# The Interstellar Medium toward the Galactic Center Source 2MASS
J17470898$-$2829561

T. R. Geballe  Gemini Observatory/NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo,
Hawai’i, 96720, USA<EMAIL_ADDRESS>Yvonne Pendleton  NASA Ames
Research Center, Moffett Field, CA 94035, USA  Jean Chiar  Diablo Valley
College, 321 Golf Club Rd, Pleasant Hill, CA 94523, USA  Alexander G. G. M.
Tielens  Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands

###### Abstract

We describe and discuss remarkable infrared spectra, covering key portions of
the $2-5$ $\mu$m wavelength interval, of the probable OH/IR supergiant 2MASS
$J$17470898$-$2829561 (2M1747), located in direction of the Sgr B molecular
cloud complex within the Central Molecular Zone (CMZ) of the Galaxy. This star
was originally singled out for examination based on its suitability for
spectroscopy of lines of H${}_{3}^{+}$ in the CMZ. Analysis of the spectra
shows that 2M1747 is deeply embedded within Sgr B1, with $A_{V}$ $\gtrsim$ 100
mag, making it the only star within Sgr B for which infrared spectra have been
obtained at present, and thereby a unique infrared probe of the dense
interstellar medium within the CMZ. Despite the high extinction, spectra of
2M1747 reveal a veiled photosphere in the $K$ band and circumstellar gas in
the $M$ band, giving clues as to its nature. Its $3.5-4.0~{}\mu$m spectrum
contains the strongest absorption lines of H${}_{3}^{+}$ observed toward any
object to date. The $4.5-4.8$ $\mu$m spectrum has impressively deep and wide
absorption lines of interstellar CO, most of which arise in dense gas within
Sgr B1. The $3-5$ $\mu$m spectrum also contains several solid-state absorption
features, which are characteristic of both dense and diffuse clouds, and which
raise questions about the identifications of some of these features. We
discuss the nature of the star, the extinction to it, the extinction law for
dust in the CMZ, and the identifications of the various solid-state features
and where they are produced along this complex line of sight.

astrochemistry — Galaxy: center — infrared: stars — ISM: lines and bands —
ISM: molecules

††facilities: Gemini: Gillett; NASA Infrared Telescope Facility††software:
Figaro (Shortridge et al., 1992), Gemini IRAF Package

##  1 Introduction

The Central Molecular Zone (CMZ) of the Galaxy (Morris & Serabyn, 1996) is a
region of diameter $\sim$300 pc and thickness $\sim$50 pc centered on the
supermassive black hole, Sgr A*, at the Galactic center (GC), here assumed to
be at a distance of 8 kpc. Geballe et al. (1999) discovered unusually high
column densities of H${}_{3}^{+}$ toward two stars in the CMZ, one located in
the Central Cluster of hot luminous stars immediately surrounding Sgr A*, the
other in the Quintuplet Cluster, 30 pc distant from Sgr A* on the plane of the
sky. Using high-resolution infrared spectroscopy of three key absorption lines
of H${}_{3}^{+}$ at 3.5337, 3.6205, and 3.7155 $\mu$m in its fundamental
($\Delta v$ = 1) vibration-rotation band together with spectroscopy of
absorption lines from low rotational levels of the first overtone ($\Delta v$
= 2) band of CO near 2.34 $\mu$m, on sightlines towards several hot stars and
dust-embedded stars within $\sim$30 pc of Sgr A*, Oka et al. (2005) and Goto
et al. (2008) established that the absorbing H${}_{3}^{+}$ in the CMZ on
sightlines toward this region is located in warm (T$\sim$200 K) and diffuse
($n\lesssim 10^{2}$ cm-3) interstellar gas within the CMZ. Based on the
observed large column densities of the H${}_{3}^{+}$, the lengths of the
absorbing columns appeared to be large fractions of the radius of the CMZ.
Although information was lacking on sightlines spread across the full
longitudinal extent of CMZ, that finding suggested that warm diffuse gas fills
a large fraction of the volume of the CMZ.

In order to test whether indeed the warm diffuse gas fills much of the CMZ, it
was necessary to find, within the CMZ but much more widely distributed in
Galactic longitude, bright stars whose intrinsic spectra are simple enough for
them to be used to accurately measure the strengths and profiles of the above
three absorption lines of H${}_{3}^{+}$. Geballe et al. (2019) completed a
low-resolution $K$-band spectroscopic survey of 500 candidate stars in the CMZ
with $L$ magnitudes brighter than 7.7, and discovered 30 such stars, scattered
across the CMZ. Almost all of these stars were previously unknown except for
their designations and $JHK$ photometry in the catalog of the Two-Micron All
Sky Survey (2MASS, Skrutskie et al., 2006) and longer-wavelength photometry of
them obtained by the Spitzer Space Telescope as part of the Galactic Legacy
Infrared Midplane Survey Extraordinaire (GLIMPSE, Ramírez et al., 2008).

Many of the newly discovered stars are emission-line stars of various types.
Some of them are embedded in optically thick and warm circumstellar shells;
many of that subset are dusty Wolf-Rayet stars. The remainder of the newly
discovered stars are red giants with warm circumstellar shells that heavily
veil their complex photospheric $K$-band absorption spectra. For each of
these, high-resolution spectroscopy of the above interstellar lines of
H${}_{3}^{+}$ produces spectra that are relatively straightforward to analyze
because they are uncontaminated by photospheric lines. The spectra of the
H${}_{3}^{+}$ lines and CO first overtone band lines toward most of these
stars revealed that the H${}_{3}^{+}$ in front of them is also situated in
long columns of warm and diffuse interstellar gas, similar to those seen
earlier toward stars nearer the center of the CMZ. This finding has
demonstrated the pervasiveness of a warm and relatively low-density
interstellar environment in the CMZ, estimated by Oka et al. (2019) to have a
filling factor of $\sim$ 2/3.

Comparison of H${}_{3}^{+}$ and CO velocity profiles toward almost all of
these objects indicates little or no absorption by CO in dense molecular gas
within the CMZ (Oka et al., 2019), with one of the few exceptions being the
subject of this paper. The lack of absorption by overtone band lines of CO in
dense clouds could be partly a selection effect. A number of giant molecular
clouds (GMCs) are located within the CMZ; the dust they contain produces
significant extinction in addition to that produced by dust in the foreground
spiral arms and in the diffuse interstellar medium (ISM) of the CMZ. Thus,
their effect is to significantly reduce the apparent brightnesses of stars
within or behind them. However, Oka et al. (2019, see their Section 1.3 and
references therein) concluded that the filling factor of clouds with
$n>3\times 10^{3}$ cm-3 in the CMZ is at most a few percent, too small to
require that their conclusion regarding the large filling factor of the warm
diffuse gas in the CMZ be modified. Thus while the name CMZ originally was
bestowed on the region because of the large mass of molecular gas in dense
clouds there, it is actually diffuse molecular gas, which has a total mass
roughly comparable to that of the gas in dense molecular clouds, which fills
most of the volume.

The reader will note that in discriminating between dense ($n\gtrsim 5\times
10^{2}$ cm-3) and diffuse ($n\lesssim 5\times 10^{2}$ cm-3) molecular gas (see
Fig. 15 of Miller et al., 2020), in addition to the above H${}_{3}^{+}$ lines
the $v$ = $2-0$ band of CO (centered near 2.35 $\mu$m) has been employed
rather than the $v$ = $1-0$ band (centered near 4.7 $\mu$m). The square of the
transition dipole moment for the CO overtone band is $\sim$260 times less than
that of the fundamental band, so that the overtone band strength is $\sim$130
times less than the fundamental (Zou & Varanasi, 2002). Due in part to the
weakness of the CO overtone, the CO fundamental band is generally used for
infrared studies of CO in circumstellar and interstellar environments.
However, because of the high extinction toward sources in the GC even the low
abundance of CO in diffuse molecular gas, where C is mostly atomic, can
produce deep absorptions in fundamental band lines, which makes it difficult
to use lines of that band for discriminating between CO in dense molecular gas
and CO in diffuse molecular gas and to determine column densities. On the
other hand, absorption in the low-$J$ lines of the overtone band by CO in
diffuse gas are very weak and often not detected, while the same absorption
lines viewed through clouds of dense gas, although optically thin, are
sufficiently strong that they can be used to obtain accurate column densities.

Figure 1: Simplified diagram of the disk-shaped CMZ and the sightline toward
2M1747 showing locations of potentially absorbing gas and dust. Diffuse
molecular gas in the CMZ is denoted by light shading, several giant molecular
clouds in it by medium shading, and representative dense cores by small dark
circles. A single representative foreground spiral arm containing all of these
components is also shown, with the same shadings to indicate diffuse and dense
molecular gas and dense cores. The low-density interarm medium is depicted
with no shading. The line of sight to 2M1747 passes through all of these
components except dense cores. Galactic center radio sources Sgr B - E and
their associated giant molecular clouds are labeled, as are the Central and
Quintuplet clusters referred to in this paper. Other known giant molecular
clouds in the CMZ are not shown. Typical gas temperatures throughout most of
the volume of the CMZ and in diffuse and dense molecular gas in the spiral
arms are given in Section 5.3. Orbital motions of the dense clouds in the CMZ
and the radial expansion of the diffuse gas in the CMZ are denoted by arrows.

##  2 2MASS $J$17470898$-$2829561

By far the most glaring exception to there being little or no absorption by
H${}_{3}^{+}$ fundamental and CO overtone lines in dense CMZ gas toward the
stars identified in Geballe et al. (2019) is the star 2MASS
$J$17470898$-$2829561, hereafter 2M1747, but referred to as Star $\iota$ in
Oka et al. (2019) and Oka & Geballe (2020). Toward this star absorption by the
strongest H${}_{3}^{+}$ lines and the low-$J$ CO overtone lines extend
continuously over a velocity range of over 200 km s-1. The profiles of both
contain a multitude of velocity components Geballe & Oka (2010). These
interstellar CO and H${}_{3}^{+}$ lines have much larger absorption equivalent
widths than observed to date in the ISM toward any star in the Galactic center
or elsewhere.

The column density of CO toward 2M1747 derived from the overtone band lines is
huge, $2.7\times 10^{19}$ cm-2 (Oka et al., 2019). In view of the strengths of
the overtone lines, almost all of this CO must reside in dense clouds, as
argued at the end of the previous section. This can also be concluded based on
extinction arguments. The ratio of dust extinction to CO column density in
dense clouds is much smaller than in diffuse clouds, where very little carbon
is in the form of CO. If much of the CO observed toward 2M1747 were in diffuse
clouds the extinction to the star would be far too large for it to be
detectable in the $K$ band. See Section 5.1 for further discussion.

In the plane of the sky 2M1747 is located 85 pc from of Sgr A* in the
direction of the Sgr B molecular cloud complex. This region, with linear
dimensions of $\sim$30 pc, is dominated by two giant molecular clouds of
roughly equal size, Sgr B1 and Sgr B2. Based on the high angular resolution
radio continuum image of Lang et al. (2010, see their Fig. 12), 2M1747 is
located within Sgr B1, but close to its northern edge. Both Sgr B1 and Sgr B2
are sites of ongoing star formation. Although this activity is considerably
more intense in Sgr B2, the presence of compact regions of ionized gas and H2O
maser emission (Mehringer et al., 1993) within Sgr B1 implies that star
formation there is still ongoing (see Barnes et al., 2017).

Unsurprisingly 2M1747, like all of the other suitable stars found by Geballe
et al. (2019), has no optical counterpart. Infrared photometry of 2M1747 by
2MASS and GLIMPSE resulted in the following values: $J$ = 17.18, $H$ = 15.35,
$K_{S}$ = 10.45, IRAC1 = 6.58, IRAC2 = 5.79, IRAC3 = 3.28 (the IRAC filters
are centered at 3.6 $\mu$m, 4.5 $\mu$m, and 5.7 $\mu$m, respectively). The
2MASS $J$-band image reveals a crowded field surrounding the coordinates of
2M1747 and does not show a discernible pointlike object at its location; thus
the above $J$ magnitude is highly suspect. 2M1747 is present in the 2MASS
$H$-band image, but also is in a crowded field with many nearby stars of
brightness similar to 2M1747, suggesting that its listed $H$ magnitude is also
affected by contamination. Only in the $K_{S}$-band image does 2M1747 stand
out clearly in the 2MASS and is photometry of it reliable.

The cause of the very red infrared colors of 2M1747 is a combination of the
star’s low photospheric temperature, high extinction, and thermal emission
from its warm circumstellar dust. Column densities of gas through the cloud
cores in Sgr B2 have been reported to be greater than $\sim$1 $\times 10^{25}$
cm-2 (Qin et al., 2011), which implies visual extinctions $\gtrsim$104 mag
assuming the standard Galactic gas-to-dust ratio (Bohlin et al, 1978), and
even higher if the metallicity in the Galactic center is supersolar, as is
suspected (see, e.g. Section A.2 of Oka et al., 2019). Sgr B1 is believed to
be more evolved and its gas more dispersed than Sgr B2. Nevertheless, the far-
infrared mapping of Lis et al. (2001, their figure axes are in 1950
coordinates) indicates that the extinction through the region of Sgr B1 where
2M1747 is located (1950 coordinates 17:43:59.01, $-$28:28:53.1), corresponding
in their Figs. $1-3$ to locations near the top left of the frames containing
positions $e$ and $f$, is only an order of magnitude less, and thus is still
formidable. The nearest compact and dense clumps of gas found in the 1.1 mm
continuum emission survey of Bally et al. (2010), their Clumps 190 and 193,
with estimated visual extinctions exceeding 103 mag, are 3 arcminutes distant.
Even though 2M1747 is not within or behind a dense molecular core, clearly it
cannot be located behind or even near the center of the more extended Sgr B1
dense gas observed by Lis et al. (2001) or it would not be so bright in the
$K$ band. Thus although deeply embedded in Sgr B1, it must reside in front of
most of the dust associated with Sgr B1 on that sightline, but not behind any
dense cores in Sgr B1.

Figure 1 is a simplified view of the line of sight toward 2M1747 showing the
various interstellar environments that might contribute absorption by
molecules in the gas phase and by solid-state matter, including both the dust
grains themeselves and ices on them. In the figure the CMZ is depicted as
described by Oka et al. (2019), with warm diffuse gas of mean density $\sim$50
cm-3 expanding radially from a location or region near the center of the CMZ
and taking up most of the volume of the CMZ. Several of the best known dense
cloud complexes within the CMZ are shown, as are representative dense cores
within them. A representative foreground spiral arm is also shown, containing
diffuse and dense clouds at lower temperatures than those in the CMZ.

The coordinates of 2M1747 are coincident to within a fraction of an arcsecond
with OH 0.548-0.059, an OH/IR star (Lindqvist et al., 1992). In addition to
the OH maser emission, Shiki & Deguchi (1997) found SiO maser emission in this
source, which Deguchi at al. (1997) identified with an infrared point source
whose $H$ and $K$ magnitudes are roughly those listed by 2MASS (albeit a
factor of 2 brighter at $H$). Sjouwerman et al. (2002) detected H2O maser
emission at a location 5 $\pm 5$ arcseconds distant and concluded it was
associated with the OH/IR star. All of the above authors identified 2M1747 as
a late-type star. Some of those authors have suggested, based on its infrared
brightness and intense SiO maser emission, that this star is in the
foreground, well outside of the Galactic center. The data presented here and
by Geballe & Oka (2010), Goto et al. (2011), and Oka et al. (2019) make it
clear that this is not the case.

This paper presents infrared spectra of 2M1747 in several spectral segments
within the $2-5~{}\mu$m wavelength range. The spectra were obtained at three
telescopes and at a variety of resolutions. Section 3 is a review of
previously published spectroscopy of 2M1747 and what it reveals about the
nature of the star and about the properties of the interstellar gas that
produces absorption in lines of H${}_{3}^{+}$ and CO. Section 4 presents and
describes more recently acquired spectra in the following order: the $K$ band,
which pertains mainly to the nature of the star; the $M$ band, which contain
features due to both interstellar gas and dust; and the $L$ band, which is
dominated by absorption features due to dust. Section 5 contains rough
calculations of the extinction to 2M1747 and the luminosity of the star, and
how these derived values interact with one another, together with discussions
of the likely locations on the line of sight to 2M1747 of the dust grains that
produce several of the solid-state absorption features in the $L$ and $M$
bands.

Figure 2: Left panel: velocity-resolved spectra of three key lines of H${}_{3}^{+}$: $R$(1,1)l, $R$(2,2)l, and $R$(3,3)l at at 3.7155 $\mu$m, at 3.6205 $\mu$m, and 3.5337 $\mu$m, respectively. Right panel: four lowest $R$-branch lines, $R$(0) - $R$(3), of the overtone vibration- rotation band of 12C16O (right) at 2.3453, 2.3433, 2.3413, and 2.3393 $\mu$m respectively. Wavelengths are in vacuo. The continuum level for each spectrum is denoted by a dotted line. The wavelength of the (marginally detected) $2-0$ band head of 13C16O is indicated by the arrow. Noise can be estimated by the fluctuations at both edges of each H${}_{3}^{+}$ spectrum and at the left edges of the spectra of the CO lines. Table 1: New Spectroscopy of 2MASS $J$17470898$-$2829561 Date | Program | Telescope | Instr | Exposure | Slit Width | Wavelength | R | Tell. Std. | Airmass
---|---|---|---|---|---|---|---|---|---
|  |  |  | sec | arcsec |  $\mu$m |  $\lambda$/$\Delta\lambda$ |  | 2M1747, tell. std
20100517 | 2010A-8 | IRTF | SpeX | 960 | 0.80 | 2.86-5.30 | 900 | HIP 84848 | 1.51, 1.57
20100722 | 2010A-C-3 | Gemini S | Phoenix | 2160 | 0.34 | 3.613-3.625 | 50,000 | HR 6879 | 1.25, 1.26
20100726 | 2010A-C-3 | Gemini S | Phoenix | 480 | 0.34 | 4.637-4.660 | 50,000 | HR 6165 | 1.04, 1.10
20160909 | 2016B-FT-12 | Gemini N | GNIRS | 800 | 0.45 | 2.19-2.37 | 4,000 | HIP 89622 | 1.66, 1.65
20160909 | 2016B-FT-12 | Gemini N | GNIRS | 480 | 0.20 | 3.03-4.02 | 950 | HIP 89622 | 1.52, 1.59

##  3 Previous Spectroscopy

Geballe & Oka (2010) found that 2M1747 has a steeply rising $K$-band
continuum, with multiple weak, CO first overtone absorption bands. These imply
intrinsically strong photospheric absorption that is being heavily veiled by
continuum emission from warm circumstellar dust. Their spectrum also contained
a prominent dip centered at the wavelength of the CO $2-0$ band center, which
encompasses lines arising from rotational levels $J\leq 3$. The intrinsically
strong CO absorption bands indicated that 2M1747 is a red giant or supergiant,
while the the dip at CO $2-0$ band center indicated that much cooler CO is
also present in foreground interstellar gas on the line of sight. The heavy
veiling by warm dust makes the source suitable for spectroscopy of
H${}_{3}^{+}$ at longer wavelengths (see Section 5.1) where the dust continuum
is much more dominant. In the course of the Geballe et al. (2019) $K$-band
survey spectra were obtained of several other bright and highly reddened stars
with sightlines close to that of 2M1747 and thus in the general direction of
Sgr B. All of them were found to be red giants with little or no veiling of
their heavily reddened photospheric spectra. Because of their prominent and
extensive photospheric absorption features, they were judged unsuitable for
study of the relatively weak lines of H${}_{3}^{+}$. At present, 2M1747 is the
only known star straightforwardly usable for spectroscopy of interstellar
H${}_{3}^{+}$ and CO along a line of sight toward and extending into the Sgr B
molecular clouds.

Figure 2 shows high-resolution spectra of three vibration-rotation lines of
H${}_{3}^{+}$ and four low-lying CO $R$-branch lines. With the exception of
the $R$(2,2)l line of H${}_{3}^{+}$, which is the average of an unpublished
spectrum (see Table 1) and a spectrum in Goto et al. (2011), these spectra
previously appeared in Geballe & Oka (2010) and in Oka et al. (2019). The
H${}_{3}^{+}$ $R$(1,1)l and $R$(3,3)l lines and each of the CO lines reveal a
multitude of absorption components extending continuously over a velocity
range of $\sim$200 km s-1. The similar strengths of the $R$(1,1)l and
$R$(3,3)l absorptions over that wide range of velocities imply that almost all
of the absorbing gas is at temperatures of $\sim$200 K, while the simultaneous
presence from the highest positive velocities to low negative velocities of
prominent absorption in the $R$(2,2)l line indicates that much of the gas at
those velocities is dense. As demonstrated later in this paper, the wam dense
positive-velocity gas is virtually entirely located within Sgr B1. The
significance of these H${}_{3}^{+}$ lines as diagnostics of gas density and
temperature has been presented in several earlier papers (e.g. Oka et al.,
2005, 2019; Oka & Geballe, 2020; Miller et al., 2020).

Two exceptions to the absorbing gas being warm are (1) the prominent sharp
absorptions centered at $-$43 km s-1, present in the $R$(1,1)l line and in the
three lowest-lying CO overtone band lines, and (2) the sharp but weaker
absorptions at $\sim$$-$25 km s-1 in the same lines. These sharp absorption
components are absent from the $R$(3,3)l line, indicating a much lower
temperature for much of the gas at those velocities. The velocity of $-$43 km
s-1 is characteristic of the cold dense gas in the foreground 3 kpc spiral
arm, which at that location produces strong absorption in the 6 cm line of
H2CO at the same radial velocity (Mehringer et al., 1995). The absorptions at
$\sim-$25 km s-1 are probably due to gas in the 4.5 kpc arm and can be seen on
many other sightlines toward stars in the CMZ (Oka et al., 2019). CO in dense
gas associated with the Local Arm may contribute in part to the absorptions
near 0 km s-1 in these lines, but does not produce sharp features, as have
been found, e.g., by Oka et al. (2019) on some CMZ sightlines.

Absorption features from the 3 kpc and 4.5 kpc arms are absent in the CO $2-0$
$R$(3) line (Figure 2, right panel, bottom trace), presumably because in
addition to the low kinetic temperatures of the clouds in these arms, the gas
densities of the clouds are not high enough to maintain LTE populations in the
$J$ = 3 level. For CO the critical density against radiative decay from the
$v$ = 0, $J$ = 3 level is $\sim 2\times$ 104 cm-3 (Yang et al., 2010).

At negative velocities the presence of prominent absorption in both the
$R$(1,1)l and $R$(3,3)l lines, coupled with the absence of absorption in the
$R$(2,2)l line indicates that at those radial velocities the sightline
contains warm and diffuse gas. Clearly this gas is part of the diffuse gas
expanding radially from the center of the CMZ observed on many other
sightlines stretching across the CMZ (Oka & Geballe, 2020).

##  4 New Observations and Results

Since obtaining the spectra shown in Figure 2, we have acquired additional
infrared spectra of 2M1747 in several key wavelength intervals using infrared
spectrographs at three telescopes. The observations are summarized in Table 1.
In 2010 we used SpeX at the NASA Infrared Telescope Facility to obtain a
2.9-5.3 $\mu$m spectrum at a resolving power, R, of 2500. That year we also
acquired a high-resolution spectrum of several of the low-lying fundamental
band lines of CO and of the $R$(2,2)l of H${}_{3}^{+}$ at 3.6205 $\mu$m (see
Figure 2), using Phoenix at Gemini South. In 2016 we used Gemini North and its
facility near-infrared spectrograph, GNIRS, to remeasure the spectrum of a
portion of the $K$ band, at much higher resolution than the 2008 spectrum. In
addition, we acquired a medium resolution $3-4$ $\mu$m spectrum using GNIRS.
In all cases the spectra were obtained in the standard ABBA manner with nods
along the slit of a few arcseconds between the A and B positions.

Data reductions, employing IRAF and Figaro packages were also standard. Each
reduction included the combining of individual spectral frames, flat-fielding,
spectrum extraction, spike removal, cross-correlating and shifting the B
spectra to align them in wavelength with the A spectra prior to combining
them, wavelength calibration using an arc lamp or telluric lines, cross-
correlating the source spectrum with that of the standard star, shifting the
source spectrum to align with that of the standard star, and finally ratioing
and conversion of the ratioed spectra to flux densities. For the $L$\- and
$M$-band spectra, hydrogen Pfund series absorption lines in the spectrum of
the standard star were removed by interpolating across them prior to ratioing.
Because of variable sky conditions during the observations and the use of
narrow slits, the accuracy of the absolute flux scale, where shown, is $\pm
30$%.

Figure 3: Spectra of 2M1747 in a long wavelength portion of the $K$ band
obtained at Gemini in 2016. The wavelengths of the first three band heads of
12C16O and the narrow wavelength interval containing lines of the $2-0$ band
from low lying rotational levels are indicated.

###  4.1 $K$-band Spectroscopy

Figure 3 displays the medium-resolution $K$-band spectrum of 2M1747. In
comparison to the five times lower resolution spectrum in Geballe & Oka
(2010), which was obtained in 2008, much more detail is seen and in
particular, many of the lines of CO are now resolved from one another. After
accounting for the difference in spectral resolution, the depths of the CO
bands are approximately 1.5 times greater in the 2016 spectrum, indicating
either decreased veiling or a change in the surface gravity of the stellar
photosphere.

The increased strengths of individual absorption lines on either side of the
CO $2-0$ band center at 2.3473 $\mu$m relative to neighboring lines is
apparent in the Figure 3. It is particularly obvious on the short wavelength
($R$-branch) side of the band, which is as expected due to the greater
intrinsic strengths of the low-$J$ lines relative to those on the $P$-branch.
In the hot and dense photosphere of the star the CO lines from low-lying
rotational levels weaken with decreasing $J$. In contrast, as pointed out in
the previous section, only the lowest few rotational levels in the ground
vibrational state are populated at low temperature and low density. One may
therefore conclude from the spectrum that CO in gas at much lower temperature
than gas in the photosphere of the star is the major contributor to the
low-$J$ lines. This is also readily apparent from the CO spectra in Figure 2.

The photospheric $2-0$ band head of 13C16O at 2.3448 $\mu$m is not evident in
Figure 3, although there is an indication of its presence in the higher-
resolution spectrum in Figure 2 (right panel, top trace). The resolution of
the spectrum in Figure 3 is too low to derive an astrophysically significant
value of or lower limit to the ratio 12C/13C. See Section 4.2 for somewhat
stronger constraints on the value of 12C/13C.

The low-resolution $K$-band spectrum in Geballe & Oka (2010) contains a broad
absorption feature at $2.26-2.27$ $\mu$m. In the 2016 spectrum (Figure 3) an
absorption is also present, but it is much weaker and can be attributed to a
triplet of photospheric Ca lines. Other weak absorption features that are
present in the 2016 spectrum also are attributable to atomic lines commonly
seen in late-type stars (e.g., see Wallace & Hinkle, 1997). Examination of the
data from 2008 indicates that the $2.26-2.27$ $\mu$m feature there is probably
spurious, and is due at least in part to partially corrupted files in the raw
data files.

Figure 4: Spectrum of 2M1747 in part of the $M$ band, observed by IRTF/SpeX at
R=900. Rotation-vibration lines in the $P$ and $R$ branches of the fundamental
bands of 12C16O and 13C16O are indicated, as are the locations of the solid-
state “XCN” and CO bands. The noise in the spectrum can be judged by the
fluctuations at wavelengths less than 4.58 $\mu$m and in the interval
4.71-4.74 $\mu$m.

###  4.2 $M$-band Spectroscopy: Low Resolution

The R=900 spectrum of 2M1747 in the short wavelength half of the $M$ band is
shown in Figure 4. The spectrum is dominated by fundamental band lines of CO.
It resembles the medium-resolution spectra taken by the Short Wavelength
Spectrometer on the Infrared Space Observatory (ISO SWS) towards stars in the
Central Cluster surrounding Sgr A* (Lutz et al., 1996; Moneti et al., 2001,
see their Fig. 1) and the spectra of individual stars in that cluster obtained
by Moultaka et al. (2009). The major differences are the greater strengths of
the CO absorption lines toward 2M1747 and the absence toward 2M1747 of the H I
Pf$\beta$ and Hu$\epsilon$ lines, which are emitted by ionized gas in the H II
region excited by the hot stars in the Central Cluster.

The fundamental band $P$\- and $R$-branch lines of 12C16O toward 2M1747 are
clearly detected in Figure 4 as individual spectral features out to $P$(4) and
$R$(5) and produce significant absorption somewhat beyond those $J$ values,
while lines of 13C16O are detected up to $R$(2) and $P$(2). The absence of
absorption from higher rotational levels implies that, as in the case of the
low-$J$ lines of the overtone band, the absorption lines arise in interstellar
gas rather than in the photosphere of 2M1747. Because of the strong dust
continuum of 2M1747, evident from both photometry and spectra, and the
presence of such deep fundamental band lines of interstellar CO, photospheric
CO absorption in 2M1747, although detected at low resolution in the $K$ band,
cannot be seen in the $M$ band in Figure 4.

Even at R=900 (corresponding to a velocity-resolution of 330 km s-1), the
fundamental band absorption lines observed toward 2M1747 are remarkably deep.
Their equivalent widths are typically a few times greater than those observed
by ISO SWS toward Sgr A* (Moneti et al., 2001) or toward individual sources in
the central parsec (Moultaka et al., 2009). As can be seen in Figure 5 the
fundamental band lines from low rotational levels are strongly saturated, and
thus the several times greater equivalent widths of the CO fundamental band
lines toward 2M1747 correspond to a much larger difference between the CO
column densities on the sightlines toward 2M1747 and Sgr A*. This indicates
that toward 2M1747 much of the CO absorption toward 2M1747 occurs within the
CMZ, a conclusion also reached in Section 2 based only on the high-resolution
spectra of low-$J$ CO overtone band lines. The strengths of the detected
13C16O lines are typically one-third those of the corresponding 12C16O lines.
Because the measured value of 12C13C in Sgr B is 24 $\pm$ 7 (Halfen et al.,
2017), this also implies on its own that large portions of the profiles of the
low-$J$ 12C16O lines are saturated.

Also apparent in Figure 4 is the presence of two well-known solid-state
absorptions. The solid CO band centered near 4.674 $\mu$m is responsible for
the absorption in the 12C16O $P$(1) and $P$(2) lines appearing deeper relative
to the low-$J$ $R$-branch lines than they do for 13C16O. The broad convex
shape of the pseudo-continuum centered at 4.62 $\mu$m is due to the “XCN”
band, often attributed to the OCN- anion (Demyk et al., 1998, but see also
Pontoppidan et al. (2003)), found within many dense clouds in the Galactic
plane (see, e.g. Pendleton et al., 1999, and references therein). For a
general review of these and other infrared solid-state absorption features see
Boogert et al. (2015).

Figure 5: High-resolution spectrum covering the lowest three lines of the
$R$-branch of the fundamental band of 12C16O, observed by Phoenix at Gemini
South. Figure 6: Velocity profile of the $2-1R$(8) line of 12C16O at 4.652
$\mu$m. The dotted line indicates the sloping pseudo-continuum, which is
affected by the wing of the strong 1$-$0 $R$(1) absorption line of 12C16O (see
Figure 5).

###  4.3 $M$-band Spectroscopy: High Resolution

The high-resolution spectrum in Figure 5 provides more detailed information
about the CO fundamental band lines. The spectrum covers the wavelengths of
the three lowest $R$-branch lines of the 12C16O fundamental, whose absorptions
are impressively strong and broad and have similar complex profiles. Five
emission lines are also present in the spectrum. Three of them are in the
$v=2-1$ band of 12C16O, one of which, the $R$(10) line, is only partially in
the observed spectrum at its short wavelength edge. The fourth emission
feature is a $1-0$ band transition of 13C16O. The wavelength of the fifth and
much weaker emission line, at 4.6392 $\mu$m, matches the wavelength of the
$1-0$ $R$(18) line of 12C18O. However, the presences of the $R$(17) (4.6459
$\mu$m) and $R$(16) (4.6526 $\mu$m) lines of this isotopomer, which should be
similar in strength to the $R$(18) line, are less clear and thus we regard the
detection of 12C18O as marginal.

Although the strong absorption lines are due to interstellar CO, the emission
lines, which originate in excited vibrational states, must be produced in hot
gas associated with the star but outside of its photosphere, as discussed
below. Lines in the $v=1-0$ bands of 12C16O and 13C16O in this narrow interval
normally would be present in emission along with the ones that are observed,
but their wavelengths coincide with and are masked by the deep and broad
interstellar 12C16O absorption lines that dominate the spectrum. The same is
true of lines of the $v=2-1$ band of 13C16O.

The relative strengths of the emission lines of the 12C16O and 13C16O
isotopomers are similar. Even though the 12C16O emission lines orginate in the
$v=2$ level and the 13C16O line is from $v=1$, their similar strengths still
indicate that the isotopic ratio 12C/13C is low, which is consistent with
2M1747 being evolved.

Close examination of the $v=2-1$ lines of 12C16O and the possible 12C18O $1-0$
$R$(18) line reveals that they have P Cygni profiles, with weak absorptions
that are blueshifted relative to the much stronger emissions. These profiles
indicate the presence of an outflow from the star. The absorption component in
the $1-0~{}R$(16) line of 13C16O near 4.643 $\mu$m, if present, is masked by
the strong interstellar absorption of the 12C16O $1-0$ $R$(2) line. However,
the full P Cygni profiles of the $2-1$ $R$(8) and $R$(9) lines of 12C16O are
unobscured. The former, shown in detail in Figure 6, extends from $v_{LSR}$
$\gtrsim$ 0 km s-1 to $-$60 km s-1, suggesting that the stellar radial
velocity is approximately $-$30 km s-1 (LSR). This is consistent with the
findings of Lindqvist et al. (1992), who determined a stellar radial velocity
of $-$32 km s-1 (LSR) and an expansion velocity of 32 km s-1 for the OH line-
emitting shell.

The negative radial velocity of 2M1747 is somewhat surprising in view of the
positive radial velocities of the Sgr B1 cloud complex. One can only speculate
as to the explanation, because little or nothing is known about the motions of
other stars within the complex. One possibility is that this highly unusual
star was ejected in a direction toward the front of Sgr B1 from a more
embedded cluster of massive stars and thereby acquired a non-representative
radial velocity.

###  4.4 Excitation of the Infrared Emission Lines

We now discuss the excitation of the CO $v$ = 1 and 2 rovibrational levels
that are observed to produce the emission lines in Figures 5 and 6. The $v$ =
$1-0$ emission line of 13C16O is from $J$ = 16 and judging from its strength
$v$ = $1-0$ line emission extends at least to somewhat higher $J$. The $v$ =
0, $J$ = 16 level is 750 K above ground. Significant thermally excited
populations of $v$ = 0, $J$ $\sim$ 16 levels require densities $n$ $\gtrsim$
106 cm-3 (Yang et al., 2010) and temperatures of at least several hundred
kelvin. Both appear to be well within the range of values for much of the gas
in the circumstellar envelope of an OH/IR star (see Figs. 2 and 4 of Goldreich
& Scoville, 1976). Although this suggests that resonant scattering of infrared
photons off of CO molecules in the ground vibrational state could contribute
to the $1-0$ line emission, a much stronger blueshifted absorption feature
would likely be present if that were the dominant cause of the emission. We
conclude that this mechanism probably is only a minor contributor to the
observed P Cygni line profiles.

Collisional excitation of $v$ = 0 levels to the observed $v$ = 1 levels, which
are more than 3,000 K above the ground vibrational state, requires
considerably higher temperatures than several hundred kelvin. Such
temperatures, perhaps up to $\sim$2,000 K, are also expected to be present in
circumstellar gas closest to the stellar photosphere (Goldreich & Scoville,
1976). We tentatively conclude that this is the cause of most of the emission
seen in the $v=1-0$ lines.

The 12C16O $v$ = 2 rovibrational levels that produce the $v$ = $2-1$ lines in
Figures 5 and 6 could be excited radiatively from $v$ = 0 and 1 levels by
resonant absorption of 2.3 $\mu$m and 4.6 $\mu$m photons, respectively. If 4.6
$\mu$m absorption dominated, one would again expect to see P Cygni profiles
with deep absorptions in the $2-1$ lines, which are not observed. However,
absorption by 2.3 $\mu$m photons, produced copiously in the stellar
photosphere and the hot innermost part of the circumstellar envelope, could be
masked by the complex CO overtone band absorption profiles in Figure 2. In
addition, those absorptions do not result in resonant emission in $\Delta$$v$
= 2 lines since the radiative decay of $v=2$ levels proceeds almost entirely
via the much faster $\Delta$$v$ = 1 transitions. Thus radiative excitation of
the $v=2$ levels by 2.3 $\mu$m photons appears to be a viable mechanism for
producing $v=2-1$ emission.

One also must consider populating CO $v$ = 2 levels from $v$ = 1 by
collisions. That requires a jump in energy of $\sim$3,000 K, similar to the
energy gap between $v$ = 0 and $v$ = 1, for any value of $J$. For this process
to contribute significantly to the $2-1$ line emission, densities $\gtrsim$
1010 cm-3 are needed to maintain CO in the $v$ = 1 states against radiative
relaxation to $v$ = 0. For OH/IR stars with high mass loss rates, those
required values of temperature and density are expected to exist in
circumstellar gas close to the photosphere (Goldreich & Scoville, 1976). We
conclude that resonant scattering of 2.3 $\mu$m photons and/or collisional
excitation, both operating in the hot and dense inner part of the envelope,
could lead to the observed emission in the $v$ = $2-1$ lines. Observations and
analysis of a wider range of rovibrational lines in 2M1747 could constrain the
relative contributions of these processes.

Finally, we note that excited vibrational levels of CO also can be radiatively
populated as a result of absorption by CO of ultraviolet (UV) radiation
followed by vibrational cascades, as proposed by Brown et al. (2013) for
protoplanetary disks. Because 2M1747 is a cool giant, however, UV excitation
is an unlikely mechanism here.

###  4.5 Detailed examination of the profiles of H${}_{3}^{+}$ and CO infrared
and millimeter lines

Figure 7 shows the velocity profile of the CO fundamental band $R$(1)
absorption line (the central strong absorptions in Figure 5). Also in the
figure and plotted on the same normalized scale for comparison are the $R$(1)
line of the 130 times less optically thick first overtone band and the
$R$(1,1)l line of H${}_{3}^{+}$ (magnified by a factor of 4). In addition, the
figure includes at top the velocity profile of the 2.6 mm $J$ = 1$-$0 line of
12C16O at a position 14″distant from 2M1747, observed in the Oka et al. (1998)
GC survey at the 45 m telescope of Nobeyama Radio Observatory, which used a
17″diameter beam (0.66 pc at the distance of the GC) and a 34″grid spacing.

The spectrum of the pure rotational CO line is dominated by emission at
positive velocities, which are as high as + 110 km s-1. The wide range of
velocities is characteristic of the Sgr B1 and Sgr B2 cloud complexes
(Scoville et al., 1975). The velocity profile of the line is surely affected
by radiative transfer effects; thus a detailed comparison of it with the
profiles of the infrared absorption lines is probably not meaningful, apart
from noting that most of the absorption equivalent width in each of the latter
is in the same range of positive velocities as the emission in the pure
rotational line.

Figure 7: Top trace: spectrum of the CO $J$ = 1$-$0 pure rotational line at
the closest position to 2M1747 obtained by Oka et al. (1998) and kindly made
available by Tomoharu Oka. The zero level is indicated by the upper dashed
line. Lower three traces: normalized spectra of the $R$(1) lines of the
fundamental and first overtone band of CO (thick and thin lines, respectively)
and the $R$(1,1)l line of H${}_{3}^{+}$ (dotted line, magnified by a factor of
4).

We now look in detail at the infrared line profiles in Figure 7. One would
expect that at wavelengths where the depth of the absorption in the CO $2-0$
$R$(1) line is more than several percent the absorption in the $1-0$ $R$(1)
line would be nearly total. This is indeed the case apart from a narrow
velocity interval centered at $-20$ km s-1. The two infrared CO lines
illustrate the difficulty in determining cloud type and temperature in the GC
from studies based largely on spectroscopy of the strong fundamental band
lines (e.g. Geballe et al., 1989; Moultaka et al., 2019). This is especially
clear at high negative velocities where there is strong absorption in the
$1-0$ $R$(1) line, but where the $2-0$ $R$(1) line is very weak and the
relative strengths of the H${}_{3}^{+}$ lines in Figure 2 show that the
absorbing gas at those velocities is warm and diffuse.

The absorption in the CO $1-0$ $R$(1) line extends from $-130$ km s-1 to
$+110$ km s-1. Its profile contains three ranges of radial velocities within
which the optical depth is 3 or higher: 0 km s-1 $<v<+90$ km s-1, $-60$ km s-1
$<v<-40$ km s-1, and $-105$ km s-1 $<v<-90$ km s-1. The first of these (at
positive velocities) is the same range as appears at this position in the
millimeter-wave CO spectrum in Figure 7. The strong absorptions in the
H${}_{3}^{+}$ $R$(2,2)l and $R$(3,3)l lines in this velocity range (Figure 2),
show that this gas is warm and mainly dense.

The deep CO $1-0$ $R$(1) absorption in the middle velocity range ($-60$ km s-1
$<v<$ $-40$ km s-1 ) is due in part to absorption outside of the CMZ by cold
dense gas in the foreground 3 kpc arm (Oka et al., 2019), which is the most
prominent absorption component in the both the overtone line of CO and the
H${}_{3}^{+}$ line. Note, however, that warm diffuse gas must also be present
in this middle velocity range, because absorption in the $R$(3,3)l line of
H${}_{3}^{+}$ is present while absorption in the $R$(2,2)l line is absent (see
Figure 2). Thus absorption by the CO fundamental band line in this velocity
range is a superposition of warm diffuse blueshifted gas, within the CMZ (see
Section 3) and cold dense gas in the 3 kpc arm.

Somewhat similarly, the absorption in the CO $1-0$ $R$(1) line at velocities
between $-30$ and 0 km s-1 contains contributions from cold dense gas in the
4.5 kpc arm and possibly also in the Local arm as was discussed in Section 3,
as well as dense and warm gas associated with Sgr B1. CO in the warm and
diffuse gas of the CMZ may also contribute absorption in the fundamental band
line at these velocities. Warm diffuse gas in this velocity range in known to
exist on many sightlines across the CMZ (Oka & Geballe, 2020). On this
sightline, however, it cannot be distinguished from the other contributors.

Finally, we conclude that the CO absorption observed at velocities more
negative than $\sim-60$ km s-1 also arises mainly in warm diffuse gas, not
only because in that velocity range absorption in the H${}_{3}^{+}$ $R$(3,3)l
line is present but absorption in the H${}_{3}^{+}$ $R$(2,2)l line is absent,
as was mentioned in Section 3, but also because absorption in the CO overtone
lines is relatively weak. Thus, blueshifted warm and diffuse gas extends from
near 0 km s-1 to $-130$ km s-1. It is part of the warm diffuse gas observed by
Oka et al. (2019) and Oka & Geballe (2020) toward numerous stars in the CMZ,
attributed by them to filling the majority of the volume of the CMZ and
undergoing radial expansion from a location near the center of the CMZ. It
includes the gas at the highest velocities, part of the “Expanding Molecular
Ring” (EMR) at the outer edge of the CMZ, first observed at radio wavelengths
by Kaifu et al. (1972) and Scoville (1972) and analyzed in detail by Sofue
(1995) and Sofue (2017).

###  4.6 $L$-band Spectroscopy

Figure 8 shows the $3-4$ $\mu$m spectrum of 2M1747, observed at R = 900. The
spectrum is an adjoining of the $2.86-3.04$ $\mu$m portion of the SpeX
spectrum and the full GNIRS spectrum (see Table 1). The figure also contains
an approximate optical depth spectrum (lower trace), which was generated using
a continuum based on a spline interpolation of the spectrum as described
below.

To create the spline we assumed that the observed data points near 3.60, 3.70,
3.78, and 4.01 $\mu$m represent the continuum. This should be a good
approximation because no known spectral features occur at those wavelengths
and because the slope of the continuum through these data points changes only
slightly between 3.60 $\mu$m and 4.01 $\mu$m. At wavelengths less than 3.60
$\mu$m several overlapping solid-state bands cover the entire observed range.
Based on the analysis of spectra toward seven stars in the Galactic center
Chiar et al. (2002) derived a mean ice-band optical depth spectrum (see their
Fig. 5). We have used that spectrum to estimate the continuum level in the
2M1747 spectrum at 3.34 $\mu$m, which corresponds to a local maximum in the
observed flux density and which is a wavelength where the wing of the ice band
is the only solid-state feature contributing significant absorption. At that
wavelength the optical depth of the mean spectrum is 0.4 times the optical
depth at the bottom of the ice band. Finally, to estimate the continuum level
at the short wavelength edge of the spectrum at 2.86 $\mu$m, which is on the
short wavelength shoulder of the ice feature, we have made use of the Lutz et
al. (1996) $2.4-45~{}\mu$m spectrum toward Sgr A*, which includes continuum at
wavelengths less than the short wavelength edge of the above absorption
feature. Based on it we estimate that the continuum level at 2.86 $\mu$m is
12% higher than the observed flux density.

Figure 8: Upper trace: $3-4\mu$m spectrum of 2M1747. Identifications and
wavelengths of lines of H${}_{3}^{+}$ are shown. Lower trace: optical depth
spectrum, with scale shown at right edge. The locations of the O-H stretching
band and C-H stretching bands are indicated.

The spectrum in Figure 8 contains six weak but clearly detected narrow
absorptions due to individual or blended lines of H${}_{3}^{+}$. A seventh
feature, due to the $R$(2,2)l line at 3.6205 $\mu$m, is only marginally
detected, but is easily seen in Figure 2. Over this rather wide wavelength
range and at this rather low spectral resolution the only interstellar gas-
phase lines detected toward this highly obscured source are those of
H${}_{3}^{+}$.

Figure 9: Comparison of the optical depth spectra of the 3.4 $\mu$m absorption
features toward 2M1747 (this paper) and the Quintuplet Cluster (a weighted
average of the spectra in Chiar et al. (2013), binned to the resolution of the
Gemini spectrum). The optical depth of the wing of the H2O ice band, which is
superimposed on the 3.4 $\mu$m feature, has been subtracted. The broad and
shallow 3.25 $\mu$m feature and three characteristic absorption peaks of the
3.4 $\mu$m feature at 3.38, 3.42, and 3.48 $\mu$m are clearly present. The
narrow absorption at 3.53 $\mu$m in the spectrum of 2M1747 is a line of
H${}_{3}^{+}$ (see Figure 8).

The spectrum also contains several broad absorption bands, most or all of
which have been detected toward other objects in and outside of the GC. Their
properties are summarized in Table 2. The strongest absorption feature,
covering the $2.86-3.60~{}\mu$m range with a peak near 3.0 $\mu$m, has been
attributed largely to the OH stretching vibration in water ice (e.g. Sandford
et al., 1991; Chiar et al., 2000). In some sources in which the water ice is
believed to be fairly pure (e.g., Hodapp et al., 1988; Geballe et al., 1988;
Smith et al, 1989), as well as in models (Chiar et al., 2002) the absorption
maximum occurs at $3.05-3.08~{}\mu$m, with a fairly sharp decrease in the
absorption on either side. Toward 2M1747, however, as well as toward other
sources in the Galactic center, a broad absorption maximum extends shortward
to 2.97 $\mu$m. The additional contribution has been attributed to impurities
in the ice, in particular NH3 (e.g. Chiar et al., 2000).

The structured absorption extending from 3.35 $\mu$m to 3.60 $\mu$m,
superimposed on the long wavelength wing of the ice band and first observed
toward the Galactic center by Soifer et al. (1976), Jones et al. (1983), and
Butchart et al. (1986), is commonly referred to as the 3.4 $\mu$m feature. It
is known to be ubiquitous in the diffuse ISM, but is not detected in dense
molecular clouds. The absorption is generally acknowledged to be due to
asymmetric C$-$H stretching modes of CH3 and CH2 groups in saturated aliphatic
hydrocarbons (Sandford et al., 1991). The three absorption maxima, at 3.38,
3.42, and 3.48 $\mu$m in the spectrum of 2M1747, better seen in Figure 9, not
only are characteristic of its profile on lines of sight to other infrared
sources in the Galactic center, but are also present in spectra toward diffuse
clouds in the Galactic plane (Sandford et al., 1991; Pendleton et al., 1994).
The 3.4 $\mu$m feature also has been observed in the interstellar dust of
external galaxies and studied extensively in one of them (Dartois et al.,
2007, and references therein).

A third, broad and much shallower absorption in Figure 8 extends from 3.22
$\mu$m approximately to 3.30 $\mu$m, and can be seen in more detail in Figure
9. The absorption profile toward 2M1747 resembles that of the mean Quintuplet
spectrum of Chiar et al. (2013, the lower trace of their Fig. 4 is reproduced
in Figure 9). In particular the maximum absorption in the above spectral
interval, at 3.28 $\mu$m, matches a distinct local absorption maximum in their
spectra, which they identify as due to the C$-$H stretch in the olefinic
constituents of hydrocarbon grains. However, this vibrational mode is expected
to only produce a narrow absorption and cannot account for the entire width of
the observed feature.

A weak absorption first reported by Sellgren et al. (1995) toward the young
stellar object (YSO) Mon R2 IRS3, and subsequently observed toward several
additional YSOs (Brooke et al., 1996, 1999; Bregman & Temi, 2001; Hardegree-
Ullman et al., 2014) also covers the $3.2-3.3$ $\mu$m spectral interval. Both
Brooke et al. (1999) and Bregman & Temi (2001) suggest that it is due to the
C$-$H stretching band in mixtures of isolated aromatic hydrocarbon molecules
in cold dense clouds. A second possible identification, ammonium
(NH${}_{4}^{+}$) in icy grain mantles, has also been suggested (Schutte &
Khanna, 2003). The optical depth of the feature observed in these sources is
typically a few times greater than toward 2M1747 and the Quintuplet stars.
Although the signal-to-noise ratio of individual spectra reported by the above
observers is low, the absorption appears to be smooth, without a narrow peak
at 3.28 $\mu$m. Thus one must entertain the possibility that the observed
$3.22-3.30$ $\mu$m absorption observed toward objects in the GC is a
superposition of two features, possibly arising in different portions of the
line of sight.

Figure 10: Expanded view of normalized spectrum of 2M1747 in Figure 8 showing evidence for broad absorption features in the $3.8-4.0\mu$m interval. The dotted horizontal line is the assumed continuum level from Figure 8. Short vertical lines denote absorption lines of H${}_{3}^{+}$ identified in Figure 8. Table 2: Absorption Features of Dust Grains Toward 2M1747  Wavelength | Identity | optical depth | Equiv. Width | Locations
---|---|---|---|---
microns |  | at peak | microns |
$2.8-3.6$ | H2O $+$ impurities | 1.25 | 0.20 | Sgr B1 dense gas, spiral arm dense gas
$3.20-3.30$ | aromatic C$-$H | 0.03 | 0.0018 |
$3.35-3.60$ | aliphatic $-$CH2, $-$CH3 | 0.24 | 0.029 | spiral arm diffuse gas, CMZ diffuse gas (?)
$3.48-3.60$ | CH3OH |  $<$0.002 |  $<$0.0003 | (not detected)
$3.80-3.86$ | $?$ | 0.005 | 0.00027 $\pm$ 0.00005 |
$3.92-4.00$ | CH3SH $?$ | 0.015 | 0.0010 $\pm$ 0.0002 |
$4.585-4.65$ | XCN (OCN-$?$) | 0.2 | 0.012 $\pm$ 0.003 | Sgr B1 dense gas (?), spiral arm dense gas (?), CNR (?)
$4.66-4.70$ | CO | 0.3 | 0.007 $\pm$ 0.003 | spiral arm dense gas, Sgr B1 dense gas (?)

Two absorption bands are present in the $3.8-4.0~{}\mu$m interval and are more
easily seen in Figure 10 than Figure 8. A weak absorption appears to be
present from approximately 3.80 $\mu$m to 3.86 $\mu$m; we hereafter refer to
it as the 3.83 $\mu$m feature. A stronger absorption feature is clearly
present between 3.92 $\mu$m and 4.00 $\mu$m; we hereafter refer to it as the
3.96 $\mu$m feature. Overall the features in Figure 10 crudely resemble those
observed previously toward the only infrared sources toward which solid-state
features have been detected in the wavelength interval, the YSOs W 33A and
AFGL 7009S, which are embedded in dense and cold molecular clouds (Geballe et
al., 1985; Geballe, 1991; Dartois et al., 1999). Although detection of the
shorter-wavelength absorption feature in W33A was not reported by Geballe et
al. (1985) there is an indication of its presence upon close examination of
their spectrum. It is more clearly seen in the Dartois et al. (1999) spectra
of the above objects.

Geballe et al. (1985) tentatively identified the longer-wavelength feature in
the two YSOs as due to solid H2S. Later, however, based on laboratory
experiments Dartois et al. (1999) identified the absorptions as due to two
overlapping bands of methanol (CH3OH) ice centered near 3.85 $\mu$m and 3.95
$\mu$m. A stronger band of CH3OH ice centered at 3.54 $\mu$m with a full width
at zero intensity of 0.10 $\mu$m is also present toward both YSOs,
strengthening their case for identifying the longer-wavelength features as due
to frozen CH3OH. The ratios of the optical depths of the features in the two
YSOs are $\tau$(3.54)/$\tau$(3.93) $\sim$ 10 and $\tau$(3.54)/$\tau$(3.83)
$\sim$ 15.

Toward 2M1747, however, the 3.54 $\mu$m band is not detected (see Figure 9);
we estimate $\tau$(3.54)/$\tau$(3.96) $<$ 0.3 and $\tau$(3.54)/$\tau$(3.83)
$<$ 1\. These limits are more than an order of magnitude less than toward W33A
and AFGL 7009S. The large difference indicates that the neither of the longer-
wavelength absorptions toward 2M1747 is due to CH3OH ice. We have considered
the possibility that the longer-wavelength feature is due to H2S frozen on
dust grains, as had been suggested earlier. The S$-$H stretching mode of solid
H2S has long been known to produce a spectral feature near 3.92 $\mu$m.
Laboratory spectra of solid amorphous H2S by Hudson & Gerakines (2018) reveal
that the full width at half maximum (FWHM) of this feature is $\sim$ 0.06
$\mu$m. Although the width of the laboratory feature is similar to that
observed toward 2M1747, the central wavelengths do not match very well. In
dirty ice mixtures the wavelengths of vibrational stretching modes are often
shifted. For example, the well-known band of solid CO at 4.67 $\mu$m is
shifted to longer wavelengths by $\sim$ 0.01 $\mu$m when in a mantle dominated
by H2O ice. A considerably larger shift than this would be needed for the
identification of the feature toward 2M1747 as H2S to be viable. Inspection of
the laboratory studies of frozen H2S (Moore et al., 2007; Soulard & Tremblay,
2019) indicates that such a shift is unlikely.

Perhaps a more plausible identification for the carrier of the 3.96 $\mu$m
feature is methanethiol (CH3SH) ice. The absorption peak of the S$-$H stretch
in amorphous CH3SH ice is 3.945 $\mu$m (Hudson & Gerakines, 2018), a much
closer match to the observed band. It is necessary to determine the size and
direction of wavelength shifts of CH3SH ice in realistic interstellar ice
mantles in order to judge whether this is an astrochemically viable
identification. At present we have no proposed identification for the much
weaker 3.83 $\mu$m feature.

##  5 Analysis and Discussion

###  5.1 An Estimated Extinction to 2M1747

There are two lines of evidence indicating that the bulk of the extinction to
2M1747 arises in dense gas within the CMZ. One is based simply on the
H${}_{3}^{+}$ and CO line profiles in Figure 2, which show that at velocities
more positive than $\sim-20$ km s-1, where most of the CO absorption occurs
the gas in which the H${}_{3}^{+}$ and CO are present is warm, indicating that
it is located in the CMZ, and that it has a mean density well in excess of the
critical density of $\sim$200 cm-3 for the (2,2)l level of H${}_{3}^{+}$,
suggesting that it is mostly found in dense clouds. The second is the huge
column density of absorbing CO, $2.7\times 10^{19}$ cm-2 found by Oka et al.
(2019), derived from the optically thin CO overtone band lines in Figure 2,
and what that would imply for the extinction to 2M1747 if even a modest
fraction of it were associated with diffuse gas (see Section 2).

Based on our conclusions that (1) the column density of CO derived from the
overtone band is virtually entirely in dense gas and (2) the sharp CO
absorptions at $-43$ and $-25$ km s-1 are also due to dense gas (in the 3 kpc
and 4.5 kpc arms), we estimate that $\sim$90% of the column density of CO
observed toward 2M1747 ($2.4\times 10^{19}$ cm-2) is located in the CMZ. Using
the ratio of gaseous CO to visual extinction derived from measured strengths
of the interstellar H2 quadrupole $S$(0) absorption line and absorption lines
of the CO first overtone lines toward stars behind the Taurus Molecular Cloud,
Lacy et al. (2017) derived $N_{gas}$(CO)/ $A_{V}$ $\sim$ $1.5\times 10^{17}$
cm-2. Lacy et al. do not give an uncertainty in this value; we estimate it to
be $0.6\times 10^{17}$ cm-2 (1$\sigma$). We note that the Lacy et al. value is
somewhat higher than other reported and derived values (Pineda et al., 2010;
Lacy et al., 2017); however it is the only direct measurement of that ratio.
Using it the observed column density of gaseous CO toward 2M1747 implies a
visual extinction of $180\pm 72$ mag due to dust in dense clouds. Were a
significant fraction of the CO in the line of sight to 2M1747 located in
diffuse gas, where the gaseous CO is heavily depleted due to
photodissociation, the above formidable value would be larger by $1-2$ orders
of magnitude, and would lead to 2M1747 being unphysically luminous, as
discussed in Section 5.2. Note that due to the higher metallicity toward the
Galactic center (for a recent summary of previous measurements and an estimate
see Section A.2 of Oka et al., 2019), the value of $N$(H)/ $A_{V}$ there is
probably considerably lower than the more local value. However, to first order
$N$(CO)/$A_{V}$ should not vary too much, as the abundances of both dust and
CO are enhanced by the higher metallicity.

The contribution of extinction toward 2M1747 due to dust in diffuse gas on its
sightline can be estimated as follows. Whittet et al. (1997) concluded that
the visual extinction due to diffuse clouds in the direction of Sgr A* is
$\sim$20 mag. As the 3.4 $\mu$m absorption feature arises in diffuse gas, one
may assume that diffuse cloud extinction on sightlines toward objects within
the CMZ scales with the strength of this feature. Comparison of the depth of
the 3.4 $\mu$m feature toward 2M1747 with the depths toward the more central
GC sources observed by Sandford et al. (1991) and Pendleton et al. (1994) and
toward the Quintuplet Cluster (Figure 8), all of which have similar values,
indicates that the extinction toward 2M1747 due to dust in diffuse clouds is
about 25% greater than toward the other objects, and thus is $\sim$25 mag.

To determine the total extinction to 2M1747 one must add to the above two
values (extinction due to dust in diffuse gas and dust associated with gaseous
CO, 25 mag and 160 mag, respectively), the extinction due to dust associated
with the solid CO. As discussed in Section 4.2, the presence of solid CO is
evident in Figure 4, but the feature itself is partially masked by the strong
interstellar CO absorption lines. If the gas-phase lines were not present the
solid CO feature, whose central wavelength (Lacy et al., 1984; Kerr et al.,
1993) is nearly coincident with the $1-0$ $P$(1) line at 4.674 $\mu$m, but
which usually possesses a long wavelength shoulder due to CO mixed with polar
molecules such as H2O, would be much more prominent.

To estimate the column density of solid CO, $N$(COsolid) we use the formula
$N$(COsolid) = $\tau$ $\Delta\nu$ / A , where $\tau$ is the peak optical depth
of the solid CO feature, $\Delta\nu$ is the FWHM of its absorption band, and A
is the band absorbance per frozen CO molecule. We estimate $\tau$ = 0.3 from
Figure 4 and adopt 8 cm-1 and $1.1\times 10^{-17}$ cm-1 molecule-1,
respectively, from Texeira & Emerson (1999). Then $N$(COsolid) = $2\times
10^{17}$ cm-2, which is less than one percent of the total column density of
gaseous CO derived above. The carrier of XCN is probably present in the same
grains containing the solid CO. CO may also effectively be locked up in solid
CO2, which produces prominent absorptions at 4.27 $\mu$m and 15.2 $\mu$m in
the spectrum of the Galactic center (Lutz et al., 1996). The strengths of the
solid CO2 features on the sightline to 2M1747 are not known. However, the
column densities of solid CO2 found by Gerakines et al. (1999) toward sources
in the Galactic center are somewhat less than that of solid CO. In addition,
the ices containing solid CO2 are also probably the same ones that contain
solid CO.

It is thus clear that it is dust associated with the gaseous CO in molecular
clouds and dust in the diffuse interstellar medium that produce the vast
majority of the extinction. Based on all of the above considerations, we
estimate that visual extinction to 2M1747 is 205 mag, with an uncertainty of
75 mag stemming almost entirely from the uncertainty in $N_{gas}$(CO)/ $A_{V}$
in dense clouds observed by Lacy et al. (2017). In addition, as mentioned
earlier, the Lacy et al. value may not hold in the Galactic center, where the
overwhelming majority of the extinction to 2M1747 arises.

###  5.2 Absolute Brightness of 2M1747, the Optical - Infrared Extinction
Curve, and a Possible Revised Extinction

At wavelengths longward of the $H$ band the infrared brightness of 2M1747 is
dominated by emission from its circumstellar dust. Nevertheless, one can
crudely estimate what the apparent brightness of the unobscured star would be
in the $K$ band based on the depth of the $2.3-2.4$ $\mu$m photospheric CO
absorption in Figure 3. To do so we assume that the photosphere of 2M1747 is
that of a late -ype giant or supergiant, whose CO band head’s depths are
$\sim$50% of the continuum at a resolving power R$\sim$3,000 (Wallace &
Hinkle, 1997). That is five times the depth of the veiled CO $2-0$ band head
in 2M1747 when smoothed to the same resolving power, which implies that warm
dust contributes $\sim$80% of the continuum at 2.3 $\mu$m). This indicates
that $K\approx\ 12.2$ mag for an unveiled 2M1747.

To estimate the intrinsic brightness of 2M1747 we use the optical$-$mid-
infrared extinction law derived by Wang & Chen (2019), noting that the
infrared portion from 1.0 $\mu$m to 2.5 $\mu$m is similar to that of the near-
infrared extinction curves of Fritz et al. (2011) and Nagatomo et al. (2019).
For $A_{V}$ = 205 mag, $A_{K}$ = 14.8 mag, implying that the unveiled $K$
magnitude of 2M1747 corrected for extinction is $\sim-2.5$. Several OH/IR
supergiants, including VY CMa and VX Sgr are among the brightest M
supergiants, which have absolute $K$ magnitudes of $-11$ to $-13$ (e.g Chatys
et al., 2019), corresponding to apparent $K$ magnitudes of $1.5-3.5$ at the
Galactic center, roughly $40-250$ times fainter than an unobscured 2M1747
would be. It seems to us unreasonable that 2M1747 would be so much brighter
than them. Alternatively, if $A_{V}$ = 130 mag toward 2M1747 (a deviation of
1$\sigma$ from the estimate in Section 5.1, the unobscured $K$ magnitude of
2M1747 would be $\sim$2.8, comparable to the brightest OH/IR supergiants).

The above alternatives suggest three possible explanations. If the absolute
$K$ magnitude of 2M1747 is comparable to those of the brightest M supergiants
then the visual extinction to 2M1747 is $\sim$130 mag, $\sim$40% lower than
the original estimate, and the mean value of $N$(COgas/ $A_{V}$ derived by
Lacy et al. (2017) for the Taurus Molecular Cloud is $\sim$40% lower than the
value for the dense gas and dust within the CMZ. We already noted that their
value of this ratio is high compared to other recent estimates. Alternatively,
the recently derived extinction laws mentioned above, while appropriate for
dust in the Galactic plane along the line of sight to the GC, do not apply to
the dust in the dense gas within the CMZ, and in particular in Sgr B1. The
final possibility is that the visual extinction of 205 mag to 2M1747 and the
extinction law are both approximately correct and 2M1747 is considerably more
than an order of magnitude more luminous than any other OH/IR star. We regard
the third possibility as unlikely, but are unable to conclude whether one of
the first two possibilities is more likely than the other.

###  5.3 Locations of the Gas and Dust Producing the Solid-state Absorption
Bands

In this subsection we attempt to link several of the well-studied solid-state
absorption features observed toward 2M1747 to environments present on its line
of sight. Our conclusions regarding their locations are summarized in Table 2.
Because of the uncertainties regarding the identifications of the absorption
features in the $3.2-3.3$ $\mu$m and $3.8-4.0$ $\mu$m regions we do not
speculate on the locations where they arise. The reader is referred back to
Section 4.5 for discussion of the issues regarding their identifications.

Based in part on the spectroscopy reported here and by Geballe & Oka (2010)
and Oka et al. (2019) one can identify several environments shown in Figure 1
that potentially contribute to the solid-state absorptions.

  * •

Warm dense molecular gas in the Sgr B complex, where 2M1747 is located.
Typical gas temperatures outside of the dense cores in Sgr B are $70-200$ K
(Lis et al., 2001, this paper). Most of the extinction toward 2M1747 is due to
dust in bf this gas.

  * •

Warm diffuse molecular and atomic gas filling much of the CMZ and thus mostly
unassociated with dense molecular clouds there. The mean density of this gas
is $\sim$ 50 cm-3 and its mean temperature is $\sim$200 K (Oka et al., 2019)

  * •

Cold and dense molecular gas in the three intervening spiral arms, with $T$
$\sim$ 20 K (Serabyn & Güsten, 1986)

  * •

Cold diffuse molecular gas in the three intervening spiral arms, with $T$
$\sim$ 50 K (Snow & McCall, 2006)

As illustrated in Figure 1 dense molecular cores are also present, in Sgr B
(Bally et al., 2010) and in the spiral arms, but are not on the line of sight
to 2M1747. Low-density interarm gas makes up a majority of the length of the
sightline, but the dust it contains probably contributes little to the solid-
state absorption features.

The circumstellar dust associated with 2M1747, which produces most of the
continuum in the $K$-band (see Section 5.2) and virtually all of the continuum
in the $L$ and $M$ bands, can be easily ruled out as a site in which any of
the solid-state featured reported here arise. The characteristic temperature
of this dust, estimated from Spitzer photometry in the IRAC1 and IRAC3 filter
bands (given in Section 2; we do not use IRAC2, as its wavelength range is
coincident with the strong interstellar CO absorption) is $\sim$300 K, far too
warm for dust mantles to be present.

####  5.3.1 Solid CO and XCN

Of the solid-state absorption features observed toward 2M1747 and listed in
Table 2, two that have been found on grain mantles in many cold dense clouds
outside of the GC are the solid CO and XCN absorptions at $4.6-4.7$ $\mu$m.
These two bands have been observed not only toward 2M1747, but also in large-
aperture spectra of the Central Cluster (Moneti et al., 2001) and toward many
individual sources within that cluster (Moultaka et al., 2009). The strength
of solid CO absorption toward 2M1747 is similar to that toward the Central
Cluster in the large ISO SWS aperture (Moneti et al., 2001), whereas the XCN
band toward 2M1747 is roughly twice as strong as its counterpart. The solid CO
and XCN bands observed by Moultaka et al. (2009) toward individual sources in
and near the Central Cluster have a wide range of strengths, some greater than
and some weaker than the bands in the spectrum of 2M1747.

Moneti et al. (2001) concluded that the solid CO absorption toward Sgr A*
arises in the foreground spiral arms. This seems reasonable in that the gas
temperatures in the spiral arms found by Serabyn & Güsten (1986) are $14-19$ K
and that dust temperatures are generally lower than gas temperatures. These
temperatures are low enough for CO to adhere strongly to grains. Moneti et al.
(2001) also concluded that the absorption by gaseous CO in its fundamental
band toward Sgr A* is mostly due to dense gas in the foreground spiral arms.
The high-resolution spectra of optically thin overtone 12CO and fundamental
band 13CO lines toward the Central Cluster sources GCIRS3 and GCIRS1W (Goto et
al., 2014; Oka et al., 2019), which are dominated by absorptions at the
characteristic radial velocities of those arms, confirm their conclusion.

The low-resolution spectra of Moultaka et al. (2009) show significant
variations in the strengths of the low-$J$ fundamental band lines of 12CO in
addition to variations in the solid CO feature. Moultaka et al. (2019) argue
that some of the source-to-source variations arise in gas and dust close to
the center. This is certainly the case for the gaseous CO, where at high
spectral resolution Goto et al. (2014) observed CO absorption at velocities
not observed on other sightlines. They attributed the additional absorption
component to gas associated with the 2 pc diameter circumnuclear ring (CNR).
The temperature of the gas and dust local to Sgr A* and the Central Cluster is
uncertain but it seems unlikely that it would be low enough for solid CO to
survive. Thus, it appears to us to be more likely that the variations in the
solid CO band strength toward the sources observed by Moultaka et al. (2009)
are entirely due to clumpy distributions of solid CO in the three intervening
spiral arms.

Toward 2M1747 the majority of the absorption by gaseous CO comes from the
front portion of Sgr B1, as discussed earlier. From the far-infrared mapping
of Lis et al. (2001), the mean dust temperature in that portion of Sgr B1 is
$\sim$35 K. Because CO rapidly sublimes from dust grains at temperatures above
20 K (Sandford & Allamandola, 1990; van Dishoeck et al., 1996), it is unlikely
that the solid CO seen toward 2M1747 arises in gas at that temperature.
However, the high extinction and absence of dense cores on this sightline
suggests that the length of the absorbing column in Sgr B1 is likely to be
several pc and thus one cannot rule out the possibility that there are small
shielded regions of lower temperature in one or more parts of the sightline
within Sgr B1, where CO ice is present.

We conclude that it is most probable that the vast majority of the solid CO
absorption arises outside of the CMZ, as we and Moneti et al. (2001) conclude
is the case for sightlines toward Central Cluster sources. Because Sgr B1 is
85 pc distant from the Central Cluster sources, apart from gas local to the
solar neighborhood the sightline to 2M1747 passes through the intervening
spiral arms tens of parsecs distant from the sightlines to the Central
Cluster. The most likely location for the solid CO on the 2M1747 sightline
would appear to be cold dense molecular gas in the 3 kpc arm, which in the
low-$J$ lines of the CO overtone band, absorbing near $-43$ km s-1, produce
much deeper absorptions than observed in the other intervening arms (see
Figure 2).

Interstellar XCN is believed to be created by chemical reactions involving CO
and/or CH3OH on the mantles of grains in dense molecular clouds. This view is
supported by a number of laboratory experiments (e.g. Lacy et al., 1984;
Bernstein et al., 1995; Schutte & Greenberg, 1997; Bernstein et al., 2000;
Palumbo et al., 2001; Hudson & Gerakines, 2018). Methanol ice is not detected
on the 2M1747 sightline, which suggests that the production of XCN on this
sightline is tied to the presence of solid CO. As discussed earlier, the high
mean dust temperature in Sgr B1 on the line of sight to 2M1747 suggests that
CO is not present in icy grain mantles there, except possibly for small
shielded pockets within the cloud. However, laboratory experiments (e.g. Lacy
et al., 1984) show that XCN survives at much higher temperatures than solid CO
and thus could be prominent in regions where solid CO is not. Chiar et al.
(2002) and Moultaka et al. (2004) have suggested that the XCN bands observed
toward sources in and near the Central Cluster arise at least in part in
molecular gas close to Sgr A*, such as the CNR.

If the XCN band observed toward 2M1747 arises in dust mantles in dense clouds
outside of the CMZ, the most likely location would appear to be in the 3 kpc
arm as argued above for solid CO. However, XCN bands tend to be weak in
quiescent cold dark clouds, with strong bands such as that observed toward
2M1747 found in absorption toward massive YSOs embedded in dense clouds and
weaker bands toward lower-mass YSOs (Whittet et al., 2001; Pontoppidan et al.,
2003). No YSOs are known in those parts of the foreground spiral arms close to
the sightline to 2M1747, but we are unaware of any searches for them. In
conclusion, we regard the location of the XCN on the sightline toward 2M1747
to be undetermined and possibly present in both the CMZ, within dense regions
in Sgr B1 and/or the CNR, and one or more foreground spiral arms.

####  5.3.2 H2O

Toward 2M1747 the broad 3 $\mu$m band of water ice plus impurities in Figure 8
has a peak optical depth of 1.25. This is 2.5 times that observed by ISO SWS
toward the Central Cluster Lutz et al. (1996) and $1.5-3$ times the optical
depths observed toward individual sources in that cluster (McFadzean et al.,
1989; Chiar et al., 2002; Moultaka et al., 2004), with the exception of the
anomalous source IRS 19. This suggests that some of the water ice on the line
of sight to 2M1747 is produced in the CMZ. In the Galactic plane the water ice
band has not been detected in diffuse molecular gas (e.g Whittet et al., 1997)
but is present in dense clouds with $A_{V}>$ 3.3 mag (e.g. Whittet et al.,
1988). Thus, the most likely locations for water ice-covered grains to be
found are dense molecular gas in the spiral arms and in Sgr B1. Whittet et al.
(1997) estimate that dust in the dense molecular gas on the line of sight to
the GC produces $\sim$10 mag of optical extinction, which suggests that the
water ice band produced in it could have an optical depth of several tenths.

Given the order-of-magnitude-larger extinction of the molecular gas in Sgr B
than the molecular gas in the foreground spiral arms and the mean dust
temperature of $\sim$35 K measured in this region of Sgr B1 by Lis et al.
(2001), it is perhaps surprising that the optical depth of its water ice band
is not much greater than the observed value. The lack of very strong
absorption might be attributable to an environment inhibiting formation of ice
mantles on grains on much of the sightline within Sgr B1. The gas temperature
in Galactic center molecular clouds sigificantly higher than in many dense
clouds in the Galactic plane. In addition, a cosmic-ray ionization rate of H2
$\sim$103 higher than in dense clouds in the Galactic plane exists throughout
much of the CMZ (Le Petit et al., 2016; Oka et al., 2019). Sputtering of water
ice off of grain mantles due to cosmic-ray - dust particle collisions (Dartois
et al., 2015) could be responsible for the lower than expected strength of the
3.0 $\mu$m ice band.

####  5.3.3 The 3.4 $\mu$m feature

In Figure 9 the optical depth spectrum of the 3.4 $\mu$m absorption toward
2M1747 is compared with the mean spectrum of six members of the Quintuplet
Cluster observed by Chiar et al. (2013). The profiles are nearly identical in
the $3.35-3.60$ $\mu$m interval, with the absorption toward 2M1747 roughly 30%
stronger. Chiar et al. (2002) and Moultaka et al. (2004) have presented
optical depth spectra of individual infrared sources in the Central Cluster.
Both papers reveal considerable source-to-source variation in the strength of
the feature. They differ in their interpretations of this. Chiar et al. (2002)
ascribe the differences in optical depth to spatial variations the abundance
of the carrier of the feature, saturated aliphatic hydrocarbons, in foreground
material, while Moultaka et al. (2004) argue that not only this absorption
feature but also the 3.0 $\mu$m water ice feature arise largely in gas within
the Central Cluster itself, within 0.5 pc (12″) of Sgr A*. The latter is in
conflict with the generally accepted view that grains with water ice mantles
are found in dense molecular gas and observational evidence that the carrier
of the 3.4 $\mu$m feature exists only in diffuse gas. Although the two groups
used similar techniques in determining the optical depth of the 3.4 $\mu$m
feature, their estimates toward the three infrared sources that both groups
observed differ considerably, in one case by almost a factor of three, with
the difference much greater than the stated uncertainties.

The peak optical depth of the 3.4 $\mu$m band toward 2M1747 was determined
using the same technique as in the above two papers. Within the uncertainties
it is the same as the average value found by Chiar et al. (2002) for all of
their measurements and is about three-fourths of the average value found by
Moultaka et al. (2004) toward their sources. Because of the roughly equal
optical depths we think it is most probable that the majority of the 3.4
$\mu$m absorption toward all of the Galactic center sources arises in the same
regions of their sightlines. The sightlines to all of these sources include
diffuse gas both within and external to the CMZ, and thus both are candidate
sites. However, laboratory studies (Godard et al., 2011, see also references
therein) suggest that the intense cosmic-ray flux and UV field in the CMZ will
inhibit formation of the carriers of the 3.4 $\mu$m feature. This suggests
that the feature arises in dust within the diffuse gas in the Galactic spiral
arms.

##  6 Summary

We have presented and analyzed a variety of infrared spectra in selected
portions of the $2-5$ $\mu$m wavelength interval toward 2M1747, a luminous
OH/IR star located in the CMZ on a sightline to the Sgr B1 giant molecular
cloud. The spectra reveal numerous gaseous absorption lines of H${}_{3}^{+}$
and CO and broad absorption features due to at least seven distinct species
associated with interstellar dust grains and their icy mantles. The gaseous
line profiles of both species are dominated by strong absorption at positive
radial velocities previously associated with Sgr B1 from radio and millimeter
wavelength studies. This implies that 2M1747 is situated within, but toward
the front of the dense and warm gas in Sgr B1, whose dust is associated with
$\sim$103 visual magnitudes of extinction. At negative velocities, absorption
in foreground spiral arms produce sharp absorption components in the lowest-
lying lines of both H${}_{3}^{+}$ and CO. The H${}_{3}^{+}$ line profiles
demonstrate that the sightline to 2M1747 also passes through a long column of
warm diffuse gas with properties including kinematics that are consistent with
those of the outwardly expanding gas observed previously on many other CMZ
sightlines (Oka et al., 2019; Oka & Geballe, 2020). The total visual
extinction to 2M1747 probably exceeds 100 mag. We propose that the 3 $\mu$m
water ice and 4.62 $\mu$m XCN bands observed toward 2M1747 arise both in Sgr
B1 and in one or more Galactic foreground spiral arms, but that the solid CO
feature arises entirely or almost entirely in the spiral arms. The 3.4 $\mu$m
hydrocarbon absorption appears to be produced almost entirely in cold diffuse
gas in the arms and not in the warm, irradiated diffuse gas of the CMZ. The
observed solid-state features also include a shallow band extending from 3.22
$\mu$m to 3.30 $\mu$m, which may be a blending of two unrelated features, and
two weak absorption bands in the $3.8-4.0$ $\mu$m interval, whose previous
identifications as CH3OH ice now appear to be in doubt as a result of these
observations.

This research is based in large part on observations obtained at the
international Gemini Observatory, a program of NSF’s NOIRLab, which is managed
by the Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation, on behalf of the
Gemini Observatory partnership: the National Science Foundation (United
States), National Research Council (Canada), Agencia Nacional de Investigación
y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación
(Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações
(Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).
We thank the staffs of the Gemini Observatory and the NASA Infrared Telescope
Facility for their support. We also thank the graphic artist, Michael
Schisler, for the creation of Figure 1. We are grateful to both Takeshi Oka
and the referee for helpful comments.

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