# Excited Hydroxyl Outflow in the High-Mass Star-Forming Region G34.26+0.15

Wei Siang Tan  Physics Department, Western Illinois University, 1 University
Circle, Macomb, IL 61455, USA.  National University of Singapore, 21 Lower
Kent Ridge Rd, Singapore 119077.  Esteban D. Araya Physics Department, Western
Illinois University, 1 University Circle, Macomb, IL 61455, USA.  New Mexico
Institute of Mining and Technology, Physics Department, 801 Leroy Place,
Socorro, NM 87801, USA.  Cade Rigg  Southeastern High School, 90 W Green St,
Augusta, IL 62311, USA.  Peter Hofner  Adjunct Astronomer at the National
Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA.
New Mexico Institute of Mining and Technology, Physics Department, 801 Leroy
Place, Socorro, NM 87801, USA.  Stan Kurtz  Instituto de Radioastronomía y
Astrofísica, Universidad Nacional Autónoma de México, Apdo. Postal 3-72,
58089, Morelia, Michoacán, Mexico.  Hendrik Linz  Max Planck Institute for
Astronomy, Königstuhl 17, 69117, Heidelberg, Germany.  Viviana Rosero
National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801,
USA.

(Accepted June 11, 2023, for publication in the ApJ.)

###### Abstract

G34.26+0.15 is a region of high-mass star formation that contains a broad
range of young stellar objects in different stages of evolution, including a
hot molecular core, hyper-compact HII regions and a prototypical cometary
ultra-compact HII region. Previous high-sensitivity single dish observations
by our group resulted in the detection of broad 6035 MHz OH absorption in this
region; the line showed a significant blue-shifted asymmetry indicative of
molecular gas expansion. We present high-sensitivity Karl G. Jansky Very Large
Array (VLA) observations of the 6035 MHz OH line conducted to image the
absorption and investigate its origin with respect to the different star
formation sites in the region. In addition, we report detection of 6030 MHz OH
absorption with the VLA and further observations of 4.7 GHz and 6.0 GHz OH
lines obtained with the Arecibo Telescope. The 6030 MHz OH line shows a very
similar absorption profile as the 6035 MHz OH line. We found that the 6035 MHz
OH line absorption region is spatially unresolved at $\sim 2$″ scales, and it
is coincident with one of the bright ionized cores of the cometary HII region
that shows broad radio recombination line emission. We discuss a scenario
where the OH absorption is tracing the remnants of a pole-on molecular outflow
that is being ionized inside-out by the ultra-compact HII region.

HII regions — ISM: molecules — radio lines: ISM — stars: formation

††facilities: VLA (NRAO), Arecibo (UCF, NSF)††software: CASA (McMullin et al.,
2007); MOLPOP-CEP (Asensio Ramos & Elitzur, 2018)

##  1 Introduction

In the turbulent core accretion model of high-mass star formation, massive
stars ($M\gtrsim 8\,$M⊙) form in Giant Molecular Clouds, where gravitational
collapse occurs in dense cores leading to the formation of a protostar
surrounded by a disk through which accretion takes place (e.g., Motte et al.
2018, Sánchez-Monge et al. 2013, Zinnecker & Yorke 2007, Beuther & Shepherd
2005). In this phase, ionized jets and molecular outflows form (e.g., Rosero
et al. 2019, Kuiper & Hosokawa 2018) while the protostar continues increasing
mass, until thermonuclear fusion begins and high energy UV photons start to
photoionize the natal environment. It is thought that this process leads to
the formation of hypercompact HII (HCHII) and ultracompact HII (UCHII) regions
(e.g., Yang et al. 2021) and eventually classical HII regions. During the
early phases of high-mass star formation, before the formation and during the
early evolution of UCHII regions, hot molecular cores (HMC) develop, which are
among the most chemically rich environments in the interstellar medium (ISM;
e.g., Kurtz et al. 2000; Cesaroni 2005; van der Tak 2005). Although most HMC
are thought to be centrally heated, others appear to be externally heated by
nearby UCHII regions (Mookerjea et al., 2007). Despite the great progress in
our understanding of high-mass star formation in the last few decades, key
aspects remain unclear, such as the evolution of the outflows/jets from their
initial launching to their later interaction with UCHII regions. UCHII and
HCHII regions characterized by broad radio recombination lines (RRLs) could be
key to reveal the evolution of outflows after photoionization begins to
dominate (e.g., Sewilo et al. 2004).

###  1.1 G34.26+0.15: A high-mass star-forming region with OH Absorption

At a distance of approximately 3.0 kpc111We adopt the trigonommetric parallax
distance of 3.0 kpc of G034.43+0.24 (Mai et al., 2023) as the distance to
G34.26+0.15. A 3 kpc kinematic distance has also been reported for G34.26+0.15
(e.g., Araya et al. 2002, Kuchar & Bania 1994). However, we note that Urquhart
et al. (2018) listed a closer distance., the high-mass star forming region
G34.26+0.15 is an excellent example where all aspects of high-mass star
formation coexist. G34.26+0.15 is part of a large scale ($\sim 30\,$pc, i.e.,
34′ at 3.0 kpc) star forming filament, including the Infrared Dark Cloud IRDC
G34.43+0.24 (Xu et al., 2016). G34.26+0.15 contains HII regions at different
stages of evolution, including two HCHII regions (see resolved images of the
HCHII regions in Avalos et al. 2009), and a HMC that appears to be externally
heated by a cometary UCHII region (Mookerjea et al., 2007). Infall signatures
of molecular gas have been detected in G34.26+0.15 (Wyrowski et al. 2012, Liu
et al. 2013, Wyrowski et al. 2016; Hajigholi et al. 2016), and outflow
activity is evident by the detection of SiO emission (Hatchell et al., 2001),
and a cone-like 4.5$\,\mu$m structure indicative of shocked gas (Sewiło et al.
2011, Liu et al. 2013). The cometary UCHII G34.26+0.15 is one of the best
examples of ionized gas with broad RRL emission (e.g., Sewilo et al. 2004;
Rodríguez-Rico et al. 2002; Gaume et al. 1994).

As it is typical of active sites of high-mass star formation, many molecular
species have been detected as masers in G34.26+0.15, including H2O (e.g.,
Hofner & Churchwell 1996, Imai et al. 2011), NH3 (Yan et al., 2022), CH3OH
(e.g., Bartkiewicz et al. 2016, Kurtz et al. 2004) and OH (e.g., Garay et al.
1985, Tan et al. 2020). In the case of hydroxyl, masers of the 18 cm ground
state have been detected (e.g., Beuther et al. 2019, Gasiprong et al. 2002,
Zheng et al. 2000), but to our knowledge, no absorption has been found (Rugel
et al., 2018). In contrast, using the 305 m Arecibo Telescope, Al-Marzouk et
al. (2012) reported the detection of 6035 MHz OH masers as well as a
relatively weak ($-65\,$mJy) and broad ($FWHM=18$ km s-1) absorption line
(6035 MHz OH masers were also detected by Caswell & Vaile 1995 and Caswell
2001, but no absorption was reported). The absorption was confirmed by Tan et
al. (2020)222Additional observations of the absorption line were presented in
the MS thesis of Kim (2014; Western Illinois University)., where we reported a
highly asymmetric blue-shifted 6035 MHz OH profile. The line has a velocity
linewidth at zero intensity greater than $\sim 50$ km s-1, indicative of
outflowing molecular gas. The 6035 MHz line belongs to the 6.0 GHz OH ladder
(6016, 6030, 6035, 6049 MHz) that corresponds to the first excited rotational
level ($J=5/2$) of the $2\Pi_{3/2}$ ladder above the well known ground state
18 cm OH transitions; the 6.0 GHz transitions are at energy levels
$E_{lower}/{k_{B}}=120\,$K333Spectroscopy information of all transitions in
this article is from Splatalogue (https://splatalogue.online/). (e.g., see
energy level diagrams in Gray 2012, Pihlström et al. 2008, Desmurs et al.
2002). Absorption of the 6035 and 6030 MHz lines has been detected in the ISM,
including toward other high-mass star forming regions (e.g., Tan et al. 2020,
Avison et al. 2016, Fish et al. 2006, Baudry et al. 1997, Guilloteau et al.
1984).

In this work we present a follow up study of the broad 6035 MHz OH absorption
line in G34.26+0.15 based on NSF’s Karl G. Jansky Very Large Array (VLA) and
305 m Arecibo Telescope data, including observations of other OH transitions
of the 6.0 and 4.7 GHz states. The 4.7 GHz OH transitions (4660, 4765, and
4751 MHz) correspond to the lowest state of the $2\Pi_{1/2}$ ladder ($J=1/2$;
$E_{lower}/k_{B}=181\,$K; Gray 2012); transitions from this state have also
been detected toward star forming regions and late type stars (e.g., Qiao et
al. 2022, Strack et al. 2019, Palmer & Goss 2005, Dodson & Ellingsen 2002,
Cohen et al. 1991). In Section 2 we report details of the observations and
data reduction; results and discussion are presented in sections 3 and 4,
respectively. We summarize our work in Section 5.

##  2 Observations

###  2.1 Arecibo Telescope

Observations of the 4.7 GHz OH (4660.2420 MHz, 4750.6560 MHz, 4765.5620 MHz)
and 6.0 GHz OH transitions (6016.74600 MHz, 6030.74850 MHz, 6035.09320 MHz,
6049.08400 MHz) were conducted with the 305 m Arecibo Telescope in Puerto
Rico, between October 2008 and January 2010 as part of the project A2411,
which was designed to investigate variability of OH masers. The maser
variability aspect of the observations was reported in Al-Marzouk et al.
(2012). The observations were conducted in position switching (1 or 2 min ON-
source followed by OFF-source observations for the same amount of time, and 10
second calibration observations by injecting noise from a diode). The WAPP
(Wideband Arecibo Pulsar Processor) spectrometer was used for the
observations, with a bandwidth of 3.125 MHz and 2048 channels per transition
(Table 1). The lines were observed in two groups, the 4.7 GHz lines using the
C-Band receiver, and the 6.0 GHz transitions using the C-Band High receiver.
Most scans included observations of the Arecibo Telescope calibrator
B1857+129, which resulted in typical system temperatures between 25 and 33 K,
and telescope gains of approximately 8 and 6 K Jy-1 at C-band and C-Band High
frequencies, respectively; pointing errors smaller than 17″, and half-power
beam widths of 56″at 4860 MHz and 44″at 6600 MHz.

A total of 23 scans were observed of the 4.7 GHz transitions and 16 scans of
the 6.0 GHz lines. Data reduction and calibration was done in IDL using the
AOIDL library provided by the observatory. After checking for consistency
between polarization and different scans, and after excluding scans affected
by radio frequency interference and technical problems (such as high system
temperatures due to a cooling failure of the C-Band High receiver), we
subtracted linear baselines obtained from line-free channels at both sides of
the spectral lines, averaged the scans, and smoothed them to final channel
widths between 1 and 8 km s-1 to enhance signal-to-noise of weak broad lines.
In the case of the 6035 MHz OH lines, we smoothed the data to a channel width
of $1$ km s-1 to avoid spectral contamination of the peak absorption with
narrow bright masers. A discussion of the 6035 MHz OH masers was reported in
Al-Marzouk et al. (2012); in this paper we concentrate on the broad absorption
lines. Further details of system checking observations, calibration and data
reduction are included in Al-Marzouk et al. (2012).

###  2.2 Very Large Array (VLA)

The 6.0 GHz OH transitions observed with the Arecibo Telescope were also
observed with the VLA in two configurations: D-Array (September 2018; project
ID 18A$-$401) and B-Array (June 2020; project ID 20A$-$311). We used the
quasar 3C286 as flux and bandpass calibrator. J1856+0610 and J1824+1044 were
used as complex gain (amplitude and phase) calibrators for the B and D
configuration observations, respectively. All lines were simultaneously
observed in different spectral windows. Further details of the VLA
observations are reported in Table 1.

Data reduction was done using the Common Astronomy Software Applications
package (CASA444https://casa.nrao.edu). The VLA pipeline in CASA 5.4.1-31
(pipeline 51-P2-B)555https://science.nrao.edu/facilities/vla/data-
processing/pipeline was used for calibration of the VLA D-configuration data.
The VLA pipeline in CASA 5.6.2-3 (pipeline 56-P2-B) was used to calibrate the
VLA B-configuration observations. Continuum subtraction was done in the
$(u,v)$ plane using the task uvcontsub. Data cubes were obtained with the task
tclean using natural weighting. We obtained velocity moments (integrated
intensity, velocity, and velocity dispersion) using the task immoments. The
cubes were smoothed to final channel widths between 0.2 and 1.5 km s-1 to
optimize the detection of weak OH absorption while minimizing spectral
contamination by the narrow and bright masers at 6035 MHz.

Table 1: Summary of OH Observations. Telescope | Arecibo | VLA-D | VLA-B
---|---|---|---
Observing Epoch | 2008 - 2010 | September 2018 | June 2020
Project ID | A2411 | 18A$-$401 | 20A$-$311
Pointing RA (J2000)(1) | 18:53:18.5 | 18:53:18.5 | 18:53:18.5
Pointing Dec (J2000)(1) | +01:14:59 | +01:14:59 | +01:14:59
Configuration |  $\cdots$ | D | B
Bandwidth | 3.125 MHz | 4 MHz | 4 MHz
Number of Channels | 2048 | 2048 | 2048
Channel Separation | 1.53 kHz | 1.95 kHz | 1.95 kHz
Beamwidth(2) | 44″, 56″ | 19.7″$\times$9.7″, $-$19∘ | 2.2″$\times$1.3″, $-$36∘
Rest Frequencies(3) | 4660.2420 MHz |  $\cdots$ |  $\cdots$
| 4750.6560 MHz |  $\cdots$ |  $\cdots$
| 4765.5620 MHz |  $\cdots$ |  $\cdots$
| 6016.7460 MHz | 6016.7460 MHz | 6016.7460 MHz
| 6030.7485 MHz | 6030.7485 MHz |  $\cdots$
| 6035.0932 MHz | 6035.0932 MHz | 6035.0932 MHz
| 6049.0840 MHz | 6049.0840 MHz | 6049.0840 MHz
(1) G34.26+0.15 pointing coordinates.
(2) The half-power beam width at 6600 MHz and 4860 MHz are reported for the
Arecibo observations. Synthesized beam at 6035 MHz
($\theta_{maj}\times\theta_{min}$, PA) is reported for the VLA observations.
(3) Rest frequencies of OH transitions with reliable detection or non-
detection. Spectroscopy information is from Splatalogue
(https://splatalogue.online/).

##  3 Results

###  3.1 Arecibo

As reported in Al-Marzouk et al. (2012), bright and narrow OH masers were
detected in the 6035 MHz transition, and a weak and broad absorption feature
was found when all scans were averaged. As shown in Figure 1 and Table
LABEL:tab:OH_Line_Parameters, absorption was also detected in the 6030 MHz OH
transition; broad and weak absorption was also detected in the 6016 MHz OH
transition. We found no evidence of absorption in the 6049 MHz OH line.

In contrast to the 6.0 GHz OH transitions that show broad absorption, no
absorption was detected in the 4.7 GHz OH transitions, instead emission was
found in the 4750 MHz and 4765 MHz OH lines, and a tentative (weak and narrow)
emission line was found in the 4660 MHz OH spectrum (Table
LABEL:tab:OH_Line_Parameters, Figure 1). In the case of the 4750 MHz and 4765
MHz OH detections, the lines have asymmetric profiles, suggesting overlapping
velocity components. Emission of these transitions (including broad and narrow
lines) has been detected toward other high-mass star forming regions (e.g.,
Qiao et al. 2022, Cohen et al. 1991, and references therein; note that Qiao et
al. 2022 did not detect the 4.7 GHz OH lines toward G34.26+0.15 due to
sensitivity). The low RMS obtained in the short observations reported in this
work (RMS $\lesssim 3\,$mJy in $\approx 20\,$min ON-source) highlights the
extraordinary sensitivity that the 305 m Arecibo Telescope was able to
achieve, and the great loss in high-sensitivity spectroscopic capabilities at
frequencies below $\sim 10\,$GHz with the demise of the telescope.

Figure 1: OH spectra obtained with the Arecibo Telescope. The left panels show
the 4.7 GHz OH transitions and the right panels show the 6.0 GHz lines (the
end of the bandpass can be seen at $-$40 km s-1 in the 6.0 GHz spectra). We
report detection of emission in the 4750 MHz and 4765 MHz transitions (the
4660 MHz may also show a narrow and weak emission line, however, only one
channel is above 3$\sigma$). Maser lines are detected in the 6035 MHz
transition (see Al-Marzouk et al. 2012), and absorption is found in the 6035
MHz, 6030 MHz and 6016 MHz transitions. The 6016 MHz line is weak; the spectra
had to be smoothed to a channel width of 7.6 km s-1 to detect the absorption.
We report no detection of the 6049 MHz transition (no clear evidence of
absorption is detected when the spectrum is smoothed).

Table 2: OH Line parameters. Spectral Line | Sν | RMS | VLSR | Width |  $\int$Sνdv | Telescope
---|---|---|---|---|---|---
| (Jy) | (Jy) | (km s-1) | (km s-1) | (Jy km s-1) |
4660 MHz | … | 0.0015 | … | … | … | Arecibo (1)
4750 MHz | 0.017 | 0.0013 | 56.3(1.4) | 30.3(2.9) | 0.28(0.03) | Arecibo
4765 MHz | 0.016 | 0.002 | 56.2(1.4) | 15.8(2.9) | 0.15(0.03) | Arecibo
6016 MHz |  $-$0.0070 | 0.0007 | 49.5(7.6) | 38.0(15.2) |  $-$0.19(0.04) | Arecibo
| … | 0.008 | … | … | … | VLA$-$D
| … | 0.009 | … | … | … | VLA$-$B
6030 MHz |  $-$0.066 | 0.002 | 61.7(1.5) | 63.7(3.0) |  $-$1.89(0.06) | Arecibo
|  $-$0.052 | 0.004 | 62.0(1.5) | 50.0(3.0) |  $-$1.48(0.11) | VLA$-$D
6035 MHz |  $-$0.062 | 0.003 | 52.7(1.1) |  $>$55 | $>|-1.6|$ | Arecibo (2)
|  $-$0.056 | 0.005 | 51.7(1.5) |  $>$49 | $>|-1.5|$ | VLA$-$D (3)
| 0.104 | 0.012 | 62.4(0.1) | 0.5(0.2) | 0.03(0.01) | VLA$-$D
| 0.091 | 0.012 | 62.0(0.1) | 0.4(0.2) | 0.02(0.01) | VLA$-$D
| 0.246 | 0.012 | 60.9(0.1) | 0.9(0.2) | 0.11(0.01) | VLA$-$D
| 0.091 | 0.012 | 60.1(0.1) | 0.5(0.2) | 0.03(0.01) | VLA$-$D
| 0.287 | 0.012 | 59.5(0.1) | 0.7(0.2) | 0.10(0.01) | VLA$-$D
| 0.496 | 0.012 | 58.5(0.1) | 0.7(0.2) | 0.20(0.01) | VLA$-$D
| 0.413 | 0.012 | 58.1(0.1) | 0.7(0.2) | 0.20(0.01) | VLA$-$D
| 0.094 | 0.012 | 56.8(0.1) | 0.6(0.2) | 0.04(0.01) | VLA$-$D
| 0.152 | 0.012 | 55.7(0.1) | 0.7(0.2) | 0.06(0.01) | VLA$-$D
|  $-$0.058 | 0.003 | 50.8(1.5) | 58.2(3.0) |  $-$1.86(0.08) | VLA$-$B
| 0.115 | 0.008 | 62.4(0.1) | 1.1(0.2) | 0.08(0.01) | VLA$-$B
| 0.187 | 0.008 | 60.9(0.1) | 1.2(0.2) | 0.14(0.01) | VLA$-$B
| 0.237 | 0.008 | 59.6(0.1) | 1.0(0.2) | 0.13(0.01) | VLA$-$B
| 0.481 | 0.008 | 58.4(0.1) | 1.7(0.2) | 0.43(0.01) | VLA$-$B
| 0.118 | 0.008 | 56.8(0.1) | 0.9(0.2) | 0.08(0.01) | VLA$-$B
| 0.140 | 0.008 | 55.8(0.1) | 0.9(0.2) | 0.08(0.01) | VLA$-$B
6049 MHz | … | 0.003 | … | … | … | Arecibo
| … | 0.008 | … | … | … | VLA$-$D
| … | 0.010 | … | … | … | VLA$-$B
We report the channel width as uncertainty in the $V_{LSR}$ peak velocity, and
twice the channel width as uncertainty in the linewidth. The linewidth is the
velocity range of the line above 3$\sigma$. The uncertainty in the integrated
flux density is the 3 $\times$ RMS $\times$ channel width $\times$ square root
of the number of channels in the line.
(1) Tentative line detected, only one channel above 3$\sigma$,
$S_{\nu}=6.0\,$mJy, $V_{LSR}=50.7$ km s-1.
(2) Multiple maser peaks overlap the absorption feature, therefore, the
linewidth and integrated flux density are lower limits. The data were smoothed
to improve signal-to-noise detection of the absorption feature, a discussion
of the masers from this dataset with high spectral resolution is presented in
Al-Marzouk et al. (2012).
(3) Multiple maser peaks overlap the absorption feature, therefore, the
linewidth and integrated flux density are lower limits.

###  3.2 Very Large Array

Figure 2 summarizes the results obtained with the VLA. The left panels show
the 6.0 GHz OH spectra obtained with the VLA in the D-configuration (blue),
the spectra obtained in the B-configuration (black) and the Arecibo spectra
(red; the 6030, 6035 and 6049 MHz are as in Figure 1, while the 6016 MHz
spectrum is shown with higher spectral resolution than the one shown in Figure
1 to facilitate comparison with the VLA spectra). The center panels show the
6.0 GHz radio continuum from the line-free channels of the respective spectral
windows obtained with the VLA in D-configuration, and the right panels show
the equivalent 6 cm radio continuum images from the line-free channels of the
VLA B-configuration spectral windows.

As it is evident from Figure 2, we obtained almost identical OH absorption
profiles of the 6030 MHz and 6035 MHz transitions with Arecibo and the VLA. In
the case of the 6035 MHz transition, the Arecibo and VLA D-configuration
profiles overlap with strong and narrow OH masers. In contrast, the 6035 MHz
OH absorption and masers are spatially separated at the VLA B-configuration
resolution. As shown in Figure 3, the masers are located toward the North core
of the cometary UCHII region and toward the two HCHII regions to the East of
the UCHII region, while the absorption is located toward the southern core of
the UCHII region (Figure 2). As this article focuses on the absorption, the
masers are not further discussed, nevertheless, the line parameters of the
masers are listed in Table LABEL:tab:OH_Line_Parameters together with the
parameters of the absorption lines. Given that the masers are spatially
separated from the absorption at the angular resolution of the VLA
B-configuration observations, we were able to obtain the absorption spectrum
without contamination by OH masers (Figure 2).

Both Arecibo and VLA observations confirm the asymmetric absorption profile of
the 6035 MHz OH line reported by Tan et al. (2020). Our data also show that
the 6030 MHz OH exhibits a very similar absorption profile, with a blue
asymmetry (line-wing) that extends from the peak absorption ($\sim 50-60$ km
s-1) to $\sim 0$ km s-1. The absorption is spatially unresolved (based on a 2D
Gaussian fit of the moment-0 VLA-B detection); we obtained consistent line
profiles from the angular resolution of the Arecibo Telescope (44″) to the VLA
B-configuration synthesized beam ($\sim 2$″). To our knowledge, these are the
spectrally broadest and most spatially compact 6035 and 6030 GHz OH absorption
lines ever detected in the ISM. We note that broad absorption lines ($>100$ km
s-1) of these OH transitions have been detected in other galaxies, but tracing
the overall ensemble of molecular clouds seen in absorption against the
continuum of the galaxy rather than from an individual site within a star
forming region (Eisner et al. 2019, Whiteoak & Wilson 1990).

Figure 2: Spectra of the 6.0 GHz OH transitions obtained with the Arecibo
Telescope (red), VLA in D-configuration (blue) and B-configuration (black) are
shown in the left panels; the spectra are sorted by frequency: 6016 MHz, 6030
MHz, 6035 MHz, and 6049 MHz from top to bottom. Radio continuum obtained from
the line-free channels of the respective spectral windows is shown in the
middle (D-configuration) and right panels (B-configuration). In the middle
panels, we show with a red circle the $\sim$44″ Arecibo beam, centered at the
pointing position; the blue contours in the 6030 and 6035 MHz panels show the
velocity integrated intensity (moment-0) images of the absorption from the VLA
D-configuration cubes (6030 MHz OH absorption contour levels: $-$10, $-$90,
$-$170, $-$250 $\times\,$3.08 mJy km s-1; 6035 MHz OH absorption contour
levels: $-$70, $-$180, $-$290, $-$400 $\times\,$2.65 mJy km s-1). No detection
was obtained in the D-configuration observations of the 6016 MHz (middle,
upper panel) nor the 6049 MHz (middle, lower panel) OH transitions; for these
transitions, we show with a blue circle the region used to obtain the blue
spectrum shown in the respective left panels. The right panels are equivalent
to the middle panels, but for the B-configuration observations; black circles
are the regions integrated to obtain the black spectra shown in the right
panels, and the moment-0 image of the 6035 MHz OH absorption is shown in black
contours ($-$60, $-$140, $-$220, $-$300 $\times\,$2.19 mJy km s-1). The
spectral window of the 6030 MHz VLA-B observations was corrupted. Figure 3:
The top-left panel shows the radio continuum image obtained from the line-free
channels of the VLA B-configuration 6035 MHz OH observations (colors).
Contours show the integrated intensity (moment 0) of the 6035 MHz OH masers
(contours levels are: 5, 15, 50, 150, 250 $\times\,$2.195 mJy km s-1). The
magenta region represents the area integrated to obtain the spectrum shown in
the top-right panel. The flux density and velocity range shown in the spectrum
were selected to highlight the masers; the spectrum highlighting the
absorption is shown in Figure 2. The velocity field (moment 1; colors) and
integrated intensity (moment 0; contours) of the 6035 OH masers are shown in
the lower-left panel. The velocity dispersion (moment 2; colors) and
integrated intensity (contours) of the 6035 MHz OH masers are shown in the
lower-right panel.

##  4 Analysis and Discussion

###  4.1 Origin of the OH Absorption: Association with Broad Radio
Recombination Line Ionized Gas

Based on Arecibo observations of H2CO and the H110$\alpha$ radio recombination
line (Araya et al., 2002), the systemic velocity of the G34.26+0.15 molecular
cloud is $\sim$60 km s-1, and the maximum flux density of the ionized gas
emission is at a velocity of 54 km s-1. As shown in Figures 1 and 2, the
stronger OH absorption detected at 6016 MHz, 6030 MHz and 6035 MHz is
approximately within the velocity range traced by H2CO and radio recombination
lines. The nature of the broad blue-shifted absorption asymmetry (line-wing)
observed in the 6030 MHz and 6035 MHz OH lines is more intriguing. As
discussed by Jacob et al. (2021), weak 3 GHz CH emission lines from molecular
clouds in the line-of-sight of G34.26+0.15 are detectable at $V_{LSR}\sim 10$
and $\sim 26$ km s-1, due to molecular clouds in the Sagittarius Spiral Arm
(see Figure 2 of Jacob et al. 2021; the 26 km s-1 feature was also detected in
HCO+ absorption by Carral & Welch 1992). We find it unlikely that the broad
blue-shifted absorption of the 6030 MHz and 6035 MHz OH lines is caused by
molecular clouds unrelated to G34.26+0.15, given the smooth line profile of
the absorption from the peak systemic velocity, and the compact nature of the
absorption (Figure 2). Instead, it is more likely that the blue-shifted line-
wing is tracing expansion/outflowing motion of the molecular gas in the
region. This is in contrast to ammonia transitions obtained with Herschel that
are indicative of infall (Hajigholi et al., 2016).

The radio continuum of the G34.26+0.15 complex has been extensively studied in
the literature (e.g., Reid & Ho 1985, Wood & Churchwell 1989, Fey et al. 1994,
Sewiło et al. 2011). This site of high-mass star formation contains a compact
HII region with a diameter of $\sim 1.5$′ (see continuum images from the VLA
D-configuration observations shown in the center column of Figure 2), which is
known as source D (e.g., Fey et al. 1994, Gómez et al. 2000, Campbell et al.
2004). At the VLA D-configuration resolution, a bright (almost unresolved)
continuum source is seen toward the North-West of source D. Arcsecond
resolution of the bright continuum source reveals a cometary UCHII region with
a diffuse tail propagating toward the West (source C) and two HCHII regions
(sources A and B) to the East of the UCHII region.666The nomenclature used
here is consistent with Sewiło et al. (2011), however, the cometary UCHII
region has also been labeled as ‘A’ in the literature (Wood & Churchwell,
1989). In Figure 4, we show in contours an 8 GHz radio continuum sub-arcsecond
angular resolution image of the cometary UCHII and HCHII regions from the VLA
image archive (https://www.vla.nrao.edu/astro/nvas/), where we label the
different continuum sources. As previously mentioned in the literature (e.g.,
Sewilo et al. 2004), the cometary UCHII region has two main continuum cores,
known as C1 and C2. It is unclear whether C1 and C2 are excited by different
stars or are simply density and/or temperature enhancements in the larger
cometary HII region.

G34.26+0.15 also harbors a HMC towards the eastern edge of the cometary UCHII
region (e.g., Heaton et al. 1989; Mookerjea et al. 2007). Campbell et al.
(2000) reported mid-infrared counterparts of the radio sources A, C, and D
(see Figures 2 and 4), no counterpart at the location of source B, and an
additional source (E) toward the South of C (we show the location and
approximate angular size of the 20$\mu m$ source E in Figure 4). We also show
in Figure 4 the integrated intensity (moment 0), velocity field (moment 1) and
velocity dispersion (moment 2) of the 6035 MHz OH absorption in the region.

We found that the broad OH absorption is closer to the C2 core. The velocity
field image suggests a slight East-West velocity gradient, and comparing the
velocity field with the moment 2 image, we note a greater velocity dispersion
toward the most blue-shifted core near the center of the OH absorption
(slightly South of C2, see Figure 4 middle and lower panels). However, we
stress that the velocity structure of the OH absorption suggested by the
moment 1 and 2 images should be considered as tentative, as the moment 0
absorption is unresolved. The coincidence of the 6.0 GHz OH absorption with C2
shows that the absorption is not directly related to the ground state (18 cm)
OH masers in the region, which appear to trace the interface between the
cometary HII region and the HMC, as well as the HCHII region B (e.g., see
Figure 3 of Zheng et al. 2000).

It is possible that the OH absorption is tracing an outflow that originates
from a source offset from the core of the cometary HII region. For instance,
the OH absorption could be tracing an outflow that originates from either
HCHII region to the East of the cometary HII region, given that Gaume et al.
(1994) proposed that stellar winds or outflows from the HCHII regions A or B
could be the responsible for the cometary morphology of G34.26+0.15C. Hatchell
et al. (2001) also pointed out that an SiO blueshifted outflow detected to the
North-West of G34.26+0.15C could be driven by G34.26+0.15B. Alternatively, as
shown in Figure 4, the C2 region is partially blended with both the source E
(to the South) and the HMC (to the East); the outflow could originate from
either source. We note that the arc of H2O masers near the HMC (Gómez et al.
2000, Fey et al. 1994) shows a blue-shifted North-South velocity gradient that
extends to $\sim 0$ km s-1. However, as shown by the VLBI observations of the
H2O masers by Imai et al. (2011) (see their Figure 5), the water masers are
distributed to the East of the cometary HII region, coincident with the HMC
and not with C2. Therefore, it is unclear whether there is a connection
between the H2O velocity gradient and the OH absorption777However, Gaume et
al. (1994) detected some H2O masers at the location of C2 (see their Figure
6).. In addition, Mookerjea et al. (2007) found evidence that the HMC is
externally heated by the cometary HII region instead of internally heated,
therefore, it is unclear whether a young stellar object could be driving an
outflow from the HMC. We note, however, that Keto et al. (1992) found evidence
for low-mass star formation at the HMC location.

An argument against the OH absorption tracing a molecular outflow originating
from the HMC (or from source E, or the HCHII regions) is the lack of spatial
elongation of the absorption profile. If the outflow originates from the HMC,
in a way that the blue-shifted cone of the flow happens to be projected toward
the cometary HII region, then one would expect to see the absorption tracing a
filament-like structure between the HMC and C2 and possibly continuing beyond
C2 (likewise, one would expect to see a North-South elongation of the
absorption if an inclined outflow were to originate from source E). This
suggests that the OH expanding gas is unlikely to be tracing an inclined
outflow originating from the HMC, source E, or the HCHII regions.

In contrast, the spatial coincidence between the OH absorption and C2 suggests
a direct association. This hypothesis is further supported by the kinematics
of the ionized gas. As discussed by Sewiło et al. (2011), the linewidth of the
H53$\alpha$ RRL toward the source C is unusually broad, with a full width at
zero intensity that ranges from $\sim 0$ to 100 km s-1, with a line center at
$V_{LSR}=49.9$ km s-1. The higher angular resolution observations of the
H76$\alpha$ line by Sewilo et al. (2004) were able to resolve the RRL emission
from C1 and C2. The C1 profile seems to be caused by the overlap of two
different velocity components, while the C2 H76$\alpha$ line has a more
symmetric (single Gaussian with center at 47.4 km s-1) profile, with a full
width at zero intensity that also extends to $\sim 0$ km s-1 (see also Garay
et al. 1986). Thus, the complete blue-shifted line-wing of the OH absorption
reported here is kinematically and spatially coincident with the blue-shifted
side of the RRL line toward C2.

The association between the broad OH absorption and C2 supports the
interpretation that broad radio recombination lines are due to kinematics
(turbulence and/or flows) of the plasma, instead of due to pressure broadening
as also concluded by Garay et al. (1986) (see also Kurtz 2005), however, the
reason why the OH absorption is only detected toward C2 is unclear. OH is a
common molecule in the ISM, if the OH gas in the G34.26+0.15 complex is due to
molecular dissociation in the interface between the molecular envelope and the
ionized gas, then it is unclear why absorption is not detected toward C1,
which has as much continuum as C2, and therefore, would show a similar
absorption profile for the same OH opacity. Of course, it is possible that, by
chance, there is a molecular core with OH overabundance that happens to be in
front of C2, and that is somehow entrained in an ionized flow from the HII
region as revealed by the broad RRLs, e.g., a proplyd-like core caught in the
ionized flow. However, such scenario seems to be highly fortuitous.

We propose an alternative hypothesis of the origin of the compact OH
absorption, in which the absorption is revealing a more general feature of
high-mass star formation instead of a curious peculiarity of a unique source.
Our idea is illustrated in Figure 5. In the canonical model of high-mass star
formation via core accretion, star formation occurs in Giant Molecular Clouds
where gravitational instabilities lead to collapse, the development of a
protostar with an accretion disk and the generation of a molecular
jet/outflow. Once the protostar reaches the Zero Age Main Sequence, high-
energy UV photons ionize the molecular environment leading to the formation of
an HII region. We propose that the ionization front can propagate through the
molecular outflow (explaining the high-velocity RRLs) and near the ionizing
front within the outflow, simple molecules like OH can be present traveling at
the high speeds of the flow. In the case of G34.26+0.15 C2, we happen to be
observing the outflow pole-on, therefore, only the foreground blue-shifted OH
is detected in absorption against the continuum of the UC HII region and
ionized outflow. We note that this interpretation does not preclude that C1 is
ionized by the same massive young stellar object as C2.

This hypothesis would explain why, as observed, the OH absorption has a small
angular size and large blue-shifted velocity dispersion due to the pole-on
orientation. Given that outflows seem to be less collimated for more evolved
and/or massive objects (see discussion in Beuther & Shepherd 2005), it would
be possible for the high velocity RRLs to have a larger angular extent (less
collimated, more evolved ionized flow) than the more collimated remnant
molecular outflow traced by OH (i.e., a ‘fossil’ signature of a less evolved
phase). We note that Garay et al. (1986) argued that the broad RRLs in
G34.26+0.15C could be caused by micro-turbulent motion of dense ionized gas
expanding towards a low-density medium; the difference with the idea proposed
here is that the RRLs are also tracing the ionized flow along a previous
molecular outflow, rather than only the champagne expansion of the HII region.
The scenario proposed here is directly related to the idea of outflow-confined
HII regions, where the outflow cavity begins to be ionized while the infalling
molecular envelope can confine the rest of the HII region; the ionized flow
results in broad RRLs (Tanaka et al. 2016, Tan et al. 2014, Tan & McKee 2003).

Higher angular resolution observations of the OH absorption and RRL velocity
distribution are needed to test this hypothesis, but if confirmed, the
combination of broad RRLs and OH absorption could give us a view into the
inside-out destruction of molecular outflows by ionized flows in the final
phases of high-mass star formation. Higher angular resolution observations
will also help to test alternative hypotheses, such as whether the OH
absorption traces the destruction of a proplyd-like molecular core from a low
mass stellar object at the edge of the HII region or whether the absorption
traces a narrow molecular jet whose blue-shifted axis happens to be in front
of the HII region, but not directly connected to an ionized flow. In addition,
high-angular resolution observations of mm RRLs like H30$\alpha$ are also
needed. Following the argument about the optical depth effects mentioned in
Sewiło et al. (2008) (see their Sect. 3.3), the lower quantum numbers would
trace deeper, denser material, not so much affected by the outer RRL gas
layers that are mainly traced by quantum numbers $>$50\. Optical depth effects
in which RRLs from different energy levels trace different ionized regions
(e.g., Sams et al. 1996) can result in biases in the peak RRL velocity used to
assess the kinematics of molecular absorption. Therefore, a detailed analysis
of RRL emission toward the OH absorption location is required. The need for
high-angular resolution observations to evaluate the interpretation of
molecular absorption is also exemplified by the single-dish detection of an
inverse P-Cygni NH3 profile toward W3(OH), which instead of tracing infalling
material, was caused by different NH3 clouds in emission and absorption (e.g.,
Wilson et al. 1978, Reid et al. 1987, Wilson et al. 1993).

Figure 4: Radio continuum image of the G34.26+0.15 HCHII regions (A and B) and
the cometary UCHII region with embedded continuum cores C1 and C2 are shown in
contours at 8 GHz in the three panels (image from the VLA image archive,
https://www.vla.nrao.edu/astro/nvas/; contours are 0.01, 0.18, 0.37, 0.69,
0.83 $\times\,$0.109 Jy b-1; synthesized beam in shown in the top right
corner). The top panel shows the 6035 MHz OH integrated intensity (moment 0)
of the absorption obtained with the VLA in B-configuration (colors); the
middle and lower panels show in colors the velocity (moment 1) and velocity
dispersion (moment 2) images, respectively. The HCHII regions A and B, and the
two continuum cores of the UCHII region (C1 and C2) are labeled in all panels.
We also show the location of the hot molecular core (HMC) and far infrared
source E from Heaton et al. (1989) and Campbell et al. (2000), respectively
(the top panel shows the approximate angular size of the HMC and infrared
source E). We note that at a distance of 3.0 kpc, 1″ is equivalent to 0.015 pc
or 3000 au. The synthesized beam of the VLA-B moment images is shown in the
lower-left corner. Figure 5: Sketch of a possible origin of the high-velocity
OH absorption. All panels show a side-view representation of the evolution of
the cometary HII region in G34.26+0.15, in all panels the observer is looking
at the star forming region from above (i.e., pole-on in case of panels 3 and
4). Star formation occurs in a Giant Molecular Cloud (panel 1) due to
gravitational collapse (panel 2). A protostar and accretion disk form, leading
to high-velocity molecular outflows/jets (panel 3). High energy UV photons
ionize the molecular gas creating an UCHII region, the ionization front
propagates through the old molecular outflow resulting in high-velocity RRLs,
the remnant of the outflow is also detected by OH absorption (panel 4). We
emphasize that this is a highly simplified sketch, intended to illustrate how
an ionization front might overtake a pre-existing molecular outflow in the
case of pole-on observations. The orientation of outflow shown here is not
intended to describe the particular geometry of the flow with respect to the
HCHII regions, the HMC or the ionized cores C1 and C2.

###  4.2 The Nature of the Excited OH Gas

The detection and non-detection of multiple OH transitions reported in this
article (Figures 1 and 2) allow us to investigate the physical conditions
needed to explain the observed spectra. We use the non-LTE radiative transfer
and line excitation code MOLPOP-CEP (Asensio Ramos & Elitzur, 2018) to model
our multi-transition data assuming homogeneous conditions. As a starting point
to set the basic parameters, we used MOLPOP-CEP to reproduce the model of 3
GHz CH emission reported by Jacob et al. (2021), which included analysis of
G34.26+0.15 observations. Specifically, we were able to reproduce the results
of their Figure A.1, including $T_{kin}=75\,$K, density $n\sim 10^{3}\,$cm-3,
CH linewidth of 6 km s-1, Cosmic Microwave Background (CMB), an interstellar
radiation field (ISRF) with optically thick dust at a temperature of 40 K
($\tau=2$), CH abundance $\chi_{CH}=[CH]/[H_{2}]=3.5\times 10^{-8}$ (A. M.
Jacob, private communication). We then modified the parameters to model the OH
lines, in particular, we explored a grid of densities from n${}_{H_{2}}$ =
$10^{3}$ to $10^{6}$ cm-3 with a step of 5$\times 10^{4}$ cm-3, linewidths
between 1 kms-1 to 60 kms-1 with a steps of 3 kms-1, and kinetic temperatures
between 30 K and 300 K (in steps of 15 K). We used the OH collision rate
coefficients reported by Offer et al. (1994) and an ortho-to-para H2 ratio of
3:1 (e.g., see Reeves & Harteck 1979, but no significant difference in our
models was obtained when using a 1:1 ortho-to-para ratio).

An example of the results of the model is presented in Figure 6, where we show
the predicted velocity-integrated flux density of the lines as a function of
OH column density (solid and dashed thick lines; the figure includes only 6.0
GHz OH transitions). Figure 6 also shows, with horizontal bands, the
integrated flux density measurement of the lines (or best upper limit) as
reported in Table LABEL:tab:OH_Line_Parameters (the 6016 MHz and 6049 MHz OH
measurements are from Arecibo, the 6030 MHz is from the VLA D-configuration
data, and the 6035 MHz is from the VLA B-configuration data). The parameters
used to generate the model were: $n_{H_{2}}=10^{5}\,$cm-3, $T_{kin}=150\,$K,
OH abundance $\chi_{OH}=5\times 10^{-6}$, a full width half maximum $FWHM=17$
km s-1 for the 6016 MHz line and $FWHM=33$ km s-1 for the other lines, and
background continuum from the UCHII region with a brightness temperature
$T_{B,cont}=3000\,$K (obtained from the background continuum of the 6035 MHz
absorption from the VLA-B observations shown in Figure 2, right panels). As
shown in Figure 6, this set of parameters reproduces (within the quoted
uncertainties) the observed OH absorption and non-detection for a column
density ($N_{OH}$) slightly below $10^{16.8}\,$cm-2. We highlight that we used
different linewidths to reproduce the observed integrated flux densities,
which suggests that the weak 6016 MHz OH absorption may not be tracing the
same gas detected in 6030 MHz and 6035 MHz OH absorption. The 6016 MHz OH line
was not imaged with the VLA, hence, we do not know if it originates from the
same projected direction. The Arecibo spectra had to be significantly smoothed
to detect the line, and the spectral line is narrower than the 6030 and 6035
MHz OH absorption (Figure 1), therefore, assuming that the 6016 MHz OH line is
tracing slightly different material is consistent with the available data. We
note that multiple velocity components were used by Hajigholi et al. (2016) to
model the infall velocity structure of G34.26+0.15, therefore, using two
different linewidths to model the OH absorption is reasonable. Moreover, we
know that different transitions of OH can trace significantly different
environments in G34.26+0.15 as the 1665 and 1667 MHz OH masers trace the
eastern edge of the cometary UCHII region; the masers are found in the arc
between the ionized gas and the HMC (as well as associated with the HCHII
region B), and no maser is found toward C2 (Zheng et al., 2000). Similarly,
the ${}^{2}\Pi_{1/2}~{}J=3/2-1/2$ rotational OH transitions observed with
SOFIA, i.e., the transitions connecting the 7.8 GHz ladder (not-observed in
this work) and the 4.7 GHz ladder (detected in this work, Figure 1) show
emission at the systemic velocity with red-shifted self-absorption, and no
indication of the blue-shifted gas reported here (Csengeri et al., 2022).

Despite the simplistic assumptions used to generate Figure 6, our model shows
that reasonable physical conditions can explain the detections and non-
detections of the 6.0 GHz OH lines. For instance, a kinematic temperature of
150 K is consistent with the temperature of molecular gas externally heated by
the UCHII region C (Mookerjea et al., 2007). The density
$n_{H_{2}}=10^{5}\,$cm-3 suggests that the OH is tracing a high-density
component of the outflow (similar to CH in outflows as discussed by Magnani et
al. 1992). We note that temperatures of $\sim 100\,$K and density values
$n_{H_{2}}\sim 10^{5}-10^{6}\,$cm-3 are also the conditions suggested by SiO
observations of the outflow in the high-mass star forming region G5.89$-$0.39
(Acord et al., 1997). Such densities in outflows are also reasonable based on
theoretical models (e.g., Tanaka et al. 2016). Likewise, while the OH column
density consistent with the observed absorption ($\sim 10^{16}-10^{17}\,$cm-2)
is greater than typical values reported for HII regions (Rugel et al., 2018),
such a high value has been obtained toward some high-mass star forming regions
based on excited OH absorption measurements (Baudry & Desmurs, 2002).

The parameters used to generate Figure 6 can also be compared to the multi-
transition modeling of several OH absorption lines (including the 6.0 GHz
lines) detected toward the star forming regions DR21 and K3-50 by Jones et al.
(1992). They found that the observed absorption profiles could be explained by
molecular gas with $T_{k}$ between $\sim$100 and 180 K, $T_{d}<90\,$K, OH
column densities between $10^{15}$ and a few times $10^{16}\,$cm-2, molecular
hydrogen densities between $10^{6}$ and $10^{8}\,$cm-3, and OH abundance
between $10^{-6}$ and $10^{-7}$. This range of physical conditions is similar
to the parameters used to generate Figure 6888We note that Jones et al. (1994)
revisited the models presented in Jones et al. (1992), and reported greater
$T_{d}$ and slightly greater $T_{k}$ values.. As pointed out by Jones et al.
(1992) and Jones et al. (1994), there is a degeneracy between OH abundance and
molecular density. If the OH gas detected here is tracing higher density gas
than $10^{5}\,$cm-3, the abundance value is expected to be smaller than
$5\times 10^{-6}$.

Finally, we note that even though the model can reproduce the observed
absorption of the 6.0 GHz lines, it does not predict emission of the 4.7 GHz
OH transitions detected with Arecibo (Figure 1). However, we found that at
higher densities ($\sim 10^{8}\,$cm-3) and slightly higher temperatures ($\sim
170\,$K), the model predicts absorption of the 6.0 GHz transitions while
population inversion is expected for the 4660 MHz OH line at a column density
of $\sim 10^{15.8}\,$cm-2, and FWHM = 17 km s-1. We note that the model of
Jones et al. (1994) (see also Jones et al. 1992) for higher density molecular
gas than the one used in Figure 6, predicted simultaneous absorption from the
6.0 GHz lines and emission (inverted populations) from the 4.7 GHz lines. We
conclude that LTE conditions cannot explain the observed line ratios. At this
point, however, further modeling of the lines detected with Arecibo (Figure 1)
would be premature, as additional observations with the VLA are needed to
obtain the location of the 4.7 GHz OH emission to establish whether a single
model should be able to explain all spectral profiles, or whether the 4.7 GHz
OH lines are tracing a different volume of gas.

Figure 6: Predicted integrated flux density of the 6016 MHz (blue solid line),
6030 MHz (green solid line), 6035 MHz (red solid line), and 6049 MHz (dark-
cyan dashed line) OH transitions as a function of column density computed with
MOLPOP-CEP (model assumes T${}_{k}=150\,$K, $n_{H_{2}}=10^{5}\,$cm-3,
$~{}\chi_{OH}={5\times 10^{-6}}$, FWHM = 33 km s-1, FWHM6016MHz = 17 km s-1).
The horizontal bands show selected integrated flux density values (including
uncertainties) as listed in Table LABEL:tab:OH_Line_Parameters. The vertical
dashed line shows a column density for which the model matches the
observations.

##  5 Summary

We report multi-transition observations of the 4.7 GHz and 6.0 GHz OH lines
toward the high-mass star forming region G34.26+0.15. The 4.7 GHz observations
were obtained with the 305 m Arecibo Telescope, while the 6.0 GHz OH lines
were observed with Arecibo as well as with the VLA in two configurations, D
and B arrays, resulting in a maximum angular resolution of $\sim 2$″. We
detected emission from the 4750 MHz and 4765 MHz OH lines, and tentative
detection of 4660 MHz OH emission. In contrast, the 6030 MHz and 6035 MHz
lines show broad OH absorption, with a linewidth at zero intensity that
expands over 50 km s-1. The absorption spectra obtained with Arecibo and the
VLA are remarkably similar, which shows that the OH is compact. Indeed, the OH
6035 MHz OH absorption was unresolved at the resolution of the VLA
B-configuration data. The absorption is found toward the ionized core C2 of
the cometary UCHII region. This core of ionized gas is known to show broad RRL
emission; the blue-shifted profile of the RRL line covers the velocity range
of the OH absorption, which strongly suggests that the broad RRL is tracing a
plasma flow that also involves a molecular outflow. We highlight the
possibility that the broad RRL could be tracing the inside-out ionization of a
remnant molecular outflow, which is detected in OH absorption seen pole-on. If
this interpretation is correct, the combination of broad RRLs and high-
velocity excited OH absorption could be indicating the final phase in the
evolution of molecular outflows as they are destroyed by ionized flows from
HII regions, i.e., these could be useful tracers to probe the final stages of
high-mass star formation. We note that other high-mass star forming regions
like DR21 (Guilloteau et al., 1984) and G45.07+0.13 (Tan et al., 2020) also
show blue-shifted 6.0 GHz OH linewings (albeit less extreme than the case of
G34.26+0.15 discussed here), and therefore, excited OH absorption could be
used to investigate the interaction between ionized flows and expanding
molecular gas in other high-mass star forming regions.

We used the non-LTE radiative transfer code MOLPOP-CEP to model the observed
lines. While we were able to find a set of physical conditions that can
explain the observed 6030 and 6035 MHz OH absorption features (and non-
detection of the 6049 MHz line), a different velocity dispersion was needed to
account for a weak 6016 MHz absorption feature, and the model did not predict
emission of the 4.7 GHz lines. The 6016 MHz OH line is significantly weaker
than the 6030 and 6035 MHz OH lines; the 6016 MHz OH line was not detected
with the VLA and the Arecibo spectrum had to be significantly smoothed to
reveal the absorption. High angular resolution and sensitivity observations of
the 6016 MHz and 4.7 GHz OH detections are needed to investigate whether the
transitions are tracing the same material as the 6030 and 6035 MHz OH
absorption lines. Such follow-up observations are needed to reliably explore
whether the OH absorption lines reported in this work are consistent with the
inside-out ionization of a remnant molecular outflow. We highlight that
detectability of the weak spectral lines reported in this work requires large
collecting areas; this work shows the potential of high-sensitivity excited OH
studies that could be conducted toward a large sample of sources using future
instrumentation, such as the ngVLA (Carilli et al., 2015) and a possible Next
Generation Arecibo Telescope (Anish Roshi et al., 2021).

We thank the review and comments from an anonymous referee that helped us
improve this manuscript. E.D.A. acknowledges partial support from NSF grant
AST-1814063 and computational resources donated by the WIU Distinguished
Alumnus Frank Rodeffer. P.H. acknowledges partial support from NSF grant
AST-1814011. This research has made use of NASA’s Astrophysics Data System.
The National Radio Astronomy Observatory (NRAO) is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc. The 305 m Arecibo Telescope was a facility of the National
Science Foundation operated under cooperative agreement by the University of
Central Florida and in alliance with Universidad Ana G. Mendez, and Yang
Enterprises, Inc. The authors would like to specially thank A. M. Jacob for
providing guidance to reproduce results of Jacob et al. (2021) using MOLPOP-
CEP. E.D.A. would like to thank the NRAO for providing access to the New
Mexico Array Science Center (NMASC) cluster for remote data reduction and
analysis, which allowed participation of the high-school student Cade Rigg in
this project. Computational resources from NSF ACCESS JetStream2 (Indiana
University) under project PHY220136 were also used by Cade Rigg. E.D.A. also
acknowledges Ms. Kassie Henry, Ms. Sue Henry and Ms. Sara Guymon for
encouraging and guiding Cade Rigg during his middle and high-school science
projects.

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