later.

Finally, HD 111774 has a FWHM of 100 km s-1 for visible lines at 4549 and
4555Å, while the NIR line has a FWHM of 88 km s-1. Overall, although there are
a few discrepant values, the FWHM values indicate that this line is stellar
and not interstellar. There is no correlation between the line strength and
reddening for these sightlines, which have a narrow range of 130 to 150 mÅ for
all but three sightlines, HD 157038 (EW=20 mÅ), HD 164875 (EW=25 mÅ) and HD
183143 (EW=8 mÅ).

####  4.1.4 1317.3 nm (possible stellar emission merged with DIB causing
inverted P-Cygni profile)

A number of sightlines show inverted P-Cygni like profiles at around 1317.3
nm, bluewards of the main DIB at 1318.1 nm. Such emission lines were predicted
in solid state carriers by Van de Hulst (1948) associated with a red wing. The
features are weakly visible in the X-shooter spectra of Cox et al. (2014)
although not commented on and much more obvious with the current higher
spectral resolution data. The Wolf–Rayet stars in the Galactic Centre observed
by Najarro et al. (2017) do not show this emission-line at 1317.3 nm, although
again the resolving power of those data is only $R$ $=$ 1200–5000 so it would
be difficult to detect (although they do clearly detect the 1318 nm DIBs).

As the features are present mainly in stars of spectra of type B2 and earlier,
there is the strong likelihood that in fact we are seeing a mix of a stellar
emission line in hot stars combined with the interstellar DIB at 1318.1 nm. We
note that one third of our sample have spectral types later than B2, in which
no inverted P-Cygni profiles are seen. Additionally, a search for emission
lines in the optical in a handful of these objects showing the 1317.3 nm
emission line feature revealed strong H$\alpha$ (Fig. 4).

Figure 4: Archival UVES spectra showing H$\alpha$ emission for four stars
towards which an emission feature is also seen near to the 1318 nm DIB.

To investigate the nature of the emission, we (a) compared the emission line
velocities with stellar and DIB velocities and (b) computed a model spectrum
of the region for an early B-type star. For the former, we determined the
stellar velocities of the sightlines where the emission feature was seen,
either using values from the literature or our own measurements from archive
data. Figure 6 shows spectra towards eight stars with emission lines bluewards
of 1318 nm, with their stellar velocities marked.

Figure 5: (a) Difference in absorption and emission line peak velocity around
1318 nm plotted against the stellar velocity. (b) Ditto for stellar velocity
minus emission line velocity. (c) Emission line velocity vs stellar velocity,
assuming the emission line has a rest wavelength of 1317.37 nm. Results with
the same stellar velocities are from repeat observations of a given star.
Purple points are from Gaia DR2 with Red points from Gaia DR3 for HD 160065

.

Figure 6: Example spectra showing the bluewards emission feature towards the
1318 nm DIB with Heliocentric stellar velocities marked by vertical black
lines. For HD 115363, HD 112272, HD 159455 and HD 160065 two spectra are shown
taken at different epochs. HD 159455 has a shift in radial velocity between
the two epochs. Kervella et al. (2019) use Gaia data to determine that this
object has a tangential velocity anomaly of 2.4 kms-1 and is hence likely a
binary although this should not affect the DIB absorption profile. The rest
wavelength of the emission line is assumed to be 1317.30 nm.

Of particular interest are three stars, HD 115363, HD 159455 and HD 160065
which have Heliocentric radial velocities of $-$70, +88 and +60 km s-1,
respectively. If the emission lines were at the stellar velocities then one
would expect an offset of approximately 0.7 nm (158 km s-1) between the
position of the emission line peaks when comparing HD 115363 and HD 159455.
Similarly, the offset between the emission line peak from the star and the
absorption line trough from the DIBs would also be expected to change. The
fact that the emission-absorption offset of 0.51 nm for HD 115363 and 0.46 nm
for HD 159455 is similar for both stars could indicate that the emission line
does not move at the stellar velocity. To investigate this further, Fig. 5
shows the difference in emission and absorption line velocities (a), the
difference in stellar and emission line velocities (b), and the emission line
velocity (c), all plotted against the stellar velocity. The simple trough or
peak of the absorption or emission lines were used for the velocity
measurements and are shown in Table 2. Additionally we performed a fit to the
data comprising an emission-line component and a 1318 nm DIB profile taken
from Fig. 12, shifted in wavelength, scaled in flux and stretched in
wavelength by up to 5 percent. The weighted wavelength was derived for the two
components by averaging the wavelength bins weighted by the absolute value of
the absorption line depth, $f(\lambda)$ vis:

$\lambda_{\rm
weighted}=\frac{\int{((1.0-f(\lambda)\times\lambda))\,d\lambda}}{\int{\lambda}\,d\lambda}$
(1)

The values of the Gaussian fit wavelengths are also included in Table 2.
Figure 7 shows the model for two stars (HD 115363 and HD 159455) as well as
the data/model residual.

The emission line minus absorption line velocity difference shown in Fig. 5(a)
shows no strong trend given the likely large uncertainties in the velocities
indicating they could be formed in the same medium. $Apriori$ it would be
expected that if the emission line were stellar in nature it should be
correlated with the stellar velocity. However, panel 5(c) implies that this is
not the case. Indeed, panel 5(b) shows that the difference in the stellar and
emission line velocity is correlated with the stellar velocity when a constant
offset may have been expected. We caution that measuring the centre of the
emission peak is not straightforward and these errors could dominate.

Figure 7: Normalised absorption and emission around 1318 nm towards HD 115363
(B1 Ia) and HD 159455 (B0.5 Ia). Black lines: data. Purple lines: Twin-
component Gaussian fits to the unidentified emission lines. Green line:
Scaled, shifted and wavelength stretched absorption line DIB profile. Red
line: Combined Gaussian fit plus DIB model. Blue line: Data divided by model.
Lines are offset in the ordinate for clarity.

A final attempt to determine the the origin of the line was performed by
computing a stellar model of a B0.5Ia blue supergiant star HD 152235 using
cmfgen (Hillier & Lanz, 2001) and comparing it with observations. A solar
metalicity was assumed, effective temperature of 21,800 K, log($g$)=2.55 and
slightly enhanced He/H=0.135 by number. In Fig. 8 we show the emission and
absorption lines from the X-shooter data and our model in the 1075 to 1105 nm
region. In particular, the model predicts emission from the He i P-Cygni
profile line at 1083 nm with an EW of 820 mÅ (integrated from 1072 to 1090
nm), somewhat lower than the observed value of 1660$\pm$100 mÅ. The
corresponding integrated line strength for the model and data for the
absorption line doublet at 1091.6 and 1092.0 nm are 1000 and 1180$\pm$70 mÅ,
and finally for the Hydrogen Paschen line at 1094.0 nm the model and data have
EWs of 2700 and 2750$\pm$100 mÅ respectively.

Figure 9 shows the stellar model and CRIRES or X-shooter data for HD 152235
for the DIB regions studied in this paper plus the 1078 and 1080 nm DIBs
studied by Cox et al. (2014). Panel (c) of this figure shows that the model
only predicts very weak emission (5 mÅ) in the bluewards emission P-Cygni
feature of the 1318 nm DIB compared with the actual value of 40$\pm$5 mÅ,
indicating a non-stellar nature. Although the model also predicts lower values
for emission in the 1075 to 1090-nm regions the difference is less pronounced
than for the 1317.3 nm feature. Given the weakness of the predicted emission
compared to that observed, a perhaps more intriguing possibility is that we
are seeing a PAH in emission near to the hot star, turning to absorption
further out. Such behaviour has been claimed towards the Red Rectangle in the
DIBs around 5797Å (Fossey 1991, Sarre 1991 although see Glinski et al. 2009).
The lack of the emission line feature in later stars in this scenario would be
explained by a higher UV flux from the earlier type stars. On the other hand,
such emission would require a large amount of cooler and dustier circumstellar
material. Additionally, Section 4.3 and previous work such as that by Hamano
et al. (2015) and Hamano et al. (2016) have found that the 1318 nm DIB acts
more like a 5780 Å DIB than the 5797 Å DIB variety, and in the Red Rectangle
no emission is seen around 5780 Å. Furthermore, in the Red Rectangle the
emission is redwards not bluewards of the DIB at 5797 Å. Hence it seems
unlikely that we are seeing PAH emission from the DIB although follow-up
observations of the red rectangle using the upgraded CRIRES or X-shooter might
be interesting. We note that circumstellar absorption has been claimed for the
1527 nm DIB towards the Red Square Nebula by Zasowski et al. (2015a). Figure 7
shows two emission line features towards HD 115363 at around –200 and +25 km
s-1, that could be explained by circumstellar material rotating the central
star and that could produce a double peak in velocity. However, the strengths
of the two peaks are different and emission line velocities are not symmetric
about the stellar velocity of 70 km s-1.

In conclusion, the fact that the emission is only seen in stars of B2 or
earlier, and that these stars also show H$\alpha$ in emission, points to a
stellar or circumstellar origin for the line, but the strength of the emission
line compared with a stellar model and lack of obvious correlation with the
stellar velocity point to an interstellar explanation. To conclusively
eliminate an ISM or circumstellar nature would need an investigation into the
wind speed of the targets.

Figure 8: Blue line: X-shooter data of HD 152235 from 1075 to 1105 nm showing
an emission line from the star at 1083 nm (He i) and absorption at 1094 nm (H
i). Red line: Stellar model in vacuum wavelength obtained using cmfgen of a
B0Ia star of solar metalicity, $T_{\rm eff}$=21,800 K, log($g$)=2.55 and
slightly enhanced He/H=0.135 by number, offset in the ordinate for clarity.
The purple line shows the model spectrum Gaussian smoothed to a FWHM of 0.91
nm ($\sim$250 km s-1) to fit the absorption-line data.

Figure 9: Red lines: The same stellar model as shown in Fig. 8, plotted at the
wavelength regions of the NIR DIBs shown in this paper plus the NIR DIBs
around 1078 and 1180 nm. Blue lines: observations of HD 152235 either from
CRIRES (panels c and e) or X-shooter (panels a, b, d, f). Table 2: Stellar
velocities, velocity of the peak of the emission line and velocity at peak of
absorption line towards stars where the 1318 nm showed strong emission. The
standard of rest is Heliocentric. Values in brackets are flux weighted
wavelengths determined using fitting as shown in Fig. 7.

HD | Stellar | Ref. | MJD | EmL, Abs. | $\delta\lambda$ | EmL, Abs. | Hel. RV
---|---|---|---|---|---|---|---
Number | velocity |  |  | Wavelength (nm) | (nm) | norm. flux peak | corr
| (km s-1) |  |  | at $|$peak$|$ |  | (abs. value) | (km s-1)
105056 | +5$\pm$6, –9.0$\pm$3.1, | (1), (4) | 56411.272 | 1317.56, 1317.95 | 0.39 | 1.020, 0.955 | +2.4
” | +14.5 $\pm$1.8 | (5) |  | (1317.61, 1318.05) | 0.44 |  |
112272 | –36$\pm$2, –38.0$\pm$4.4 | (1) | 56440.172 | 1317.46, 1317.89 | 0.43 | 1.025, 0.918 | –6.0
” | –37.1$\pm$1.8 | (5) |  | (1317.47, 1318.02) | 0.55 |  |
” |  |  | 56732.333 | 1317.38, 1317.79 | 0.41 | 1.025, 0.917 | +14.7
” |  |  |  | (1317.39, 1317.92) | 0.53 |  |
113904 | +7$\pm$5, +4.4$\pm$1.5 | (1), (5) | 56459.143 | 1317.56, 1317.97 | 0.41 | 1.010, 0.965 | –10.5
” |  |  |  | (1317.58, 1318.07) | 0.49 |  |
115363 | –70$\pm$2, –71.0$\pm$4.5 | (1), (4) | 56465.029 | 1317.41, 1317.92 | 0.51 | 1.023, 0.929 | –12.4
” | –70.4$\pm$1.4 | (5) |  | (1317.39, 1318.02) | 0.63 |  |
” |  |  | 56732.346 | 1317.30, 1317.80 | 0.50 | 1.021, 0.929 | +15.8
” |  |  |  | (1317.31, 1317.90) | 0.59 |  |
115842 | –21$\pm$2 –3.0$\pm$4.3 | (1), (4) | 56465.058 | 1317.61, 1317.95 | 0.34 | 1.020, 0.980 | –15.7
” | –21.1$\pm$1.7 | (5) |  | (1317.57, 1318.07) | 0.50 |  |
152235 | –25$\pm$1, –35.2$\pm$2.3 | (1), (4) | 56467.098 | 1317.43, 1317.91 | 0.48 | 1.015, 0.913 | –7.2
” | –35.9$\pm$1.8 | (5) |  | (1317.50, 1318.01) | 0.51 |  |
159455 | 87.5$\pm$1.9 | (2) | 56435.263 | 1317.44, 1317.90 | 0.46 | 1.029, 0.880 | +11.5
” |  |  |  | (1317.46, 1318.00) | 0.54 |  |
” |  |  | 56764.186 | 1317.33, 1317.78 | 0.45 | 1.032, 0.875 | +25.4
” |  |  |  | (1317.34, 1317.93) | 0.59 |  |
160065 | 59.9$\pm$6.4, 10.4$\pm$2.7 | (2), (6) | 56434.396 | 1317.33, 1317.83 | 0.50 | 1.022, 0.873 | +12.0
” |  |  |  | (1317.32, 1317.94) | 0.62 |  |
” |  |  | 56746.394 | 1317.27, 1317.75 | 0.48 | 1.022, 0.870 | +29.7
” |  |  |  | (1317.22, 1317.68) | 0.46 |  |
169754 | 32.0$\pm$7.4 | (3) | 56497.240 | 1317.34, 1317.91 | 0.57 | 1.025, 0.904 | –12.4
” |  |  |  | (1317.37, 1318.04) | 0.67 |  |

References: (1) POP, UVES pop survey (Bagnulo et al., 2003) (2) Gaia DR2 (3)
Kharchenko et al. (2007) (4) Gontcharov (2006) (5) This paper with the stellar
velocity measured using He i at $\lambda$=4387 Å (6) Gaia DR3 Vallenari
(2022). There is a large (50 km s-1) difference in the Gaia DR2 and DR3
velocities for the one star (HD 160065) for which data are available for both
releases.

####  4.1.5 1529.9 nm (absorption line)

A single sightline, the magnetically chemical peculiar star HD 145501
(B8V+B9VpSi) displays an absorption line at 1529.9 nm. Given the fact that it
is the only time the line is seen in the sample, with the reddening towards
the star a ”normal” value of $E(B-V)$=0.22 mag., the line is very likely
stellar. The line is not seen towards a close companion.

####  4.1.6 1555.7 nm (emission and absorption lines)

The 16–4 Brackett line at 1555.65 nm is observed in many spectra777See
http://www.gemini.edu/sciops/instruments/nearir-resources/astronomical-
lines/h-lines for a list of Hydrogen lines., both in absorption and emission.
The lines have a range of shapes including: strong, symmetric absorption (HD
105071, B8Ia), asymmetric absorption (HD 183143, B7Ia), twin absorption
components (possibly from super-imposed central emission in HD 168607, B9
Iaep) and double-peaked emission in the Be-type objects (HD 88661 and HD
148184), while several stars do not show the line at all (e.g. HD 111774, B8V;
HD 93130, O6.5 III).

####  4.1.7 1562.4 nm, 1563.0 nm and 1563.5 nm (absorption)

The B5V star HD 147932 ($\rho$ Oph C), displays several absorption lines
around 1563 nm, the strongest being at 1562.4, 1563.0 and 1563.5 nm with FWHM
widths of 49 km s-1. This compares with the FWHM of the Mg ii line at 4481 Å
of 180 km s-1, indicating that if the lines are stellar they likely originate
from a cool spectroscopic binary companion as indicated from Gaia observations
(Kervella et al., 2019). The star is magnetic (Alecian et al., 2014) and shows
possible variability over timescales of 20.7 hours (Koen & Eyer, 2002)
although with possible contamination of the spectra by the brighter $\rho$ Oph
(David et al., 2019).

####  4.1.8 1566.1 nm (emission)

Weak emission at 1566.1 nm is seen towards the supergiant and emission-line
stars HD 148184 (B2Vne; broad and hard to fit the continuum) HD 168607
(B9Iaep, FWHM=0.26 nm), HD 183143 (B7Iae, and often used as a ‘template’ for
DIBs), and HD 318014 (B8Iab).

####  4.1.9 1610.9 nm (absorption and emission)

A number of spectra show absorption or emission near to 1610.9 nm, which is
the 13–4 Hydrogen Brackett line.

####  4.1.10 1616.4 nm (absorption line)

An absorption feature at 1616.4 nm is visible towards the supergiant stars HD
112272 (B0.5Ia), HD 148379 (B2Iab), HD 148688 (B1Iae), HD 168987 (B1Ia), HD
169454 (B1Ia), HD 318014 (B8Ia), close to the emission lines noted in the next
section.

####  4.1.11 1616.7, 1625.3, 1626.1 and 1634.2 nm (emission lines)

The Wolf–Rayet star HD 94910 shows a rich set of emission lines at 1616.7,
1626.1 and 1634.2 nm, with the supergiant HD 148688 (B1Ia) showing an emission
line at 1625.3 nm.

####  4.1.12 1633.8 nm (absorption line)

Absorption line features are seen at 1633.8 nm towards HD 90706 (B2.5Ia), HD
94910 (WN11 type star, near emission-line feature, creating P-Cygni profile),
HD 105071 (B8Ia), HD 106068 (B8Ia/Iab), HD 112272 (B0.5Ia), HD 148379
(B9Iaep), HD 148688 (B1Iaeqp), HD 164865 (B9Ia), HD 168607 (B9Iaep, twin
absorption line profiles), HD 168625 (B6Iap), HD 169454 (B1Ia), HD 170938
(B1Ia), HD 183143 (B6Ia), HD 318014 (B8Ia). At a similar wavelength several of
the stars in the sample display prominent emission (Fig. 3 and Sect. 4.1.11).
Neither Geballe et al. (2011) or Cox et al. (2014) identified this feature. It
is likely stellar although we could not identify the line. Both N ii (1634 nm)
and Ne i were candidates but not visible in the B0.5Ia model (nor in a model
for a cooler B8Ia star).

###  4.2 Likely interstellar or circumstellar features

A number of insterstellar or cirucumstellar features were detected and are
described below.

####  4.2.1 Wavelengths of detected features

Interstellar or circumstellar features were detected at the following
wavelengths; 1318.1 nm, 1527.9 nm (partly contaminated by stellar absorption
as seen in the cmfgen model of HD 152235 in Fig. 9), 1561.1 nm, 1564.4 nm
(broad and weak, possibly stellar), 1565.1 nm (also partly contaminated by
stellar absorption), 1567.0 nm and possibly at 1634.2 nm although it seems
likely stellar (see Sect. 4.1.12). We had hoped to detect the broad DIB at
1622.7 nm as previously observed towards the highly-reddened Galactic centre
(Geballe et al., 2011). This DIB was also observed at 1622.7 nm in 4U 1907+09,
the most heavily reddened object in the sample of nine stars observed by Cox
et al. (2014). We do not detect the absorption in any of our higher resolution
CRIRES spectra nor the more highly-reddenened X-shooter Cepheid data; there is
nearby weak telluric absorption at 1623.3 nm but no obvious DIB absorption.

####  4.2.2 Shape of the DIB profiles and Gaussian fitting

Example fits to the sample are shown in Fig. 10, using one and two Gaussian
components. For the DIB at 1318 nm the majority of the lines are best fitted
by twin Gaussians due to the extended red wing. A single-fit Gaussian leaves
larger residuals.

An attempt was made to search for correlations between the ratio of the
integrated flux in the twin Gaussian components and the difference in their
peak wavelengths with 5780, 5797 Å equivalent widths,
$\lambda\lambda$5780/5797 DIB ratio and reddening $E(B-V)$. No obvious
correlations were found.

For the 1318 nm line there is a range of FWHM fitted with a single component
Gaussian of 0.36$\pm$0.01 nm (82$\pm$2.5 km s-1) for HD 188293 to
0.53$\pm$0.01 nm (120$\pm$2.5 km s-1) for HD 170938. This compares with FWHM
values of 0.45$\pm$0.12 nm from Cox et al. (2014) and 0.40$\pm$0.05 nm from
Joblin et al. (1990) in lower-resolution spectra. For the current
observations, the instrumental resolution of CRIRES was 6 km s-1, so the DIB
lines appear to be always resolved. Historically, the blue wing seen towards
the 4428 Å DIB was thought to provide evidence for a solid state carrier
(Wickramasinghe, 1968), although this explanation has fallen out of favour for
other reasons, principally being the lack of polarisation seen in DIBs. In
contrast, the steep blue side (or red wing) seen in many DIBs is currently
explained by the short-wavelength spectral limit of the R-branch being reached
in molecular carriers (e.g., Sarre, 2014). In the current data even the
single-component dominated sightlines show the wing in the 1318 nm DIB.
Finally, we note that no substructure is seen in the 1318 nm DIB at a spectral
resolution of 50,000. In the optical DIB at 6614 Å, such structure has been
used to infer the rotational constant, temperature and possible excitation of
the DIB via rotational contour modelling (Kerr et al., 1996; Cami et al.,
2004; Marshall et al., 2015; MacIsaac et al., 2022). Higher spectral
resolution CRIRES observations currently being analysed by Cox et al.
($R$=100,000) may be useful in finding such substructure.

Figure 11 shows the 1527 nm DIB towards four sightlines. At the S/N ratio and
resolution of the current observations, a Gaussian fit is sufficient to fit
the profile, although in each case the trough of the profile is slightly
deeper than the fit implies and in the highest S/N spectra towards HD 148937
there are some residuals present.

The three DIBs at 1561, 1565 and 1567 nm have been extensively studied by
Elyajouri et al. (2017) at a spectral resolution of 21,500. They measure mean
FWHM values for the three DIBs of 0.44, 0.57 and 0.37 nm, compared with the
current work of 0.53, 0.53 and 0.35 nm, respectively. As for the 1318 nm DIB,
the current work finds that the 1561 and 1567 nm DIBs are better fitted with
twin rather than single Gaussian components.

Figure 12 shows a number of individual profiles for the 1318, 1527, 1561, 1565
and 1567 nm DIBs plus the co-added profile for spectra with the highest S/N
ratios. For the DIBs from 1561–1567 nm the spectrum appears to show an extra
broad component at around 1564.4 Å, but the S/N ratio is low. Continuum
placement is hence difficult, and such a feature is not mentioned in the case
of the higher S/N but lower spectral resolution data of (Elyajouri et al.,
2017). On the other hand, a close look at their Figure 4 shows that a
similarly broad and shallow absorption is not precluded. The feature is not
telluric in nature.

Figure 10: CRIRES DIB observations around 1318 nm corrected for telluric
absorption. Blue lines: one component Gaussian fit plus the residual
(1.0+data-model). Red lines: ditto for the two component Gaussian fits. The
spectral resolution is $\sim$0.026 nm.

Figure 11: CRIRES DIB observations around 1527 nm corrected for telluric
absorption. Blue lines: data. Red lines: one component Gaussian fit. Green
lines: residual (1.0+data-model), offset in the ordinate for clarity. The
spectral resolution is $\sim$0.030 nm. We recall that HD 145502 is a binary.

Figure 12: CRIRES DIB spectra around 1318, 1527 and 1561-1567 nm. Main panels:
Observations towards all stars, shifted in wavelength to 1318, 1527 and 1567
nm, respectively and the absorption line trough normalised to 0.2. Top black
line: Model of telluric absorption assuming PWV values of 1.3, 2.5 and 7.5 mm,
respectively with $R$=50,000. Inset panels: averaged profiles for the DIBs
with the strongest lines and for the 1318 nm DIBs excluding sightlines showing
inverted P-Cygni type profiles.

###  4.3 Comparison of Near IR DIB strengths with other DIBs, atomic lines and
reddening

Correlation studies between DIBs and other DIBs, atomic and molecular features
serve two purposes. Firstly, to search for lines that near-perfectly
correlate, providing evidence that the carrier producing the two lines is the
same, and secondly to determine the physical and/or chemical conditions in
which certain DIBs exist and hence can be used in other work as probes of the
environmental conditions. We note that just as in atomic lines a 1:1
correlation does not mean that the carriers are the same (as in the case of
the near perfect correlation between Na i and K i equivalent width), there is
some evidence that the lack of a 1:1 correlation between different lines at
different wavelength may not rule out a common carrier as in the case of the
postulated C${60}^{+}$ DIBs at 9577 and 9632 Å due to issues in telluric line
fitting, possible overlapping bands and stellar contamination (Walker et al.,
2017).

We searched for optical data Advanced Data Products (ADPs) towards our NIRDIB
sightlines using the ESO archive888archive.eso.org for the FEROS, HARPS (Pepe
et al., 2000) and UVES instruments. Some ADPs did not exist for FEROS
observations so the data were re-reduced. Additionally we performed a search
of the NARVAL/ESPADONS online archives but no relevant data were found. One
star (HD 157778) was used from historical observations using the SOPHIE
spectrometer at Observatoire the Haut Provence (Perruchot et al., 2008). A
total of 57 out of 76 of the objects for which we have CRIRES spectra had
optical data.

In each sightline we measured the EW of the strongest 50 DIBs in the catalogue
of DIBs in the spectrum of HD 204827 (Hobbs et al., 2008, 2009) as well as the
EWs derived from the current spectra. Many of the weaker DIBs were not
detected in the archive spectra due to insufficient S/N ratios. Telluric
correction was performed by using scaled versions of synthetic spectra
produced by skycalc. CSV files containing the results of all of the equivalent
width measurements and derived correlations are available in CDS/Vizier. Aside
from the DIBs, we also measured a number of atomic, ionised and molecular
lines. The full list is shown in Table 3 with Fig. A2 displaying the Na i
(5895 Å) and K i (7698 Å) data for sightlines where available, as well as the
DIBs at 5780, 5797, 6196, 6203, 6269, 6283, 6379 and 6614 Å.

Table 3: Optical lines for which equivalent widths were measured for available spectra. Species, $\lambda$(Å) | Species, $\lambda$(Å) | Species, $\lambda$(Å)
---|---|---
NaI 3302.36 | DIB 5236.18 | DIB 6204.49
NaI 3302.97 | DIB 5418.87 | DIB 6269.85
TiII 3383.75 | DIB 5450.60 | DIB 6283.84
FeI 3719.93 | DIB 5452.40 | DIB 6376.08
FeI 3859.91 | DIB 5494.10 | DIB 6379.32
CNI 3873.99 | DIB 5542.20 | DIB 6397.01
CNI 3874.60 | DIB 5545.40 | DIB 6439.48
CNI 3875.76 | DIB 5546.90 | DIB 6445.28
CaII 3933.66 | DIB 5705.08 | DIB 6449.22
KI 4044.14 | DIB 5711.60 | DIB 6521.30
C3 4051.60 | DIB 5763.20 | DIB 6597.40
CaI 4226.72 | DIB 5766.10 | DIB 6614.00
CH+ 4232.54 | DIB 5780.48 | DIB 6993.13
CH 4300.31 | DIB 5797.06 | DIB 7224.03
DIB 4501.79 | DIB 5849.81 | DIB 7562.25
DIB 4726.83 | NaI 5889.95 | KI 7698.96
DIB 4762.61 | NaI 5895.92 | C2 7714.20
DIB 4780.02 | DIB 6089.85 | C2 7724.20
DIB 4963.88 | DIB 6113.18 | Rb 7800.30
DIB 4984.79 | DIB 6195.98 | C2 8757.68
DIB 4987.42 | DIB 6203.05 | C2 8761.19
DIB 5074.47 | DIB 6204.49 | C2 8763.74
DIB 5176.04 | DIB 6269.85 | C${}_{60}^{+}$ 9632.10

In work on optical DIBs, Moutou et al. (1999) classified a Pearson correlation
coefficient $r$ of less than 0.7 as weak, 0.7 to 0.95 as good, and anything
exceeding 0.95 as strong (also see Fan et al., 2017; Friedman et al., 2011,
who suggest two quantities may have a physical relation if $r$ exceeds
0.86–0.88). This is due to the fact that within the Galaxy increasing distance
along the line of sight will typically result in more absorbing material, so
some correlation is natural. Most DIBs correlate (if only weakly), although to
date the only near perfect 1:1 correlation between two DIBs is for those at
6196.0 and 6613.6Å (McCall et al., 2010).

Table 3 shows the Pearson correlation coefficients for those DIBs that showed
at least one instance of a correlation $r$ exceeding 0.87 with another feature
(the criteria of Friedman et al. (2011) to indicate that ”two quantities are
correlated at a significant level”) for the whole sample, plus some optical
transitions. Note that the correlation coefficients should be considered lower
limits as when sightlines with only one dominating component are considered
(SCDs), typically the correlation coefficient improves as shown in Fig. 13 and
Table 4. The disadvantage of such sightlines is that they are few in number in
the current sample. Figure A3 shows the correlations themselves where
$r\geq$0.87.

Following the referees suggestion, the method of partial correlation
coefficients was used to investigate the confidence level in the derived
correlation coefficients. Partial correlation coefficients take into account
the fact that even when there is correlation between two variables, it may be
partly caused by a third variable, the so-called covariate or controlling
variable (Vio et al., 2020). For the current work the reddening $E(B-V)$ was
used as the covariate. The implementation of Vallat (2018) was used to derive
the 95 percent confidence limits around the partial correlation coefficient.
These are also shown in Tables 3 and 4. If the 95 percent ranges are large
then the chance of the two main variables not being physically correlated is
increased. We note that results for correlations are shown only where there
are 5 data points or more and with so few samples the significance is low.
This is particularly the case for the sightlines which are single cloud
dominated where there are fewer data available.

Figure 13: Top panel: Comparison of correlation coefficients derived using the
whole sample (abscissa) with those determined for the Single Cloud Dominated
(SCD) sightlines (ordinate). There is a tendency for the correlation
coefficients for the latter to be higher. The stars that we consider to be SCD
from their K i profiles are HD numbers 36861, 36862, 88661, 89587, 93576,
137753, 144217, 145501, 145502, 147932, 148184, 156738, 164438, 164865,
167264, 169454, 188294 and 318014. Blue and orange symbols: greater than or
equal to 10 or 15 datapoints, respectively. Bottom panel: Correlation
coefficients comparing the 1318 nm sample excluding emission line stars with
the whole sample.

####  4.3.1 Correlations between NIRDIB strength and reddening

Figure 15 shows the correlation between DIB EWs and reddening for the lines
studied in this paper. In the optical wavelength range, at low values of
$E(B-V)$ the correlation between EW and reddening is reasonable (e.g. Lan et
al., 2015), but reaches saturation at higher values, partly due to the skin
effect by which DIB carriers appear to disappear in the centre of dense clouds
Snow & Cohen (1974). One DIB relatively well correlated with reddening is the
”Gaia DIB” at 8620.4Å (Munari, 2000) although even it shows some intrinsic
scatter.

There are reasonable correlations for the CRIRES-observed targets with
reddening for all of the the NIRDIBs studied. Correlations between the 1318,
1527, 1561, 1565 and 1567 nm DIBs and $E(B-V)$ are in the range 0.76 to 0.92
for the full sample and 0.74 to 0.96 for the single cloud dominated sample.
These are similar to the values observed by Cox et al. (2014) for the 1318 and
1527 nm DIBs. We note that in the current dataset the scatter in the
measurements above $E(B-V)$ of around 2-3 mags becomes larger. Of course, this
could partly be due to stellar contamination of the lines in the high-
reddening X-shooter Cepheid sample. Stellar contamination in the 1318 nm and
1527 nm lines for the Cepheid variables was estimated from X-shooter spectra
of six Cepheid variable stars taken from the X-shooter spectral library of
Chen et al. (2014). Five of these are in the Galactic disc with reddening
$E(B-V)$ values between 0.04 and 0.89 and one towards the low-metallicity
cluster $\omega$ Cen with $E(B-V)$=0.12 .

Figure 14 shows the Chen et al. spectra (at a range of reddenings) and two
targets from Minniti et al. (2020). The EW values of the stellar lines around
1318 nm and 1527 nm DIBs for the Galactic plane Cepheid with $E(B-V)$ of 0.12
are 368 and 651 mÅ, respectively. Hence, at $E(B-V)$ $\approx$ 3 magnitudes
the stellar contribution in the Cepheid data could be up to 20 percent in both
1318 nm and 1527 nm DIBs. We note that the Cepheid towards the low-metallicity
cluster Omega Cen (NGC 5139 LEID 32029) with $E(B-V)$=0.12 has a 1318 nm EW of
135 mÅ although the region around 1527 nm is very crowded with lines with the
total EW from 1524 to 1530 nm of $\sim$1.5 Å.

Figure 14: X-shooter spectra of Cepheid variables around 1318 nm (left panels)
and 1527 nm (right panels) taken from Chen et al. (2014) and Minniti et al.
(2020). Reddening increases downwards. (A) HD 052973, $E(B-V)$=0.04, EW(1318
nm)=368$\pm$30 mÅ, EW(1527 nm)=450$\pm$50 mÅ. (B) HD 101602, $E(B-V)$=0,25,
EW(1318 nm)=419$\pm$30, EW(1527 nm)=840$\pm$50 mÅ. (C) HD 178287,
$E(B-V)$=0.40, EW(1318 nm)=470$\pm$20 mÅ, EW(1527 nm)=570$\pm$60 mÅ. (D) HD
179315, $E(B-V)$=0.59, EW(1318 nm)=691$\pm$50 mÅ, EW(1527 nm)=890$\pm$70 mÅ.
(E) HD 139717, $E(B-V)$=0.89, EW(1318 nm)=913$\pm$30 mÅ, EW(1527
nm)=2300$\pm$600 mÅ. (F) B19, $E(B-V)$=3.27, EW(1318 nm)=2300$\pm$250 mÅ,
EW(1527 nm)=1400$\pm$300 mÅ. (G) D20, $E(B-V)$=3.57, EW(1318 nm)=2120$\pm$110
mÅ, EW(1527 nm)=3300$\pm$100 mÅ.

These results compare to Cox et al. (2014) who found $r$=0.91 for the 1318.1
nm DIB (with fewer datapoints). No correlation coefficients are shown for
$E(B-V)$ in the paper of (Elyajouri et al., 2017) for this DIB.

Figure 15: Reddening $E(B-V)$ (mag.) vs. equivalent width in mÅ for the DIBs
in the current sample. Blue points show all of the data with orange points
showing only single cloud dominated sightlines. For the 1318 and 1527 nm DIBs
for reddening exceeding 3.13 mag. the data are from the X-shooter Cepheid
sample which suffer from stellar contamination of around 500 mÅ for unreddened
sightlines. The solid red line is the best fit y=mx+c with the dashed red
curves showing the upper and lower 1 sigma error bounds.

####  4.3.2 Correlation between strength of NIR DIBs

Figure 16 shows correlations between the line strength of DIBs studied in this
paper. The correlations between the 1318, 1527, 1561, 1565 and 1567 nm DIBs
range from 0.84 to 0.92 for the entire sample and from 0.18 (few stars) to
0.98 for the single cloud dominated sample. Recall that these are lower limits
due to the S/N limitations and difficulty in the baseline fitting especially
for the 1561 nm DIB. Cox et al. (2014) determined $r$=0.93 when comparing the
1318 and 1527 nm DIBs, similar to our value of 0.90.

Figure 16: Correlations in equivalent width in mÅ for the NIR DIBS in the
current sample. Both the whole sample (blue points) and the Single Cloud
Dominant subsample (orange points) are shown. The dashed red curves show the
upper and lower 1 sigma error bounds.

####  4.3.3 Correlation between 1318 nm DIB strength, H i, H2 and f(H2)

Figure 17 shows normalised X-shooter spectra, corrected for telluric
contamination using molecfit for early-type stars for which $N$(H i), $N$(H2)
and f(H2) data were available from the literature (Fan et al. (2017) and refs
therein). To these data we added our CRIRES 1318.1 nm observations, using
either literature values for H i, H2 and f(H2), or, in the case where these
did not exist, estimates of H2 from the CH(4300 Å) line derived from
log($N$(H2)) = 0.8875 x log($N$(CH)) + 8.88 using data from Welty et al.
(2006). The DIB at 5780 Å was used to estimate $N$(H i) via log($N$(H i)( =
0.82 x log(EW(5780 Å DIB)) + 19.34 (Friedman et al., 2011). The results are
shown in Table 4.

Figure 17: X-shooter data around 1318 nm corrected for telluric contamination
using molecfit for which f(H2) is available in the literature.

Figure 18 shows either the EW of the 1318 nm line or the EW divided by the
$E(B-V)$ plotted against $N({H\textsc{i}})$, $N(H_{2})$ and f(H2). The aim of
these plots was to search for the lambda-shaped trend which shows that atomic
and diffuse molecular clouds show dramatically different balances in the
creation and destruction of DIBs (Fan et al., 2017). Although there are hints
of such a trend when plotting the EW of the 1318 nm DIB against molecular
fraction, when this is normalised by the reddening, the shape disappears and
what is left is a noisy but slowly decreasing EW(1318 nm)/$E(B-V)$ trend with
increasing molecular fraction. In the straight correlation we find better
correlation between the 1318 nm DIB and H i than with H2, although still with
a lot of scatter.

Figure 18: EW(1318 nm) (plots a,c,e) or EW(1318 nm)/$E(B-V)$ (plots b,d,f) plotted against $N$(H i), $N$(H2) and f(H2). Stars with emission lines near 1318 nm were excluded. Blue circles: CRIRES data with $N$(H i), $N$(H2), f(H2) from the literature, red squares: CRIRES data with H i calculated from the 5780 DIB and N(H2) from CH, purple pentagons: X-shooter with $N$(H i), $N$(H2), f(H2) from the literature. Outlier HD 105071 is labelled.  Table 4: Values of the equivalent widths of the 1318 nm DIB, CH(4300 Å) and the 5780 Å DIB for CRIRES (C) and X-shooter (X) sightlines for which f(H2) is available or can be estimated.  HD | Inst. | EW(1318) | EW(CH4300) | EW(5780) | N(H2) | N(H2)c | N(Hi) | NHic | fH2 | fH2c
---|---|---|---|---|---|---|---|---|---|---
|  | (mÅ) | (mÅ) | (mÅ) | (cm-2) | (cm-2) | (cm-2) | (cm-2) | (cm-2) | (cm-2)
35912 | X | … | … | … | 20.35 | … | 20.35 | … | … | 0.12
36408 | C | Det. (HWV) | … | … | … | … | … | … | … | …
36861 | C | $<$ | 1.76 | 48.29 | 19.65 | 19.11 | 20.77 | 20.72 | 0.14 | 0.04
36862 | C | … | 2.44 | 34.27 | 19.80 | … | … | 20.60 | 0.24 | …
36959 | C |  $<$ (HWV) | … | … | … | … | 20.40 | … | … | …
52266 | X | 170.00 | … | … | 21.16 | … | 21.16 | … | … | 0.14
53367 | X | … | … | … | 21.32 | … | 21.32 | … | … | 0.51
57060 | X | … | … | … | 20.78 | … | 20.78 | … | … | …
62542 | X | … | … | … | 20.90 | … | 20.90 | … | … | 0.62
88661 | C |  $<$ (EmL) | … | 13.21 | … | … | … | 20.26 | … | …
90706 | C | 308.10 | … | … | … | … | … | … | … | …
92740 | C | 156.30 | 6.38 | … | 20.26 | 19.97 | 21.20 | … | … | 0.11
93130 | C | 250.20 | 6.76 | 237.90 | 20.28 | … | 21.58 | 21.29 | 0.17 | …
93632 | C | 258.40 | 12.55 | 266.51 | 20.58 | … | … | 21.33 | 0.26 | …
94910 | C | 448.10 | 10.76 | 430.55 | 20.50 | … | … | 21.50 | 0.17 | …
97707 | C | 190.80 | … | … | … | … | … | … | … | …
105056 | C | 202.00 | 3.36 | … | 19.95 | … | 21.15 | … | … | …
105071 | C | 312.20 | 4.18 | … | 20.06 | … | … | … | … | …
106068 | C | 264.60 | 6.19 | 234.96 | 20.24 | … | … | 21.28 | 0.15 | …
108927 | X | … | … | … | 20.86 | … | 20.86 | … | … | 0.46
111774 | X | … | 1.00 | … | 19.38 | … | … | … | 1.00 | …
112272 | C | 362.70 | 39.21 | 441.12 | 21.12 | … | … | 21.51 | 0.45 | …
113904 | X | 144.20 | 1.66 | 126.35 | 19.62 | 19.84 | 21.08 | 21.06 | 0.07 | 0.10
114213 | X | 370.00 | … | … | 21.35 | … | 21.35 | … | … | 0.56
115088 | C | $<$ | … | 13.75 | … | … | … | 20.27 | … | …
115363 | C | 331.10 | 27.87 | … | 20.95 | … | … | … | … | …
115842 | C | 81.20 | 21.83 | 234.59 | 20.84 | … | 21.15 | 21.28 | 0.42 | …
116119 | C | 471.00 | 8.17 | 449.75 | 20.37 | … | … | 21.52 | 0.13 | …
125007 | C | … | … | 18.28 | … | … | … | 20.37 | … | …
125241 | C | 514.10 | 19.10 | 532.96 | 20.78 | … | … | 21.58 | 0.24 | …
142468 | C | 589.80 | … | 461.15 | … | … | … | 21.52 | … | …
144217 | C | 225.70 | 2.15 | 156.24 | 19.74 | 19.82 | 21.09 | 21.14 | 0.07 | 0.10
144218 | C | 244.60 | 9.95 | 157.34 | 20.47 | … | … | 21.14 | 0.30 | …
145501 | C | 244.30 | 2.04 | 182.11 | 19.72 | … | … | 21.19 | 0.06 | …
145502 | C | 226.40 | 3.80 | 177.17 | 20.01 | 19.88 | 21.15 | 21.18 | 0.12 | 0.10
147889 | X | … | … | … | 21.80 | … | 21.80 | … | … | 0.48
147932 | C | 284.30 | 16.59 | 207.86 | 20.71 | … | … | 21.24 | 0.37 | …
148184 | X | 125.70 | 23.21 | 100.11 | 20.87 | 20.62 | 21.14 | 20.98 | 0.61 | 0.38
148379 | C | 546.70 | 15.60 | 427.06 | 20.68 | 20.41 | … | 21.50 | 0.23 | …
148688 | C | 318.10 | 13.20 | … | 20.60 | … | … | … | … | …
148937 | X | 601.60 | 17.85 | 356.50 | 20.74 | … | 21.60 | 21.43 | 0.29 | …
151932 | C | 247.40 | 23.27 | 336.08 | 20.87 | … | 21.39 | 21.41 | 0.36 | …
152235 | X | 406.30 | 29.13 | 418.07 | 20.98 | 21.00 | 21.42 | 21.49 | 0.38 | 0.43
152386 | C | 378.00 | 19.97 | 352.84 | 20.80 | … | … | 21.43 | 0.32 | …
154368 | X | $<$ | … | … | 21.00 | … | 21.00 | … | … | 0.74
154811 | C | 409.70 | 15.96 | … | 20.69 | … | … | … | … | …
154873 | C | 384.60 | 12.18 | … | 20.56 | … | … | … | … | …
Table 4: continued.

HD | Inst. | EW(1318) | EW(CH4300) | EW(5780) | N(H2) | N(H2)c | N(Hi) | NHic | fH2 | fH2c
---|---|---|---|---|---|---|---|---|---|---
|  | (mÅ) | (mÅ) | (mÅ) | (cm-2) | (cm-2) | (cm-2) | (cm-2) |  |
156134 | X | 283.70 | 23.21 | 320.89 | 20.87 | … | … | 21.40 | 0.37 | …
156201 | C | 306.20 | … | 318.77 | … | … | … | 21.39 | … | …
156212 | X | 322.10 | 30.49 | 375.79 | 21.00 | … | … | 21.45 | 0.41 | …
156738 | C | 413.00 | 49.81 | 494.35 | 21.23 | … | … | 21.55 | 0.49 | …
157038 | C | 559.60 | 17.40 | 447.04 | 20.73 | … | … | 21.51 | 0.25 | …
157778 | C | $<$ | … | 34.26 | … | … | … | 20.60 | … | …
164438 | C | 320.90 | 21.13 | 391.36 | 20.82 | … | … | 21.47 | 0.31 | …
164865 | C | 485.20 | 19.28 | 383.28 | 20.78 | … | … | 21.46 | 0.30 | …
165319 | C | 374.60 | 23.65 | 533.85 | 20.88 | … | … | 21.58 | 0.29 | …
166734 | X | 566.50 | 46.12 | 720.74 | 21.19 | 21.25 | 21.63 | 21.68 | 0.39 | 0.45
167264 | X | 122.90 | 6.01 | … | 20.23 | 19.97 | 21.20 | … | … | 0.11
167971 | C | 745.00 | 30.89 | 563.33 | 21.00 | 20.85 | 21.60 | 21.60 | 0.34 | 0.26
168076 | X | 390.00 | … | … | 21.65 | … | 21.65 | … | … | 0.18
168607 | C | 877.80 | 47.60 | 859.65 | 21.21 | … | … | 21.75 | 0.37 | …
168625 | C | 809.30 | 41.27 | 842.82 | 21.14 | … | … | 21.74 | 0.34 | …
168987 | C | 902.30 | 20.74 | 752.62 | 20.81 | … | … | 21.70 | 0.21 | …
169034 | X | 759.60 | 36.03 | 689.54 | 21.08 | … | … | 21.67 | 0.34 | …
169454 | X | 527.80 | 28.68 | 482.95 | 20.97 | 20.98 | 21.51 | 21.54 | 0.35 | 0.37
170740 | X | 130.00 | … | … | 21.04 | … | 21.04 | … | … | 0.57
170938 | C | 774.30 | 20.30 | 476.37 | 20.80 | … | … | 21.54 | 0.27 | …
171722 | C | $<$ | 0.47 | 20.88 | 19.02 | … | … | 20.42 | 0.07 | …
183143 | C | 629.40 | 38.99 | 807.49 | 21.11 | … | … | 21.72 | 0.33 | …
214080 | X | … | … | … | 20.58 | … | 20.58 | … | … | 0.01
219188 | X | 100.00 | … | … | 20.75 | … | 20.75 | … | … | 0.07
318014 | C | 813.00 | 42.88 | 675.39 | 21.16 | … | … | 21.66 | 0.39 | …
319699 | C | 534.00 | 39.22 | 363.85 | 21.12 | … | … | 21.44 | 0.49 | …
319702 | C | 541.70 | 37.45 | 521.04 | 21.09 | … | … | 21.57 | 0.40 | …

Notes: A ”¡” corresponds to no detection. The log of the column densities of
H2 and Hi taken from the literature or derived from CH (N(H2)c) and the 5780 Å
DIB (N(Hi)c) are also shown, along with the fractional H2 content or its value
derived from CH and the 5780 Å DIB, (fH2c). HWV refers to sightlines that had
high water vapour affecting the 1318 nm DIB measurements and EmL observations
affected by emission lines in the DIB.

####  4.3.4 Correlation between optical DIB strengths

There is an extensive literature concerning correlations between the strengths
of optical DIBs (e.g. Cami et al. 1997, Moutou et al. 1999, McCall et al.
2010, Kos et al. 2013 amongst many others).

We first check on the accuracy of our new measurements and estimation of
correlations. Figure 19 shows the ratio of the EWs measured for several
sightlines in the current paper with values taken from the literature. The
current measurements have a median value some 4.6 percent larger than the
literature values ($N$=86) with 76/86 of the measurements lying within 20
percent of each other. On the other hand, the correlation coefficient of the
two lines of the Na i UV doublet and the Na i vs K i are (as expected) close
to 1.00, respectively, indicating no gross error in our analysis.

We next consider optical to optical correlations with values of $r\geq$0.95
and more than or equal to six data points. These include the 6614 Å DIB
correlated with 5780 Å, 5797 Å, 6269 Å, 6376 Å, 6439 Å and 6693 Å. Of these,
5780, 6196, 6203, 6269, and 6284 Å were historically classified as a DIB
“family” (Krelowski & Walker 1987 although see Cami et al. 1997). Additionally
the 6439 Å DIB correlates extremely well with 6376 Å and the NIRDIB at 1527 nm
although for the latter with only six data points.

Finally, a number of DIBs show negative correlations when compared with other
optical or IR DIBs. For correlations with greater than or equal to 10
datapoints these include the 4501.8 DIB compared with the infrared DIBs
described in this paper, the 4762.6 nm, 5074.5 nm and 6993.1 nm DIBs compared
with more than 10 other DIBs, and finally the 4780.0 DIB compared with 5418.9
DIB. The correlations between all DIBs are available online in VIZIER.

Figure 19: Ratio of the equivalent width from the current paper divided by
values taken from the literature. Values are shown for several DIBs. The unity
line is shown.

####  4.3.5 Correlation between NIRDIB and Optical DIB strengths

Of particular interest is the correlation between the NIRDIBs and
”C${}_{60}^{+}$”-DIBs whose wavelengths were measured in the lab by Campbell
et al. (2015, 2016) of 9348.4, 9365.2, 9427.8, 9577.0, and 9632.1 Å with an
uncertainty of 0.2 Å. The strongest lines are the reddest two. In this work we
only measured the 9632.2 Å DIB as it is less affected by telluric
contamination. That said, there is a nearby Mg ii stellar line that causes
contamination of the DIB (Galazutdinov et al. 2017b; Lallement et al. 2018).
The correlation coefficient between the ”C${}_{60}^{+}$” 9632 Å DIB and 1318
nm DIB is only $r$=0.50 (N=10), not correcting for possible stellar
contamination.

The 1318, 1561 and 1567 nm DIBs show good correlation ($r=0.88,0.86,0.85$)
when compared with the 5780 Å DIB (Cox et al. (2014) determine $r$=0.98 for
the 1318 nm DIB but with fewer data points) and similar values when comparing
with the 6283 Å DIB ($r$=0.91, 0.85, 0.89), 6203 Å DIB ($r$=0.86, 0,85, 0.88),
5705 Å DIB ($r$=0.84, 0.83, 0.80), 6196 Å DIB ($r$=0.81, 0.88, 0.80), 6269 Å
DIB ($r$=0.79, 0.85, 0.81) and 6993 Å DIB ($r$=0.78, 0.85, 0.83). For the 5797
Å DIB the correlation is slightly lower with the NIRDIBs, being $r$=0.71, 0.77
and 0.73 for the 1318, 1561 and 1567 nm DIBs, respectively with typically
around 30 data points in the samples. These results indicate that the NIRDIBs
are more sigma type than eta type and supports the conclusion of Cox et al.
(2014) from the absence/weakness of NIRDIBs in the HD 147889 sightline probing
a translucent cloud. The correlation plots are shown in Fig. 20.

Figure 20: Correlations in equivalent width in mÅ for the 1318 nm and 1561 nm
DIBs in the current sample towards the optical DIBs where the highest
correlation coefficient was observed. The dashed lines show upper and lower
error bounds obtained by fitting with a second order polynomial.

####  4.3.6 Correlations between DIB ratios and $\lambda\lambda$5797/5780, Ca
i/Ca ii, CH/CH+ and Na i/Ca ii

The most commonly studied optical DIBs are at 5797 Å and 5780 Å. The former,
like the blue 4429 Å DIB, is known to trace the neutral ISM with the latter
probing more ionised regions (e.g. Farhang et al., 2015; Bailey et al., 2016).
The 5797 Å/5780 Å DIB equivalent width ratio is therefore used probe the UV
field, with values of less 0.35 probing regions with higher UV, and higher
values being shielded from UV (e.g. Cami et al. 1997; Vos et al. 2011 and refs
therein).

Hence we also searched for correlations between ratios of the 1318 nm DIBs to
a number of the stronger optical DIBs and reddening, the first part as
presented by Hamano et al. (2015, 2016) and Elyajouri et al. (2017) in order
to determine the relative dependence of the 1318 nm band on the UV radiation
field. This was performed by plotting in Fig. 21 the ratio of equivalent width
of the 1318 nm DIB divided by the 4984, 5780, 5797, 6196, or 6283 Å EW against
the $\lambda\lambda$5797/5780, ratio. As noted above, the latter ratio is an
indicator of UV radiation field with the 5780 Å DIB preferring regions where
the UV radiation field is strong (the so-called $\sigma$ DIBs) and the 5797 Å
DIB where the field is weaker (the $\zeta$ DIB). Although our results are
somewhat scattered, they are in line with previous work in that the 1318 nm
DIB appears to favour regions of higher UV field and perhaps are cation
molecules (Hamano et al., 2016).

Finally, in the same figure we plot the 1318 nm / optical DIB ratio against a
number of line ratios. The first is Ca i/Ca ii which has been used as an
indicator of electron density (e.g. McEvoy et al. 2015). The use of this line
ratio has many issues, in particular the presence of perhaps dominating Ca iii
(Sembach et al., 2000) and the assumption that Ca i and Ca ii trace the same
parts of the ISM (Crawford, 2002) (see also Welty et al. 2003). Secondly, we
show correlations with CH/CH$+$ which is an indicator of turbulent dissipation
and warm H2 (Valdivia et al., 2017). The last is Na i/Ca ii which is an
indicator of shocks and dust content via the Routly-Spitzer effect (Routly &
Spitzer, 1952; Vallerga et al., 1993). We note that saturation can be an issue
in these lines and that there are no convincing trends in any of the
relationships.

Figure 21: Ratio of the equivalent width of the 1318 nm NIRDIB divided by the
EW of optical DIBs or $E(B-V)$ plotted against (1) $\lambda\lambda$5797/5780
ratio, (2) $\lambda\lambda$Ca i/Ca ii, (3) $\lambda\lambda$CH/CH+ and (4) Na i
(5895 Å)/Ca ii (3933 Å). Blue dots are for the whole sample with orange points
showing the single cloud dominated DIBs. The dashed lines show uppper and
lower error bounds obtained by fitting with a second order polynomial.

###  4.4 Search for correlations in normalised DIB spectra

Cami et al. (1997) and more recently Fan et al. (2021) searched for
correlations in DIB strengths by using the normalised DIB strength
(EW/$E(B-V$)). One of the goals of the latter work was to search for anti-
correlated DIBs and compare the changes in the correlation coefficients across
DIB families. In the current dataset we find that no NIRDIBs show anti-
correlations less than $\sim$–0.7 with more than ten datapoints. The best
correlations are found between the 1318 and 1527 nm reddening-normalised DIB
EWs where $r$ exceeds 0.9. The correlations between all DIBs are available
online in VIZIER.

###  4.5 Search for small scale structure and time-variability in NIRDIBs

####  4.5.1 Search for small-scale structure

In neutral species such as Na i and K i, small-scale (pc-) structure is
ubiquitous in the interstellar medium (Meyer & Lauroesch, 1999), with
variations also seen (but smaller) in singly ionised elements such as Ca ii
and Ti ii (Smoker et al., 2011). Variations have also been detected in five
optical DIBs between 5780 and 6614 Å by Cordiner et al. (2013) towards the
stellar binary $\rho$ Oph A and B, separated by 344 au and with variations in
the DIB strengths of 5–9 per cent. Sometimes the variations seen are greater
than in the corresponding difference between atomic and diatomic lines such as
Na i. Similarly, van Loon et al. (2013) observed DIBs in both Galactic and LMC
gas, finding that small-scale structure increased from Na i to the 5780 Å DIB
to the DIB at 5797 Å. In detail, in Milky Way gas the 5797 Å DIB displayed
variation of order 10 times greater than that seen in Na i on scales of 0.04
pc. Wendt et al. (2017) discuss small-scale variations over the face of NGC
6397 (a globular cluster), which are of the order of a few thousand au.

Eleven of our CRIRES sightlines are in double, triple or quadruple systems.
Table 5 shows the stars that have companions within 1′ that were observed to
search for small-scale structure in DIBs. The subsample was derived in part
from observations by Watson & Meyer (1996) and Lauroesch & Meyer (1999) who
studied many of the sightlines in either Na i or K i and partly by chance when
another object was visible on the slit viewer image. Of our sample of eleven
fields, five show no DIB at 1318.1 nm and four show no difference in profile
between the two closely separated objects. The spectra are shown in Figs. 23
to 25.

In particular, three stars have bright companions within 20 arcseconds; HD
144217 and HD 144218 that are separated by 13$\aas@@fstack{\prime\prime}$6 and
have similar values of parallax (8.07$\pm$0.78 and 7.75$\pm$0.40 mas), the HD
154873 A/B pair with separation of 8$\aas@@fstack{\prime\prime}$2 and similar
parallaxes of 0.97$\pm$0.06 and 1.00$\pm$0.08 mas respectively and HD 318014
which is a line of sight companion to TYC 7380-1056-1 at
20$\aas@@fstack{\prime\prime}$8\. The parallaxes of these stars are somewhat
different, being 0.42$\pm$0.05 and 0.18$\pm$0.06 mas, respectively. None of
the stars above show convincing differences in the 1318 nm EW compared with
their line of sight companions. In the case of the HD 144217 and HD 144218
pair some optical data exist in the ESO archives (see Fig. 22). Similarly,
there are no convincing small-scale variations observed in the optical lines.

This leaves three fields in which possible variations in the 1318 nm line are
seen. The first is the HD 145501 / HD 145502 field. This comprises the two
main objects, each of which has a companion. The two main objects were
observed in K i by Lauroesch & Meyer (1999) as many of the lines were
saturated in Na i. They found a density contrast of between 1.1$\pm$0.1 and
1.8$\pm$0.8 in three different components observed, with the HD 145501
sightline having stronger K i lines. This contrast in the K i components is
not seen in data we downloaded from the ESO archive and shown in Fig. 23,
likely due to the lower spectral resolving power in the archive data.

In the 1318 nm spectra, the main component, HD 145501 A ($\pi$=7.09$\pm$0.07
mas), separated by 41 arcs or $\sim$7200 au from HD 145502 A, has a marginally
deeper line profile than HD 145502 A/B with EW of 25$\pm$1 mÅ compared with
22$\pm$1 mÅ. The secondary component HD 145501 B, separated by
2$\aas@@fstack{\prime\prime}$5, has a different profile shape and stronger EW
of 31$\pm$1 mÅ. However, its spectral type and parallax are unknown and the
proper motion of $-$8.0$\pm$0.4, $-$24.0$\pm$0.3 mas yr-1 is different to that
of HD 145501 A which has proper motion of $-$11.9$\pm$0.1 $-$24.4$\pm$0.1 mas
yr-1. Hence HD 145501 B may well be a line-of-sight ”companion”. The blue
absorption line components at 1315.5 and 1316.8 nm are only seen in the HD
145501 B spectrum and likely stellar. Finally, there are some indications that
the DIB at 1527 nm is also stronger in HD 145501 A/B than HD 145502 A/B,
although the data are rather noisy.

The two components of spectroscopic binary HD 145502 (nu Sco A and nu Sco B)
are separated by 1$\aas@@fstack{\prime\prime}$35 and no obvious difference is
visible in their 1318.1 nm DIB profiles which have equivalent widths of
22$\pm$1 mÅ. Their proper motions are $-$7.65$\pm$0.71 and $-$23.71$\pm$0.47
mas yr-1 with parallax unknown for HD 145501 and 6.88$\pm$0.76 mas for HD
145502, both from Hipparcos data. The stars have no parallax or proper motion
data in Gaia DR2. Figure 23 shows the available K i, 5780 Å 5797 Å and 1318 nm
data towards the field with Table 6 showing the NIR DIB EW measurements.

Comparing with optical data, the variation in between the main components of
HD 145501 and HD 145502 in the 1318 nm DIB are also reflected in the 5780 Å
DIB, but not in the K i line and only marginally in the 5797Å DIB. We recall
that in the data from Cordiner et al. (2013), the 5797 Å DIB showed more
variation than the 5780 Å feature. The observed small-scale structure seen in
HD 145501 A/B may just be caused by the objects being at different distances.
A spectroscopic parallax for this object would be useful.

The second field where possible variation is seen in the 1318 nm line is the
HD 160065 field. HD 160065 has $E(B-V)$ of 1.06 mag. and another star with
separation of $\sim$18$\aas@@fstack{\prime\prime}$6 was put on the slit. The
”companion” has no reference in SIMBAD. The spectrum of the second star is far
noisier than towards HD 160065 but a clear difference in the DIB strength is
seen. Follow-up work on this star would be needed to determine its distance.

The final field where there is some evidence for infrared DIB structure is the
region that contains the stars HD 168607 ($\pi$=0.6438$\pm$0.0603 mas) and HD
168625 ($\pi$=0.6212$\pm$0.0640 mas). These stars have similar reddenings
($E(B-V)$=1.54 and 1.47, respectively) and are separated on the sky by 67′′ or
around 0.52 pc. Possible variation in the 1318 nm line is seen. Clear
variation is also seen in the K i line and possible variation in the 5797 Å
DIB. Although the variation of this line presented in Fig. 24 looks
convincing, the baseline fitting is not straightforward. No clear variation is
seen in the 5780 Å DIB.

Tables 6 and 7 show the equivalent widths and percentage variations detected
in the stars discussed above.

In conclusion, possible variation in the 1318 nm DIB is only seen towards two
double systems; HD 145501 and HD 145502 with separation of $\sim$7200 au, and
towards HD 168607 and HD 168625 with separation of around 0.52 pc or 107,000
au.

Table 5: Objects with companions within one arcminute that were observed.
CRIRES reference wavelengths are as follows: (a) 1318.1 nm (b) 1527.9 nm (c)
1568.8 nm (d) 1574.4 nm (e) 1624.2 nm.

HD | Alternative | $l,b$ | Companion | Separation | NIRDIB | Reference and comments
---|---|---|---|---|---|---
number | names | (deg.) |  | (arcs, au) | setting(s) |
|  |  |  |  | observed |
36408 | HR 1847 | 188.4942 -08.8854 | HD 36408B | 9.5, 2100 | a,c | WM96. No NIRDIB detection.
36959 | YSO | 209.5670 -19.7206 | HD 36960 | 36.0, 21000 | a,b,c | WM96. No NIRDIB detection.
36861 J | lam Ori A | 195.0519 -11.9951 | HD 36862 | 3.5, 2500 | a,b | WM96. No NIRDIB detection.
139891 | HR 5834 | 058.6978 +53.4129 | HD 139892 | 6.3, – | a,b | LWV03. No NIRDIB detection.
144217 | HR 5984, bet Sco | 353.1929 +23.5996 | HD 144218 | 13.6, 2900 | a,b,c | WM96. NIRDIB detection.
145501 | HR 6026 | 354.6140 +22.7107 | HD 145501 B | 2.5, – | a,b,c | NIRDIB detection.
145501 | HR 6026 | 354.6140 +22.7107 | HD 145502 A | 41.0, 7200 | a,b,c | LM99. NIRDIB detection.
145502 | HR 6027, nu Sco A | 354.6087 +22.7002 | HD 145502 B | 1.4, – | a,b,c | LM99. NIRDIB detection.
154873 A | HIP 84010 | 341.3454 -04.1096 | HD 154873 B | 8.0, – | a,b | No optical data. NIRDIB det.
157779 | HR 6485, rho Her A | 061.4422 +32.7103 | HD 157778 | 4.3, 480 | a,b,c | WM96. No NIRDIB detection.
160065 | LS 4314 | 355.3166 -01.3562 | – | 18.6, – | a | Companion has no SIMBAD
|  |  |  |  |  | reference. NIRDIB detection.
168607 |  | 014.9679 -00.9397 | HD 168625 | 67.4, 108289 |  | NIRDIB detection.
188293 | 57 Aql A | 032.6612 -17.7609 | HD 188294 | 35.6, 5700 | a,b,c | WM96. NIRDIB detection.
318014 | LS 4255 | 355.0738 -00.7062 | LS 4256 | 20.5, – | a | NIRDIB detection.

WM96 (Watson & Meyer, 1996), LM99 (Lauroesch & Meyer, 1999), LWV03 (Lallement
et al., 2003).

Table 6: Equivalent widths of sightlines with companions within one arcminute that had a NIRDIB detection. Star | EW | EW | EW | EW | EW | EW | EW
---|---|---|---|---|---|---|---
| Ca ii | Na i | K i | DIB(5780 Å) | DIB(5797 Å) | DIB(1318 nm) | DIB(1527 nm)
| (mÅ) | (mÅ) | (mÅ) | (mÅ) | (mÅ) | (mÅ) | (mÅ)
HD 144217 | 40$\pm$1 | 148$\pm$1 | – | 157$\pm$1 | 16$\pm$1 | 22$\pm$1 | 8.3$\pm$ 0.4
HD 144218 | 38$\pm$2 | 152$\pm$2 | – | 148$\pm$2 | 16$\pm$1 | 24$\pm$1 | 9.9$\pm$ 0.4
HD 145501 A | 58$\pm$2 | – | 35$\pm$1 | 175$\pm$3 | 31$\pm$1 | 25$\pm$1 | –
HD 145501 B | – | – | – | – | – | 31$\pm$1 | –
HD 145502 A | 62$\pm$1 | – | 37$\pm$1 | 172$\pm$2 | 33$\pm$1 | 22$\pm$1 | –
HD 145502 B | – | – | – | – | – | 22$\pm$1 | –
|  |  |  |  |  |  | –
HD 160065 | – | – | – | – | – | 60$\pm$2 | –
No name | – | – | – | – | – | 110$\pm$12 | –
HD 168607 | 768$\pm$3 | 695$\pm$2 | 220$\pm$2 | 880$\pm$5 | 359$\pm$5 | 87$\pm$1 | –
HD 168625 | 793$\pm$3 | 683$\pm$2 | 182$\pm$2 | 887$\pm$5 | 305$\pm$5 | 80$\pm$1 | –
HD 188293 | 19$\pm$1 | – | – | – | – | 6.4$\pm$0.4 | 4.2$\pm$0.6
HD 188294 | 17$\pm$1 | 80$\pm$1 | 30$\pm$1 | – | – | 4.1$\pm$0.6 | 3.5$\pm$0.9
HD 318014 | 808$\pm$15 | – | 194$\pm$3 | 643$\pm$5 | – | 91$\pm$2 | –
LS 4256 | – | – | – | – | – | 88$\pm$1 | –
Table 7: Variation in equivalent width in percent for sightlines where NIRDIBs were detected. Where available, optical data are shown. Main | Companion | Var. | Var. | Var. | Var. | Var. | Var. | Var.
---|---|---|---|---|---|---|---|---
object | star | Ca ii | Na i | K i | DIB (5880 Å) | DIB (5897 Å) | DIB (1318 nm) | DIB (1527 nm)
|  | (%) | (%) | (%) | (%) | (%) | (%) | (%)
HD 144217 | HD 144218 | 5$\pm$7 | –3$\pm$2 | – | 6$\pm$2 | 0$\pm$12 | –9$\pm$9 | –16$\pm$9
HD 145501 A | HD 145501 B | – | – | – | – | – | –20$\pm$7 | –
HD 145501 A | HD 145502 A | –6$\pm$5 | – | –6$\pm$5 | 2$\pm$3 | –6$\pm$6 | 14$\pm$8 | –
HD 145502 A | HD 145502 B | – | – | – | – | – | 0$\pm$9 | –
HD 160065 | No name | – | – | – | – | – | –45$\pm$14 | –
HD 168607 | HD 168625 | –3$\pm$1 | 2$\pm$1 | 20$\pm$2 | –1$\pm$1 | 18$\pm$3 | 9$\pm$2 | –
HD 188293 | HD 188294 | 11$\pm$10 | – | – | – | – | 56$\pm$20 | 20$\pm$40
HD 318014 | LS 4256 | – | – | – | – | – | 4$\pm$3 | –

####  4.5.2 Search for time-variability

Seventeen of our sightlines were observed twice by CRIRES, with the epochs
separated by six to fourteen months, with the aim of searching for time-
variable structure in NIRDIBs. Additionally, two of our X-shooter sightlines
observed in conditions of low water vapour in August 2019 were previously
observed in 2010 and published by Cox et al. (2014). These are additionally
used to search for time variation in the 5780, 5797 and C${}_{60}^{+}$ 9577
and 9632 Å DIBs.

Table 8 lists the stars that were observed at more than one epoch in order to
check for time-variability in the ISM. The transverse distances derived from
the Hipparcos proper motions in these time range from 1.7$\pm$0.3 to 24$\pm$3
au. Time variability, although uncommon, has been found in over 20 cases in
atomic lines such as Na i and singly ionised lines such as Ca ii (e.g. Hobbs
et al., 1991; Crawford et al., 2000; Smoker et al., 2011); see Lauroesch
(2007) and Stanimirović & Zweibel (2018) for reviews). In the optical, a twin-
epoch study of more than 100 early-type stars by McEvoy et al. (2015) found
that in spectra with $R$ = 80 000 to 160 000 and S/N ratios of several
hundreds, typically a few percent of sightlines (corresponding to less than 1
percent of all components) in the diffuse interstellar medium show such time
variation in optical lines on timescales from 6–20 years. In their survey,
time-variable structure was only visible in sightlines with upper limits of 50
au or more of transverse separation. Previous claims of time-variability in
DIBs have been claimed by Law et al. (2017) and, more recently, by Farhang (in
preparation) towards the DIBs at 4735, 5547, 6196 and 6602 Å, in high
resolution UVES, FEROS and HARPS data towards early-type stars in the EDIBLES
sample. The changes are typically very close to the noise level and will have
to be followed-up by long-term monitoring. Non-detection of time variability
in the 5780, 5797, 6613 Å DIBs towards $\zeta$ Oph was recently reported by
Cox et al. (2020).

Four examples of our CRIRES spectra and two observed with X-shooter at twin
epochs are shown in Fig. 26. No time-variability is detected in the 1318.1 nm
DIB in the CRIRES data or the 5780.5, 5797.1 Å, 1318.1 and 1527.4 nm DIBs.
Tentative time variation is observed in the ”C${}_{60}^{+}$” DIBs at 9577 and
9632 Å towards HD 183143 although very close to the noise level and with
confirmation required at a higher spectral resolution as continuum fitting and
telluric line corrections are not straightforward. Although the transverse
distance travelled in 9.9 years of 124 AU is the largest in our sample,
variation is not seen in the $\lambda\lambda$5780, 5797 DIBS.

Figure 22: Spectra of the binary stars HD 144217 and HD 144218 field in the Ca
ii, Na i line and 5780 Å, 5797 Å 1318 nm, 1527 nm and 1567 nm DIBs.

Figure 23: Spectra of the HD 145501 / HD 144502 field in the Ca ii and K i
lines and 5780, 5797 Å and 1318.1 nm DIBs. We recall HD 145502 is a close
binary.

Figure 24: Spectra of the HD 168607 / HD 168625 field in the Ca ii, K i lines
and 5780, 5797 Å and 1318 and 1568 nm DIBs.

Figure 25: Spectra of binary and multiple systems observed around 1318 nm.

Figure 26: Examples of normalised spectra for which twin epochs were taken separated by more than six months. Top panel: 5797 and 5780 Å DIBs observed with X-shooter at low PWV (4U 1907+09 not shown due to low S/N ratio). Middle panel: Ditto C${}_{60}^{+}$ DIBs observed with X-shooter. Bottom panel: 1318.1 nm NIRDIBs. The black line is the earlier epoch and the blue line the later epoch. For HD 115363, HD 116119, HD 142468, HD 144969 and HD 183143 the transverse distances moved by the star between the two epochs are 24.0$\pm$3.4, 19.5$\pm$2.9, 7.2$\pm$0.7, 6.4$\pm$3.1 and 138$\pm$20 au, respectively. The parallax and proper motions for 4U 1907+09 are not available in SIMBAD. Table 8: Objects with repeat observations at 1318 nm (for CRIRES data ’C’), and with X-shooter low water vapour data (’XL’) at 1318 nm and 1527 nm (for 4U 1907+09 and HD 183143 only) to search for time-variability in the ISM. Parallaxes and proper motions are from Gaia. The transverse distance travelled by the star between the first and last epoch is shown in the last column. HD | Observation | Inst. | PM (mas/year) | $\pi$ | Transverse.
---|---|---|---|---|---
number | dates |  | RA DEC | (mas) | dist. (au)
4U 1907+09 | 2010-Jul-02 2019-Aug-10 | XL | – | – | –
HD 106068 | 2013-May-27 2014-Mar-16 | C | -5.18$\pm$0.06 -0.24$\pm$0.06 | 0.58$\pm$0.04 | 7.1$\pm$1.9
HD 112272 | 2013-May-28 2014-Mar-16 | C | -8.49$\pm$0.05 -7.93$\pm$0.05 | 0.63$\pm$0.04 | 14.5$\pm$0.9
HD 115363 | 2013-Jun-22 2014-Mar-16 | C | -8.81$\pm$0.05 -1.92$\pm$0.05 | 0.27$\pm$0.04 | 24.1$\pm$3.4
HD 116119 | 2013-Jun-22 2014-Mar-17 | C | -5.71$\pm$0.04 -1.71$\pm$0.05 | 0.22$\pm$0.03 | 19.6$\pm$2.9
HD 125241 | 2013-May-16 2014-Mar-20 | C | -6.92$\pm$0.04 -1.99$\pm$0.06 | 0.41$\pm$0.03 | 14.6$\pm$1.3
HD 142468 | 2013-May-16 2014-Mar-20 | C | -2.52$\pm$0.07 -3.05$\pm$0.06 | 0.46$\pm$0.05 | 7.2$\pm$0.7
HD 144969 | 2013-May-16 2014-Mar-20 | C | 3.31$\pm$0.63 -1.85$\pm$0.32 | 0.50$\pm$0.21 | 6.3$\pm$3.1
HD 147932 | 2013-Jun-24 2014-Jun-17 | C | -9.34$\pm$0.15 -21.94$\pm$0.09 | 7.46$\pm$0.07 | 3.1$\pm$0.1
HD 148379 | 2013-May-16 2014-Mar-22 | C | -2.97$\pm$0.19 -4.04$\pm$0.11 | 0.33$\pm$0.13 | 12.7$\pm$5.2
HD 148937 | 2013-May-21 2014-Mar-23 | C | 0.88$\pm$0.08 -3.38$\pm$0.04 | 0.88$\pm$0.05 | 3.3$\pm$0.3
HD 154873 | 2013-Sep-04 2014-Mar-17 | C | 2.57$\pm$0.09 -3.17$\pm$0.07 | 0.97$\pm$0.06 | 2.2$\pm$0.2
” | 2013-Sep-04 2014-Mar-30 | C | ” | ” | 2.4$\pm$0.2
HD 157038 | 2013-May-22 2014-May-15 | C | -0.20$\pm$0.11 -0.60$\pm$0.08 | 0.36$\pm$0.05 | 1.7$\pm$1.0
HD 159455 | 2013-May-23 2013-Jun-25 | C | 0.06$\pm$0.11 -0.90$\pm$0.08 | 0.34$\pm$0.06 | 0.2$\pm$0.0
” | 2013-May-23 2014-Apr-17 | C | ” |  | 2.4$\pm$0.3
HD 160065 | 2013-May-22 2014-Mar-30 | C | 1.18$\pm$0.11 -1.46$\pm$0.08 | 0.47$\pm$0.06 | 3.4$\pm$0.6
HD 162168 | 2013-Jun-02 2014-Apr-27 | C | -0.70$\pm$0.11 -1.13$\pm$0.09 | 0.70$\pm$0.06 | 1.7$\pm$0.3
HD 183143 | 2013-Apr-11 2014-Jun-23 | C | -1.09$\pm$0.06 -5.57$\pm$0.08 | 0.41$\pm$0.05 | 16.6$\pm$2.1
” | 2010-Jul-02 2019-Aug-09 | XL | ” | ” | 124$\pm$16
HD 318014 | 2013-May-13 2014-Apr-17 | C | 0.16$\pm$0.10 -1.40$\pm$0.08 | 0.43$\pm$0.05 | 3.0$\pm$2.0

##  5 Summary and future work

We have undertaken a mini-survey of several Near-Infrared Diffuse Interstellar
Bands (NIRDIBs) using CRIRES with limited wavelength coverage. These data were
supplemented by lower S/N and resolution X-shooter observations towards ten
highly reddened Cepheid variables, nineteen archival X-shooter targets and
towards four stars observed at very low values of water vapour in order to
investigate time variability of the “C${}_{60}^{+}$”-DIBs and correlations
with the NIRDIBs. Our findings can be stated as follows:

1) The NIRDIBs studied are reasonably well correlated with the strong optical
bands, in particular those at 5705, 5780, 6203, 6283 and 6269 Å. The 5797 Å
DIB is less well correlated with NIRDIB strengths. Using DIB ratios, and as
found by previous authors, this implies that the NIRDIBs are $\sigma$ type
(unshielded) DIBs as opposed to the $\zeta$ DIB that are affected by UV
radiation. The NIRDIBs are not well correlated with the “C${}_{60}^{+}$”-DIB
at 9632.1 Å, neither is the 5797 Å DIB although numbers are small.

2) Correlations with reddening are fair to good with $r$ values for the whole
dataset of $\sim$0.89 for the 1318 nm DIB.

3) The 1318 nm DIB does not have a Gaussian profile shape but has a red wing
that can be fitted adequately using two Gaussian functions. The integrated
flux ratio of the two components is not correlated with reddening or the
$\lambda\lambda$5780/5797 flux ratio.

4) Towards stars of type earlier than B2, the 1318 nm DIB often displays an
emission feature to the blue. It is unclear if this is stellar, circumstellar
or interstellar in nature.

5) In spectra with S/N ratios of several hundreds and spectral resolving power
of $\sim$50,000, variation in the 1318 nm DIB was found towards two near
equidistant stellar systems (HD 145501/HD 145502 and HD 168607/HD 168625).
Variations of $\sim$14 and $\sim$9 percent in the 1318 nm line were observed
with transverse separations of 7200 au and 0.52 pc. Two further sightlines (HD
145501 and line of sight companion separated by 2.5 arcsec, and HD 165501 with
line of sight companion separated by 18.6 arcsec) also show small scale
variation in the 1318 nm line, but the distance to the companion object is
uncertain in both cases. The remaining stellar pairs show no difference in
their 1318 nm profiles.

6) No time variation was detected towards the 1318 nm DIB on timescales of
6–14 months towards 17 sightlines and towards two other stars observed with
X-shooter over epochs separated by 9.9 years. One of these two objects (HD
183143) showed tentative time variation for the “C${}_{60}^{+}$”-DIBs at 9577
and 9632 Å although very close to the noise level.

Future work will use the upgraded CRIRES spectrometer (Dorn et al., 2014) with
its expanded wavelength coverage to look for new DIBs in the infrared, check
for emission in the 1318 nm DIBS towards the red rectangle, search for
polarisation in the 1318 nm absorption and emission line DIBs and observe more
sightlines in the 1527 nm band at high spectral resolution.

###### Acknowledgements.

We thank the referee Ed Jenkins for many useful corrections and suggestions to
the paper. JVS thanks the UK Astronomy Technology Centre for hospitality. AMI,
KTS and ME thank ESO for funding under the auspices of the Director General’s
Discretionary Fund. AMI acknowledges support from the Spanish MINECO through
project AYA2015-68217-P. Observations were performed as ESO programmes
091.C-0655, 093.C-0480 and 0103.C-0766(A) and archival data taken from ESO
programmes 072.D-0286, 081.C-0475, 266.D-5655, 194.C-0833, 082.C-0566,
083.C-0503, 194.C-0833, 074.D-0240, 194.C-0833, 081.D-2002, 194.C-0833,
284.D-5048, 60.A-9036, 079.C-0597 and 095.A-9029. This research has made use
of the SIMBAD database operated by CDS in Strasbourg.

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##  Appendix A Appendix material to go on CDS

Table 9:  Stellar sample, sorted alphabetically and split into CRIRES and X-shooter subsections.  Target | Inst. | Spectral |  $l$,$b$ | $m_{B}$ | $m_{V}$ | $m_{H}$ | $(B-V)_{0}$ | $E(B-V)$ | $N_{\text{Obs}}$
---|---|---|---|---|---|---|---|---|---
|  | type | (deg.) | (mag.) | Fitz. | Weg. |  | a b c d e
HD 36408 A | C | B7-8IV-V | 188.49 $-$08.89 | 6.10 | 6.09 | 6.02 | $-0.13$ | … | … | 1 1 1 – –
HD 36408 B | C | B7II | 188.49 $-$08.89 | 6.53 | 6.49 | 6.28 |  $-$0.13 | … | 0.17 | 1 1 1 – –
HD 36861 | CAO | O8III | 195.05 $-$12.00 | 3.48 | 3.66 | 3.77 |  $-$0.30 |  $-$0.30 | 0.12 | 1 1 – – –
HD 36862 | CAO | B0.5V | 195.05 $-$11.99 | … | 6.32 | … | … | … | … | 1 1 – – –
HD 36959 | C | B1VvYSO | 209.57 $-$19.72 | 5.44 | 5.67 | 6.24 |  $-$0.26 |  $-$0.23 | 0.01 | 1 1 1 – –
HD 36960 | C | B1/2Ib/II | 209.56 $-$19.71 | 4.50 | 4.72 | 5.38 |  $-$0.17 |  $-$0.16 |  0.031 | 1 1 1 – –
HD 89587 | C | B3III | 279.84 +05.19 | 6.74 | 6.87 | 7.20 |  $-$0.20 | … | 0.07 | 1 – – 1 –
HD 88661 | C | B5Vne | 283.08 $-$01.48 | 5.66 | 5.75 | 5.57 |  $-$0.16 |  $-$0.15 | 0.06 | 1 – – 1 1
HD 90706 | C | B2.5Ia | 284.52 +00.00 | 7.56 | 7.10 | 5.84 |  $-$0.15 |  $-$0.14 | 0.61 | 1 – – 1 1
HD 92740 | C | WN7h+ | 287.17 $-$00.85 | 6.50 | 6.42 | 5.58 | … | … | 0.36 | 1 – – 1 1
| C | O9III-V |  |  |  |  |  |  |  |
HD 93130 | C | O6.5III | 287.56 $-$00.86 | 8.31 | 8.04 | 7.26 | … | … |  0.562 | 1 – – 1 –
HD 93632 | C | O5I | 288.03 $-$00.87 | 8.57 | 9.10 | 7.06 | … | … |  0.623 | 1 – – 1 –
HD 94910 | C | WN11h | 289.18 $-$00.70 | 7.57 | 6.96 | 5.08 | … | … | 0.59 | 1 – – 1 1
HD 97707 | C | B11b | 291.26 $-$00.11 | 8.57 | 8.07 | 6.77 | … | … | 0.67 | 1 – – 1 1
HD 100841 | C | B9III | 294.47 $-$01.40 | 3.10 | 3.14 | 3.10 |  $-$0.08 | … | 0.04 | 1 – – 1 1
HD 105056 | C | ON9.7Ia | 298.96 $-$07.06 | 7.48 | 7.34 | 7.14 |  $-$0.27 |  $-$0.27 | 0.41 | 1 – – 1 –
HD 105071 | C | B8Ia | 298.24 $-$03.09 | 6.45 | 6.31 | 5.67 |  $-$0.01 |  $-$0.03 | 0.16 | 1 – – 1 1
HD 106068 | C | B8Ia/Iab | 298.51 $-$00.42 | 6.28 | 5.98 | 5.04 |  $-$0.01 |  $-$0.03 | 0.32 | 2 – – 1 1
HD 111774 | C | B8V | 303.04 +23.19 | 5.89 | 5.97 | 6.21 |  $-$0.11 |  $-$0.11 | 0.03 | 1 – – 1 1
HD 112272 | C | B0.5Ia | 303.49 $-$01.49 | 8.03 | 7.39 | 5.36 |  $-$0.22 |  $-$0.20 | 0.85 | 2 – – 1 1
HD 113904 | C | WC5+B0III | 304.67 $-$02.49 | 5.50 | 5.53 | … | … | … |  0.254 | 1 – – 1 1
| C | O9IV |  |  |  |  |  |  |  |
HD 115088 | C | B9.5/A0V | 304.18 $-$17.17 | 6.31 | 6.33 | 6.47 |  $-$0.04 |  $-$0.06 | 0.03 | 2 – – 2 1
HD 115363 | C | B1Ia | 305.88 $-$00.96 | 8.32 | 7.82 | 6.13 |  $-$0.19 |  $-$0.19 | 0.69 | 2 – – 1 1
HD 115842 | C | B0.5Ia | 307.08 +06.83 | 6.39 | 6.09 | 5.30 |  $-$0.22 |  $-$0.20 | 0.51 | 1 – – 1 1
HD 116119 | C | A0Ia | 306.62 +00.63 | 8.58 | 7.87 | 5.88 |  0.00 |  0.00 | 0.71 | 2 – – 1 1
HD 125007 | C | B9V | 319.45 +17.49 | 7.00 | 7.03 | 7.03 |  $-$0.07 |  $-$0.07 | 0.04 | 1 – – 1 –
HD 125241 | C | O8.5Ib | 313.54 +00.14 | 9.08 | 8.60 | 6.99 |  $-$0.29 |  $-$0.30 | 0.77 | 2 – – 1 1
HD 137753 | C | B7IV | 325.93 +03.34 | 6.66 | 6.69 | 6.68 |  $-$0.13 | … | 0.10 | 1 – – 1 1
HD 139891 | C | B9V | 058.69 +53.41 | 5.82 | 5.93 | … |  $-$0.07 |  $-$0.07 | 0.07 | 1 1 – – –
HD 139892 | C | B7V | 058.69 +53.41 | 4.87 | 4.98 | 5.30 | … | … | … | 1 1 – – –
HD 142468 | C | B1Ia/Iab | 327.95 $-$00.76 | 8.48 | 7.90 | 6.41 |  $-$0.19 |  $-$0.19 | 0.77 | 2 – – 1 1
HD 144217 | CAO | B1V | 353.19 +23.60 | 2.55 | 2.62 | 2.80 | … | … | 0.20 | 1 1 1 – –
HD 144218 | CAO | B2V | 353.20 +23.60 | 4.87 | 4.89 | 4.47 | … | … | … | 1 1 1 – –
HD 144969 | C | B1Ia | 333.18 +02.01 | 9.32 | 8.39 | 5.68 |  $-$0.19 |  $-$0.19 | 1.12 | 2 – – 1 1
HD 145482 | C | B2V | 348.12 +16.84 | 4.42 | 4.57 | 5.01 |  $-$0.24 |  $-$0.21 | 0.07 | 2 – – 2 2
HD 145501A | CAO | B8V+ | 354.61 +22.71 | 6.43 | 6.30 | 5.69 |  $-$0.09 |  $-$0.09 | 0.22 | 1 1 1 – –
| CAO | B9VpSi |  |  |  |  |  |  |  |
HD 145501B | CAO | … | 354.61 +22.71 | 7.54 | 7.23 | … | … | … | … | 1 1 1 – –
HD 145502A | C | B2V | 354.61 +22.71 | 4.35 | 4.35 | … |  $-$0.24 |  $-$0.21 | 0.28 | 1 1 1 – –
HD 145502B | C | … | 354.61 +22.70 | 6.72 | 6.60 | … | … | … | … | 1 1 1 – –
HD 146331 | C | B9V | 350.41 +17.53 | 8.69 | 8.30 | 7.19 |  $-$0.07 |  $-$0.07 | 0.46 | 1 – – 1 –
HD 147932 | CAO | B5V | 353.72 +17.71 | 7.59 | 7.27 | 5.92 |  $-$0.16 |  $-$0.15 | 0.48 | 1 – – 1 1
HD 148184 | C | B2Vne | 357.93 +20.67 | 4.71 | 4.43 | 3.26 |  $-$0.24 |  $-$0.21 | 0.51 | 1 – – 1 1
HD 148379 | C | B2Iab | 337.25 +01.57 | 5.95 | 5.37 | 3.83 |  $-$0.17 |  $-$0.16 | 0.74 | 2 – – 1 1
HD 148688 | C | B1Ia | 340.72 +04.34 | 5.78 | 5.39 | 4.24 |  $-$0.19 |  $-$0.19 | 0.58 | 1 – – 1 1
HD 148937 | C | O6f?p | 336.37 $-$00.22 | 7.12 | 6.71 | 5.74 | … | … | 0.66 |
HD 151932 | C | WN7h | 343.22 +01.43 | 6.72 | 6.49 | 5.27 | … | … |  0.505 | 1 – – 1 1
HD 152235 | C | B0.5Ia | 343.31 +01.10 | 6.92 | 6.38 | 4.94 |  $-$0.22 |  $-$0.20 | 0.75 | 1 – – 1 1

Table 1: ctd Target | Inst. | Spectral |  $l$,$b$ | $m_{B}$ | $m_{V}$ | $m_{H}$ | $(B-V)_{0}$ | $E$ | $N_{\text{Obs}}$
---|---|---|---|---|---|---|---|---|---
|  | type | (deg.) | (mag.) | Fitz. | Weg. | $(B-V)$ | a b c d e
HD 152386 | C | O5/6I:fek | 341.11 $-$00.94 | 8.64 | 8.13 | 6.62 |  $-$0.32 |  $-$0.31 | 0.83 | 1 – – 1 1
HD 154811 | C | OC9.7Iab | 341.06 $-$04.21 | 7.33 | 6.93 | 5.85 |  $-$0.27 |  $-$0.27 | 0.67 | 1 – – 1 1
HD 154873A | C | B1Ib | 341.35 $-$04.10 | 6.98 | 6.70 | … |  $-$0.19 |  $-$0.19 | 0.47 | 3 1 2 2 1
HD 154873B | C | … | 341.34 $-$04.11 | 8.49 | 8.23 | … | … | … |  | 3 1 2 2 1
HD 156134 | C | B0Iab… | 351.20 +01.36 | 8.70 | 8.05 | 6.41 |  $-$0.24 |  $-$0.22 | 0.88 | 1 – – 1 1
HD 156201 | C | B0.5Ia | 351.51 +01.48 | 8.70 | 8.01 | 6.10 |  $-$0.22 |  $-$0.20 | 0.90 | 1 – – 1 1
HD 156212 | C | B0Iab | 357.59 +05.82 | 8.34 | 7.95 | 6.57 |  $-$0.24 |  $-$0.22 | 0.62 | 1 – – 1 1
HD 156738 | C | O6.5III | 351.18 +00.48 | 10.27 | 9.35 | 6.92 |  $-$0.32 | … | 1.24 | 1 – – 1 –
HD 157038 | C | B3Iap | 349.95 $-$00.79 | 7.20 | 6.44 | 4.29 |  $-$0.13 |  $-$0.13 | 0.89 | 2 – – 1 1
HD 157778 | C | A0Vn | 061.22 +32.71 | 5.40 | 5.40 | … | … | … | … | 1 1 1 – –
HD 157779 | C | B9.5III | 061.44 +32.71 | 4.49 | 4.56 | … |  $-$0.05 | … |  0.005 | 1 1 1 – –
HD 159455 | C | B0.5Ia | 355.23 $-$00.61 | 9.11 | 8.44 | 6.25 |  $-$0.22 |  $-$0.20 | 0.88 | 3 – – 2 2
HD 160065A | C | O9.5Iab | 355.31 $-$01.35 | 9.41 | 8.60 | 6.17 |  $-$0.27 |  $-$0.24 | 1.06 | 2 – – 1 1
HD 160065B | C | … | … | … | … | … | … | … | … | 1 – – – –
HD 161289 | C | A0V | 027.50 +16.03 | 6.60 | 6.54 | 6.38 |  0.00 |  0.00 | 0.07 | 1 – – – –
HD 161291 | C | B0.5Iab | 001.49 +00.85 | 9.61 | 8.88 | 7.04 |  $-$0.22 |  $-$0.20 | 0.94 | 1 – – 1 –
HD 162168 | C | B0.5Iab | 357.13 $-$03.08 | 9.04 | 8.42 | 6.84 |  $-$0.22 |  $-$0.20 | 0.83 | 2 – – 1 1
HD 164438 | C | O9.2IV | 010.35 +01.78 | 8.89 | 8.76 | 6.65 | … | … | 0.65 | 1 – – 1 1
HD 164865 | C | B9Iab | 006.21 $-$01.19 | 8.47 | 7.63 | 4.93 |  0.00 |  $-$0.01 | 0.85 | 1 – – 1 1
HD 165319 | C | O9.7Ib | 015.12 +03.33 | 8.63 | 8.04 | 6.36 |  $-$0.27 |  $-$0.24 | 0.85 | 1 – – 1 1
HD 166734 | C | O7.5Iab | 018.92 +03.62 | 9.49 | 8.42 | 5.53 |  $-$0.31 |  $-$0.30 | 1.37 | 1 – – 1 1
HD 167264 | C | O9.7Iab | 010.45 -01.74 | 5.42 | 5.37 | 5.21 |  $-$0.27 |  $-$0.27 | 0.32 | 1 – – 1 1
HD 167971 | C | O8I+O5 | 018.25 +01.68 | 8.27 | 7.50 | 5.32 | … | … |  1.056 | 1 – – 1 1
HD 168607 | C | B9Iaep | 014.97 $-$00.93 | 9.82 | 8.28 | 3.88 |  0.00 |  $-$0.01 | 1.54 | 1 – – 1 1
HD 168625 | C | B6Iap | 014.97 $-$00.95 | 9.78 | 8.37 | 4.54 |  $-$0.07 |  $-$0.06 | 1.47 | 1 – – 1 1
HD 168987 | C | B1Ia/ab | 014.99 $-$01.44 | 8.76 | 8.02 | 5.53 |  $-$0.19 |  $-$0.19 | 0.93 | 1 – – 1 1
HD 169034 | C | B2Ia | 017.66 $-$00.06 | 9.42 | 8.24 | 4.77 |  $-$0.17 |  $-$0.16 | 1.35 | 1 – – 1 1
HD 169454 | C | B1Ia | 017.53 $-$00.66 | 7.61 | 6.71 | 4.09 |  $-$0.19 |  $-$0.19 | 1.09 | 1 – – 1 1
HD 169754 | C | B0.5Ia | 020.02 +00.23 | 9.64 | 8.60 | 5.81 |  $-$0.22 |  $-$0.20 | 1.25 | 1 – – 1 1
HD 170938 | C | B1Ia | 016.84 $-$03.04 | 8.84 | 7.99 | 5.51 |  $-$0.19 |  $-$0.19 | 1.04 | 1 – – 1 1
HD 171722 | C | B9V | 324.68 $-$24.68 | 7.21 | 7.24 | 7.29 |  $-$0.07 |  $-$0.07 | 0.04 | 1 – – 2 –
HD 183143 | C | B7Iae | 053.24 +00.62 | 8.08 | 6.86 | … |  $-$0.04 |  $-$0.04 | 1.26 | 2 – – 2 2
HD 188293 | C | B7Vn | 032.66 $-$17.76 | 5.63 | 5.71 | 5.88 |  $-$0.13 |  $-$0.13 | 0.05 | 1 1 – – –
HD 188294 | C | B8V | 032.65 $-$17.76 | 6.42 | 6.44 | 6.51 |  $-$0.11 |  $-$0.11 | 0.09 | 1 1 – – –
HD 303188 | C | B3 | 286.91 $-$00.30 | 9.90 | 9.19 | 6.64 | … | … | 0.75 | 1 – – 1 1
HD 312980 | C | B0 | 010.70 $-$01.29 | 10.31 | 9.63 | 7.19 | … | … |  1.157 | 1 – – 1 –
HD 318014 | C | B8Iab: | 355.07 $-$00.70 | 10.24 | 8.78 | 4.21 |  $-$0.02 |  $-$0.03 | 1.49 | 2 – – 1 1
HD 319699 | C | O5V | 351.32 +00.91 | 10.32 | 9.63 | 7.45 |  $-$0.32 |  $-$0.30 | 1.00 | 1 – – 1 –
HD 319702 | C | O8III | 351.34 +00.60 | 10.94 | 10.12 | 7.56 | … | … |  1.308 | 2 – – 2 –
LS 4825 | C | B1Ib | 001.67 $-$06.62 | 12.04 | 11.99 | … |  $-$0.19 |  $-$0.19 | 0.24 | 1 – – 1 –

Table 1: ctd.

Target | Inst. | Spectral |  $l$,$b$ | $m_{B}$ | $m_{V}$ | $m_{H}$ | $(B-V)_{0}$ | $E$ | $N_{\text{Obs}}$
---|---|---|---|---|---|---|---|---|---
|  | type | (deg.) | (mag.) | Fitz. | Weg. | $(B-V)$ | a b c d e
4U 1907+09 | XL | O8.5Iab | 043.74 +00.48 | 19.41 | 16.35 | … | … | … | 3.48 | – – – – –
ALS 18752 | XL | O3.5III(f*) | 353.17 +00.89 | 13.33 | 11.84 | 7.28 | … | … |  1.929 | – – – – –
ALS 19692 | XL | O5.5IV(f) | 353.11 +00.65 | 13.53 | 11.82 | 6.93 | … | … |  2.269 | – – – – –
B02 | X | T2C | 001.93 +01.44 | … | … | 14.00 | … | … | 3.35 | – – – – –
B09 | X | CC | 355.22 $-$00.19 | … | … | 14.20 | … | … | 6.51 | – – – – –
B19 | X | CC | 354.37 $-$00.44 | … | … | 13.76 | … | … | 3.27 | – – – – –
B21 | X | T2C | 358.86 $-$00.85 | … | … | 13.45 | … | … | 4.10 | – – – – –
B22 | X | T2C | 359.29 $-$00.71 | … | … | 12.75 | … | … | 4.27 | – – – – –
B24 | X | T2C | 003.13 $-$00.97 | … | … | 13.42 | … | … | 3.66 | – – – – –
D07 | X | CC | 339.26 $-$00.32 | … | … | 14.67 | … | … | 5.51 | – – – – –
D20 | X | CC | 323.33 $-$00.13 | … | … | 13.07 | … | … | 3.57 | – – – – –
D17 | X | CC | 346.13 +01.00 | … | … | 13.79 | … | … | 3.43 | – – – – –
D18 | X | T2C | 346.16 +00.31 | … | … | 13.84 | … | … | 3.13 | – – – – –
HD 35912 | XA | B2/3V | 201.88 $-$17.84 | 6.21 | 6.38 | 6.83 | … | … |  0.0610 | – – – – –
HD 52266 | XA | O9.5IIIn | 219.13 $-$00.67 | 7.22 | 7.23 | 7.24 | … | … |  0.3210 | – – – – –
HD 52973 | XA | CC | 195.75 +11.90 | 4.58 | 3.79 | 2.03 | … | … |  0.0411 | – – – – –
HD 53367 | XA | B0IV/Ve | 223.70 $-$01.90 | 7.40 | 6.96 | 6.22 | … | … |  0.7410 | – – – – –
HD 57060 | XA | O7Iafpvar | 237.82 $-$05.36 | 4.80 | 4.95 | 5.19 | … | … |  0.1710 | – – – – –
HD 62542 | XA | B3V | 255.91 $-$09.23 | 8.21 | 8.03 | 7.58 | … | … |  0.3510 | – – – – –
HD 108927 | XA | B5V | 301.91 $-$15.35 | 7.83 | 7.76 | 7.58 | … | … |  0.2210 | – – – – –
HD 101602 | XA | CC | 294.95 $-$00.92 | 9.60 | 8.30 | 6.96 | … | … |  0.2511 | – – – – –
HD 114213 | XA | B1Ib | 305.18 +01.31 | 9.88 | 8.96 | 6.46 | … | … |  1.1210 | – – – – –
HD 139717 | XA | CC | 325.65 $-$00.16 | 10.68 | 9.22 | 5.18 | … | … |  0.8911 | – – – – –
HD 147889 | XA | B2III/IV | 352.85 +17.04 | 8.73 | 7.90 | 4.94 | … | … |  1.0710 | – – – – –
HD 152235 | XA | B0.5Ia | 343.31 +01.10 | 6.92 | 6.38 | 4.94 | … | … |  0.6910 | – – – – –
HD 154368 | XA | O9.5Iab | 349.97 +03.21 | 6.63 | 6.13 | 4.85 | … | … |  0.7810 | – – – – –
HD 168076 | XA | O4III(f) | 016.93 +00.83 | 8.72 | 8.25 | 6.66 | … | … |  0.7410 | – – – – –
HD 170740 | XA | B2/3II | 021.05 $-$00.52 | 5.96 | 5.72 | 5.25 | … | … |  0.4810 | – – – – –
HD 178287 | XA | CC | 028.20 $-$07.12 | 8.88 | 7.79 | 5.14 | … | … |  0.4011 | – – – – –
HD 179315 | XA | CC | 039.15 $-$02.57 | 9.01 | 7.78 | 4.80 | … | … |  0.5911 | – – – – –
HD 183143 | XL | B6Ia | 053.24 +00.63 | 8.08 | 6.86 | … | … | … |  1.2610 | – – – – –
HD 214080 | XA | B1/2Ib | 044.80 $-$56.91 | 6.81 | 6.93 | 7.25 | … | … |  0.0610 | – – – – –
HD 219188 | XA | K0III | 083.02 $-$50.17 | 6.90 | 7.06 | 7.43 | … | … |  0.1310 | – – – – –

Notes: Targets are identified by HD number or alternative catalogue
designations for those not in the HD catalogue. The instrument used is denoted
by C for CRIRES (no adaptive optics), CAO for CRIRES using adaptive optics, X
for data from Minniti et al. (2020), XA for archival X-shooter data, and XL
for the X-shooter low-water-vapour sample. Spectral types, Galactic
coordinates and apparent magnitudes in the $B$, $V$ and $H$ bands are taken
from SIMBAD. CC indicates a classical Cepheid and T2C a type 2 Cepheid; the
intrinsic colours of these stars vary over the pulsation period. Cepheid names
are from Minniti et al. (2020), where ”B” refers to a Bulge Cepheid and ”D” to
a Disc Cepheid. Intrinsic ($B-V$)0 colours for the spectral type are taken
from Fitzgerald (1970) and Wegner (1994); where the values differed we took an
average. Reddening $E(B-V)$ was estimated by comparing the observed $(B-V)$
colours to the intrinsic colours, except these entries taken from the
literature shown in the footnote and towards the Cepheids as explained in the
text. $N_{\text{Obs}}$ indicates the number of observations in each of the
five wavelength regions observed using CRIRES, with central wavelengths (a)
1318.1 nm (b) 1527.9 nm (c) 1568.8 nm (d) 1574.4 nm and (e) 1624.2 nm. All
wavelengths were observed in a single setting for X-shooter. There are no
SIMBAD data for 145501 ”B” nor 160065 ”B” that were detected using CRIRES.
Published reddening values taken from: (1) Sonnentrucker et al. (2018); (2)
Weselak et al. (2014); (3) Crowther et al. (2006); (4) Zhekov et al. (2014);
(5) Swihart et al. (2017); (6) Xiang et al. (2017); (7) Wegner (2003); (8)
Russeil et al. (2012) (9) Maíz Apellániz et al. (2015) (10) Fan et al. (2017)
(11) Chen et al. (2014).

Figure A1: Normalised CRIRES spectra around 1318, 1528, 1555 or 1565 nm offset
in the ordinate for clarity.

Figure A1: CRIRES spectra ctd.

Figure A1: CRIRES spectra ctd.

Figure A1: CRIRES spectra ctd.

Figure A1: CRIRES spectra ctd.

Figure A1: CRIRES spectra ctd.

Figure A1: CRIRES spectra ctd.

Figure A1: CRIRES spectra ctd.

Table 2: Equivalent width values for the NIR DIBs studied in this paper as well as the optical DIBs at 5780 and 5797 Å. Columns with no entries either had no data taken or were affected by stellar lines or severe telluric absorption. HD | EW | EW | EW | EW | EW | EW | EW
---|---|---|---|---|---|---|---
number | (1318 nm) | (1527 nm) | (1561 nm) | (1565 nm) | (1567 nm) | (578.0 nm) | (579.7 nm)
| (mÅ) | (mÅ) | (mÅ) | (mÅ) | (mÅ) | (mÅ) | (mÅ)
13989 | $<$ | $<$ | … | … | … | … | …
35912 | … | … | 55 | $<$ | $<$ | … | …
36408 | 170 | 55 | $<$ | $<$ | $<$ | … | …
36861 | … | $<$ | … | … | … | 48 | 16
36862 | … | … | … | … | … | 34 | 17
36959 | … | $<$ | $<$ | $<$ | $<$ | $<$ | 8.2
36960 | $<$ | $<$ | $<$ | $<$ | $<$ | 15.0 | 5.6
52266 | 170 | … | … | … | … | … | …
88661 | … | $<$ | $<$ | $<$ | $<$ | 13.2 | 8.2
89587 | … | $<$ | … | $<$ | $<$ | … | …
90706 | 308 | … | 33 | 34 | 36 | … | …
90772 | … | … | … | … | … | 222 | …
92740 | 156 | … | … | … | … | … | …
93130 | 250 | … | 22 | … | 20 | 237 | 46
93576 | … | … | … | … | … | 199 | 78
93632 | 258 | … | 19 | … | 16 | 266 | 48
94910 | 448 | … |  | … | … | 430 | 92
97707 | 191 | … | 24 | 25 | 29 | … | …
100841 | $<$ | … | $<$ | $<$ | $<$ | … | …
101065 |  | … |  | … | … | … | …
105056 | 202 | … | 23 | 15 | 28 | … | …
105071 | 312 | … | 24 | 15 | 37 | … | …
106068 | 265 | … | 32 | 17 | 35 | 235 | 47
111774 | $<$ | … | … | … | … | … | $<$
112272 | 363 | … | 38 | … | … | 441 | 143
113904 | 144 | … | 8 | 11 | 35 | 126 | 24
114213 | 370 | … | … | … | … | … | …
115088 | $<$ | … | … | … | … | 13.8 | 0.0
115363 | 331 | … | 34 | 20 | 38 | … | …
115842 | 81 | … | 21 | 17 | 22 | 234 | 108
116119 | 471 | … | … | … | … | 449 | 125
125007 | $<$ | … | … | … | … | 18 | ¡$<$
125241 | 514 | … | … | … | … | 533 | 109
137753 | 78 | … | … | … | … | … | …
138014 | 869 | … | … | … | … | … | …
139892 | $<$ | … | … | … | … | … | …
141850 | … | … | … | … | … | … | …
142468 | 590 | … | 32 | 38 | 72 | 461 | 118
144217 | 226 | 86 | … | … | 23 | 156 | 16
144218 | 245 | 100 | … | … | … | 157 | 20
144969 | 458 | … | … | 29 | 51 | … | …