# Tracing the layers of Photodissociated gas in Trifid Nebula

Bhaswati Mookerjea Department of Astronomy & Astrophysics, Tata Institute of
Fundamental Research,
Homi Bhabha Road, Mumbai 400005, India<EMAIL_ADDRESS>Göran Sandell
Institute for Astronomy, University of Hawai‘i at Manoa, Hilo,
640 N. Aohoku Place, Hilo, HI 96720, USA

###### Abstract

Photodissociated gas bears the signature of the dynamical evolution of the
ambient interstellar medium impacted by the mechanical and radiative feedback
from an expanding H ii region. Here we present an analysis of the kinematics
of the young Trifid nebula, based on velocity-resolved observations of the
far-infrared fine-structure lines of [C ii] at 158 µm and [O i] at 63 µm. The
distribution of the photodissociated regions (PDRs) surrounding the nebula is
consistent with a shell-like structure created by the H ii region expanding
with a velocity of 5 km s-1. Comparison of ratios of [C ii] and [O i]63 µm
intensities for identical velocity components with PDR models indicate a
density of 104 cm-3. The red- and blue-shifted PDR shells with a combined mass
of 516 $\mathrm{M}_{\odot}$ have a kinetic energy of $\sim 10^{47}$ erg. This
is consistent with the thermal energy of the H ii region as well as with the
energy deposited by the stellar wind luminosity from HD 169442A, an O7 V star,
over the 0.5 Myr lifetime of the star. The observed momentum of the PDR shell
is lower than what theoretical calculations predict for the radial momentum
due to the shell being swept up by an expanding H ii region, which suggests
that significant mass loss has occurred in M20 due to the dispersal of the
surrounding gas by the advancing ionization front.

ISM: clouds – ISM: kinematics and dynamics – submillimetre: ISM – ISM:
structure – stars: formation – ISM:individual (M20)

††journal: APJ

##  1 Introduction

The Trifid nebula (M20) is an H ii region with an age of approximately 0.5 Myr
(Lefloch & Cernicharo, 2000) ionized by the O7 V star HD164492A (Lynds et al.,
1985) located in the Sagittarius spiral arm at a distance of 1180$\pm$25pc
(Kuhn et al., 2021). Figure 1 shows a composite three-color optical image of
M20 observed by Wolfgang Promper from his remote observatory in Namibia with
narrowband filters, O III ($blue$), H$\alpha$ ($green$) and S II ($red$). The
image shows both the large-scale filamentary dust feature and the denser gas
towards the south-west that lie in front of the ionized gas in the H ii region
created by HD 164492A.

The Infrared Space Observatory (ISO) and Hubble Space Telescope (HST)
observations (Cernicharo et al., 1998; Lefloch & Cernicharo, 2000) show the
Trifid to be a dynamic star-forming region containing young stellar objects
(YSOs), many of them undergoing episodes of violent mass ejection or
transferring mass and energy to the nebula in the form of jets. Comparison of
a high-resolution radio continuum image at 20 cm and a CO map suggest that in
its expansion the H ii region has encountered a large dense molecular cloud to
the south-west (SW), possibly triggering the formation of new stars
(Cernicharo et al., 1998). From the Spitzer Infrared Array Camera
(Spitzer/IRAC) and Multi-Band Imaging Photometer for Spitzer (MIPS) images of
the Trifid nebula (M20), Rho et al. (2006) identified about 160 YSOs at
different evolutionary stages distributed around the whole nebula, and beyond.
These cover all stages of proto-stellar evolution, from Class 0 protostars to
more evolved young stellar objects (YSOs). The youngest (Class 0/I protostars
TC0–TC8) of the sources in the region are located within the dense far-IR and
millimetre cores identified in the 1.3 mm continuum map observed by Lefloch et
al. (2002). In some of the dense cores evidence of jets originating from
protostars have been discovered through narrow-band line imaging in the
optical with the HST. Sticking out of the boundaries of one of the cores, TC2
is a 22-arcsec-long irradiated jet HH 399, ending close to a bow shock $\sim
1.5$′ away, in a low density environment (Cernicharo et al., 1998; Rosado et
al., 1999; Yusef-Zadeh et al., 2005). Millimeter-wavelength molecular-line
observations toward the nebula reveal several cloud components at velocities
of v${}_{\rm LSR}\sim$ 2 km s-1 (corresponding to the dark lanes crossing the
optical nebula), v${}_{\rm LSR}\sim 8$ km s-1 (corresponding to dark features
at the edge of the optical nebula), and v${}_{\rm LSR}\sim 18$ km s-1 (Kuhn et
al., 2022; Cernicharo et al., 1998; Torii et al., 2011, 2017). A bridging
feature between the 2 and 8 km s-1 clouds is seen coincident with the optical
nebula and indicates that these clouds are physically interacting and may have
collided (Haworth et al., 2015; Torii et al., 2017; Kalari, 2021).
Alternatively, the cloud kinematics could be interpreted as either an
expanding H ii region or turbulence (Fukuda & Hanawa, 2000; Rho et al., 2008).
Kuhn et al. (2022) used the 13CO(2–1) channel maps to suggest that the 18 km
s-1 cloud which is of much larger spatial extent is likely to be in the
background. Until now studies of photodissociated gas in M20 have only been
for a small region around the TC2 globule using the far- and mid-infrared
fine-structure lines detected with the LWS and SWS onboard ISO (Lefloch et
al., 2002).

Here we present a study of the photodissociated gas in an approximately
400′$\times$400′ region using the far-infrared (FIR) lines of [C ii] at 158
$\mu$m and [O i] at 63 µm that account for most of the cooling. The datasets
used here not only cover a much larger area compared to that studied by
Lefloch et al. (2002) but also have the advantage of having higher spatial and
more importantly spectral resolution compared to the ISO data. Recent
velocity-resolved studies of Galactic PDRs have shown that both the [C ii] and
[O i]63 µm emission are often moderately optically thick and sometimes
significantly self-absorbed (Graf et al., 2012; Mookerjea et al., 2018;
Guevara et al., 2020; Mookerjea et al., 2021). We use the velocity-resolved [O
i]63 µm and [C ii] spectra to characterize the properties and distribution of
the PDR gas vis-a-vis location of these components with respect to the
ionizing source by segregating the major velocity components of both the
diffuse and the dense PDR gas.

##  2 Observational Datasets

For this work, we have used several sets of publicly available data, which are
described below.

###  2.1 GREAT/SOFIA observations

We have retrieved observations of the 2P3/2$\rightarrow$2P1/2 fine structure
transition of ionized carbon (C+) at 1900.5369 GHz (157.74 µm) and the
3P${}_{1}\rightarrow^{3}$P0 transition of atomic oxygen, [O i]63 µm, at
4,744.77749 THz of a region centered at the location of the TC2 globule
(($\alpha_{J2000}$: $18^{h}~{}02^{m}~{}27.22^{s}$, $\delta_{J2000}$:
$-23^{\circ}04^{\prime}$ 27${}^{\prime\prime}\mskip-7.6mu.\,$5) in M20 from
the data archive of the Stratospheric Observatory for Infrared Astronomy
(SOFIA; Young et al., 2012). The observations (Id: 06_0041; PI: M. Kaufman)
were carried out using the German REceiver for Astronomy at Terahertz
frequencies (GREAT; Heyminck et al., 2012) on 2018 June 08. The GREAT was in
the Low Frequency Array/High Frequency Array (LFA/HFA) configuration with both
arrays operating in parallel. The LFA was tuned to [C ii] and the HFA was
tuned to [O i]63 µm. The beam sizes for the [C ii] and [O i]63 µm were
14$\farcs$1 and 6$\farcs$3 respectively. The [C ii] map extends over a region
of 400″$\times$400″, while the usable part of the [O i]63 µm is closer to
300″$\times$ 300″. We used the Level 3b data available on the SOFIA archive
for the region. All data were resampled to a spectral resolution of 0.5 km s-1
resulting in a median rms of 0.5 K for both [C ii] and [O i]63 µm data. The [O
i]63 µm map was smoothed with a Gaussian kernel and regridded to the same
spatial resolution as the [C ii] map, i.e. 14$\farcm$1(Fig. 2).

###  2.2 12CO(3–2) and HCO+(4–3) & (1–0)

For comparison with our observations, we used maps of the $J$=3–2 transition
of 12CO at 345795.9899 MHz at and the $J$=4–3 transition of HCO+ at
356734.2230 MHz, both observed with the James Clerk Maxwell Telescope (JCMT)
using the Heterodyne Array Receiver Program (HARP) receiver with a beam size
of 14″. The observations were performed as part of the proposal M07AH24A. The
spectra were downloaded directly from the JCMT archive at the Canadian
Astronomical Data Centre (CADC). The JCMT maps have a median rms of 0.3 K at a
velocity resolution of 0.5 km s-1. Additionally, HCO+(1–0) spectra observed
with the IRAM 30m telescope as part of the Large Programme (LP017) for Master2
internships from Grenoble University, available at
https://www.iram.fr/ILPA/LP017/ were also used for comparison with the
HCO+(4–3) data.

###  2.3 SEDIGISM

We have used the data cubes for $J$=2–1 transition of 13CO and C18O at
220398.68 and 219560.36 MHz respectively, that were observed as a part of the
large-scale (84 deg2) spectroscopic survey of the inner Galactic disk, named
Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium
(SEDIGISM; Schuller et al., 2021). These data were observed with the APEX
telescope between 2013 and 2016 with an angular resolution of 30″ and a
1$\sigma$ sensitivity less than 1.0 K at 0.25 km s-1 velocity resolution. The
SEGIDISM data were also used by Kuhn et al. (2022).

Figure 1: Composite three-color optical image of M20 observed by Wolfgang
Promper from his remote observatory in Namibia with narrowband filters, OIII
($blue$), H$\alpha$ ($green$) and SII ($red$). The authors have obtained the
permission to use the image available at
https://www.astrobin.com/2c61wh/B/?q=%22HD164294%22 Figure 2: Contours of [C
ii] and [O i]63 µm intensities overlaid on color images of 8 µm Spitzer/IRAC
region. Both maps are integrated between -4 to 14 km s-1. The [C ii] contours
(in K km s-1) correspond to 60 to 280 (in steps of 20) and the contours for [O
i]63 µmare at 0.5, 3 to 30 (in steps of 3), 35, 40, 45, 50\. The [C ii] map is
at the original resolution of 14″, and the [O i]63 µm map is smoothed to the
same resolution. The colorscale (in MJy sr-1) for the 8 µm map is shown to the
extreme right. The three solid straight lines correspond to directions along
which position-velocity diagrams are analyzed (Fig. 7). Other important
sources in the region are marked on both panels and labeled on the the left
panel.

##  3 Molecular Gas Distribution in the region

Figure 3: Map of 13CO(2–1) emission in 1.5 km s-1 wide channels shown as
contours overlaid on the Spitzer/IRAC 8 µm color image. The numbers in square
brackets in each panel indicate the velocity range over which the emission was
integrated. The bottom rightmost panel is an exception that shows intensity
integrated over a wider velocity range of 17 to 22 km s-1. The contours (in K
km s-1) are drawn at 1.5, 3, 4.5, 6, 7 to 28 in steps of 3. For the bottom
right panel the levels (in K km s-1) are at 6 to 18 in steps of 1. The color
bar to the right shows the 8 µm intensity scale in units of MJy sr-1. HD164492
is marked with a star symbol and the sources TC1 and TC2 are marked with ‘+’
signs.

Figures 3 and 10 respectively, show the velocity-channel maps for the
13CO(2–1) and 12CO(3–2) emission in this region. The 13CO(2–1) channel maps
show emission mainly from the rims of the nebula, from the pillar containing
TC2 and at velocities $>$14 km s-1. The 12CO(3–2) channel map shows similar
emission features except for the component beyond 14 km s-1. The 13CO(2–1)
SEDIGISM channel maps were also analyzed by Kuhn et al. (2022), who write that
the 13CO emission is seen at almost any velocity between -4 to 28 km s-1 but
the emission does not necessarily originate from gas at the same distance. In
the Galactic spiral arm model from Reid et al (2016), gas near the Sagittarius
Arm is expected to have vLSR $\approx$ 8 km s-1 and gas in the near Scutum Arm
would have vLSR$\approx$ 18 km s-1. Kuhn et al. (2022) argued that given the
18 km s-1 matches the expected velocity of the near Scutum Arm it is likely
that the CO emission in the velocity range vLSR$\approx$ 13–28 km s-1 comes
from a molecular cloud far behind Trifid. These authors concluded that the
multiple peaks seen in the average spectrum of the region lying between 0 to
13 km s-1 are compatible with the acceleration of dense cloudlets during the
shredding of the molecular cloud by the expanding H ii region created by the
radiative feedback due to HD 164492. They further noted that the observed
velocities and structure are comparable to those obtained from numerical
simulations of cloud disruption by radiative feedback (Ali et al., 2018;
Fukushima et al., 2020).

##  4 Velocity Distribution of Diffuse and Dense PDR Gas

The fine structure lines [C ii] at 158 µm and [O i] at 63 µm are widely
regarded as the bona-fide tracers of the PDR gas. Owing to the significantly
different excitation conditions, the two spectral lines are also useful to
discriminate between the diffuse and dense PDR gas. The [C ii] at 158 µm with
an upper energy level of 91 K and a critical density of 3000 cm-3 is easily
excited under average Galactic ISM conditions in which C+ are found. The [O
i]63 µm line has an upper energy level of 227.7 K and a critical density of 2
$\times 10^{5}$ cm-3. Thus, although atomic oxygen is as abundant as carbon
the [O i]63 emission arises primarily from high density PDRs. Utilising the
velocity-resolved spectra for both tracers we attempt to localize the diffuse
and dense PDR gas in a velocity-coherent manner.

Figure 2 suggests an overall similarity between the distribution of the
velocity-integrated emission of [C ii] and [O i]63 µm when both are integrated
between -4 to 14 km s-1. Both intensity maps trace the southern part of the
region hosting the pillar and the TC2 globule and part of the the dust lanes
seen in the optical image across M20. In order to investigate the kinematics
and distribution of the gas contributing to the emission of [C ii] we examine
the [C ii] spectrum averaged over the entire mapped region (Fig. 4). Using a
multi-component Gaussian fit to the average [C ii] spectrum we identify three
distinct velocity components approximately centered at 2.6, 10.4 and 18.9 km
s-1 – these components match well with the velocity components already
identified in the molecular line data (Fig. 4).

Figure 4: The [C ii] spectrum averaged over the entire mapped region. The red
curve shows a fit to the spectrum using a three-component Gaussian profile
centered at 2.6, 10.4 and 18.9 km s-1. The centers of the Gaussian components
are marked by the vertical dashed lines.

The distribution of the PDR gas is traced more clearly in the channel maps of
[C ii] and [O i]63 µm (Fig. 5, 6). All the three velocity components are seen
in the [C ii] map. The [O i]63 µm map primarily traces the emission of the 8
km s-1 component, but also shows some of the 2 km s-1 velocity component, in
particular the PDR emission from the dark dust lane bisecting the image. This
PDR emission stands out in both [C ii] and [O i]63 µm in the channels from
-2.5 to +0.5 km s-1, in contrast to the 13CO(2–1) and 12CO(3–2) emission which
show very little emission at velocities $<$ -1 km s-1. The velocity component
at +18 km s-1 is strongly contaminated by telluric [O i] emission, which masks
any [O i] emission in that velocity range. The northern ridge containing TC1
was only partially observed in the [O i]63 µm (Fig. 6). The 8 km s-1 component
is clearly visible in [C ii], [O i]63 µm, 13CO(2–1) and 12CO(3–2) channel
maps. Both [C ii] and [O i]63 µm emission trace the pillar-like structure,
with the TC2 globule at its head, at velocities between 5 to 9.5 km s-1,
whereas the emission from the velocities between 8 to 13 km s-1appear to be
also tracing the denser gas located to the south-west of the map. The
velocity-resolved observations suggest that the [C ii] and [O i]63 µm emission
as a function of velocity resemble emission from different photo-irradiated
layers of the molecular cloud impacted by the expanding H ii region. Such a
scenario is further substantiated by the position-velocity ($p$-v hereafter)
diagrams for both [C ii] and [O i]63 µm (Fig. 7) along the three directions
shown in the integrated intensity maps (Fig. 2). The clear circular shell
generated in the [C ii] $p$-v diagram by the 2 km s-1 and 8 km s-1 clouds is
characteristic of the position-velocity diagram due to shell created by an
expanding H ii region. Additionally we notice that for the direction along the
pillar with TC2, while the [O i]63 µm emission is centered at 8 km s-1, the [C
ii] emission shows two branches with the branch around 10 km s-1 being
brighter. The 12CO(3–2)emission traces warmer and denser gas than the
13CO(2–1) emission and though the features seen in the position-velocity
diagrams are closer to the structures seen in the [O i]63 µm, the blue-shifted
half of the shell clearly detected in [C ii] is also partially detected in
12CO(3–2) (Fig. 11).

Figure 5: Map of [C ii] emission in 1.5 km s-1 wide channels shown as contours
overlaid on the Spitzer/IRAC 8 µm color image. The numbers in square brackets
in each panel indicate the velocity range over which the emission was
integrated. The contours are drawn at 0.5, 2.5, 4.5, 6 to 48 (K km s-1) in
steps of 6 K km s-1. The lowermost panel is an exception that shows intensity
integrated over a wider velocity range of 16 to 22 km s-1 with contours drawn
at 15 to 23 K km s-1 in steps of 2 K km s-1. The color bar on the top shows
the 8 µm intensity scale in units of MJy sr-1. HD164492 is marked with a star
symbol and the sources TC1 and TC2 are marked with ‘+’ signs. .  Figure 6: Map
of [O i]63 µm emission in 1.5 km s-1 wide channels shown both a color map and
contours. The numbers in square brackets in each panel indicate the velocity
range over which the emission was integrated. The contours (in K km s-1) are
drawn at 0.5, 1.0, 1.75, 2.5, 3.25, 5 to 20 in steps of 1. HD164492 is marked
with an asterisk and the sources TC1 and TC2 are shown as ‘+’. The color bar
on the top shows the 8 µm intensity scale in units of MJy sr-1. HD164492 is
marked with a star symbol and the sources TC1 and TC2 are marked with ‘+’
signs.  Figure 7: Position-velocity diagrams of [C ii] emission along the cuts
shown in Fig. 2. The contours are for the $p$-v diagram for [O i]63 µm and
correspond to the levels (Left) 1 to 14 K in steps of 1 K, (Middle) 1 to 30 K
in steps of 1 K, and (Right) 0.5, 1 to 35 K in steps of 1 K. The colorbar on
the top of each panel shows the [C ii] intensities in K.

In order to explore whether the split in emission in the 8 km s-1 cloud seen
in [C ii] close to the bright emission regions arises due to opacity effects,
we compared the [C ii] and [O i]63 µm spectra with 13CO(2–1) SEDIGISM spectra
and HCO+(1–0) spectra at twelve selected positions (Fig. 8). We find that at
most positions where the [C ii] spectra are double-peaked in the 8 km s-1
component, the [O i]63 µm, 13CO(2–1) and HCO+ tend to align with one of the
peaks and do not appear at the velocity where the [C ii] spectra show the dip.
Although restricted by the noise level, the C18O(2–1) spectra at those
positions also show the peak to be aligned with one of the peaks, thus
indicating that the two peaks correspond to emission from clouds at two
velocities. The overlapping but distinctly different structures which manifest
in the velocity ranges 6.5 to 8 km s-1 and 9.5 to 11 km s-1 in the [C ii]
channel map strongly suggest that the two peaks in the [C ii] spectrum arise
from different layers of the PDR separated in velocity. Fitting the 8 km s-1
feature in the [C ii] spectra in this region using Gaussian components we
obtain velocities around 8.2 and 11 km s-1 for the two components. The [O i]63
µm spectra at these positions also show two components centered around 7.8 and
10.8 km s-1 while the 13CO(2–1) profiles can be fitted with a single component
lying between 7 and 8.2 km s-1 as the RA offset ($\Delta\alpha$) changes from
-42 to 28″. Of the two velocity components, the $\sim$ 7 km s-1 component
appears to be due to denser gas, since it is detected in both [O i]63 µm and
HCO+(1–0). The $\sim$ 11 km s-1 component in contrast is dominantly seen in
the [C ii] emission and hence is likely to be from more diffuse PDR gas.

Figure 8: Comparison of [C ii] (black), [O i]63 µm (red), 13CO(2–1) (blue) and
HCO+(1–0) (maroon) spectra at selected positions. The positions are shown on
the [C ii] intensity map integrated over -3 to 14 km s-1. The contour levels
in intensity map correspond to 60 to 280 K km s-1 in steps of 20 km s-1 and
the offsets in are with respect to the same center as in Fig. 5. The y-axis
for each spectrum corresponds to $T_{\rm mb}$. The [C ii], 13CO(2–1) and
HCO$+$(1–0) spectra are scaled by factors of 0.11, 0.3 and 1.5 respectively.
The [C ii], [O i]63 µm and 13CO(2–1) spectra are shifted along the y-axis for
better visibility. Vertical dashed lines correspond to 2, 8 and 18 km s-1,
respectively.

###  4.1 Gaussian Decomposition of Spectra

We attempt to identify velocity-coherent features associated with the primary
velocity features in the [C ii], [O i]63 µm and CO datasets. For this purpose
we chose to use [C ii] because of its sensitivity to both diffuse and dense
PDR gas, hence tracing the overall kinematics better than the [O i]63 µm
emission. We have used the FUNStools.Decompose
111https://github.com/radioshiny/funstools to decompose the [C ii] data cubes
into multiple Gaussian components. The tool is based on an algorithm that
primarily decides the number of components and their velocity positions in the
smoothed spectrum using the first, second, and third derivatives based on the
idea that a genuine velocity component is continuous with the surroundings.
The results of the first fitting are given as the initial guess to a
subsequent round of Gaussian fitting in an iterative manner. The outcome are
the parameters such as the central velocity and the linewidth for each
velocity component identified at each pixel in the [C ii] datacube.
Subsequently, we identify coherent structures corresponding to each of the
velocity components so identified by applying the friends-of-friends (FoF)
technique to the central velocity of each of the components. The algorithm
works in an iterative manner in which it first selects the decomposed Gaussian
seed component with the maximum amplitude and gives the structure a number.
Subsequently the other Gaussian components in the neighboring pixels whose
pixel distance is less than $\sqrt{2}$ from the seed component are selected,
and the velocity differences of the seed and other components in the
neighboring pixels are checked. If the the velocity difference between the
seed and other components in the neighboring pixel is less than the velocity
dispersion of the seed ($\sigma_{\nu}^{\rm seed}$) the neighboring component
is identified as a friend of the seed and assigned the same group number. If
more than one velocity component in a neighboring pixel is within the range of
velocity dispersion from the velocity of the seed, then only the closest one
becomes the friend of the seed. In the next iteration the assigned friends of
the seed become the seeds of the structure and the same procedure is repeated
until there are no more friends to assign.

Figure 9: Velocity coherent structures identified in the region based on the
[C ii] spectra using Gaussian decomposition followed by the identification of
groups using a friend-of-friend algorithm are shown as contours. The color
image corresponds to the velocity-integrated [C ii] emission and the
colorscale is shown at the top of the figure. In each panel we indicate the
mean velocity of the group along with the standard deviation in km s-1 on the
left and the contour levels in units of K km s-1 on the right.

Using this method of identification of velocity-coherent structures in the [C
ii] emission, we identify 5 main velocity components with velocities (in km
s-1) centered at -0.1$\pm$0.7, 2.8$\pm$1.5, 6.8$\pm$0.7, 7.7$\pm$0.1,
10.8$\pm$1.4. Additionally we detect the component at 18.7$\pm$0.8 km s-1,
which as mentioned earlier is unrelated to M20 and most probably associated
with the Scutum arm. While the outcome is consistent with the channel maps,
decomposition of the spectra into Gaussian components helps us to isolate the
overlapping features. This is particularly true for the features with mean
velocities at 6.8, 7.7 and 10.8 km s-1.

A combination of the features seen in the channel maps of [C ii], and [O i]63
µm and grouping of the components identified in the Gaussian decomposition of
[C ii] spectra provides us with a three-dimensional view of the multiple
layers of PDR shells associated with HD164492A.

##  5 Discussion

###  5.1 Properties of the PDR shells

The velocity-resolved spectra of [C ii] and [O i]63 µm have enabled us to
identify the red- and blue-shifted parts of the far-ultraviolet (FUV; 6-13.6
eV) irradiated molecular shell created by the expanding H ii region in M20.
The structure seen particularly in the [C ii] $p$-v plots (Fig. 7) along
selected directions suggests that the emission between -5 to 5 km s-1 belongs
to the blue-shifted (foreground) part of the shell, while the 5 to 15 km s-1
emission corresponds to the red-shifted part. The Gaussian decomposition of
the spectra followed by identification of velocity-coherent emission features
identifies two and three sub-structures in the blue- and red-shifted parts of
the shell, respectively (Fig. 9). The detection of brighter [O i]63 µm
emission from the red-shifted part of the shell indicates that it has higher
density than the blue-shifted part. Previous studies of Galactic PDRs have
shown that the [C ii] emission is often optically thick, while the [O i]63 µm
spectra are affected by foreground low-excitation gas with atomic oxygen.
Since we did not detect [13C ii], which would have enabled us to the determine
the optical depth of the [C ii] emission, we instead use the results from
Lefloch et al. (2002), who estimated that the kinetic temperature of the PDR
gas was 300 K, to derive the column density $N$(C+). Since it is likely that
the 5–15 km/s PDR emission has high density, for this part of the shell we
adopt an optical depth of 2, a value typically seen in Galactic PDRs
(Mookerjea et al., 2018). We assume the -5 to +5 km/s PDR emission to be
optically thin. Equation (26) from Goldsmith et al. (2012) when modified for
optically thick emission for the red-shifted shell, is as follows:

$N(C^{+})={\rm 2.91\times 10^{15}\left[1+0.5{\rm
e^{91.25K/T_{kin}}}\left(1+\frac{A_{ul}}{C_{ul}}\right)\right]\frac{\tau}{1-e^{-\tau}}\int
T_{\rm mb}d\upsilon}$ (1)

where $A_{\rm ul}$ = 2.3$\times 10^{-6}$ s-1, $T_{\rm kin}$ is the gas kinetic
temperature, the collision rate is $C_{\rm ul}$ = $R_{\rm ul}n$ with $R_{\rm
ul}$ being the collision rate coefficient with H2 or H0, which depends on
$T_{\rm kin}$, and $n$ is the volume density of H. Since the critical density
of the [C ii] transition is $n_{\rm cr}$=3000 cm-3, and since it is likely
that most of the [C ii] detected could be at such densities along with some
emission arising from clumps with densities exceeding 105 cm-3, we assume a
density of 104 cm-3 to estimate the $N$(C+) density distribution of the PDR
gas. For this calculation we consider excitation of [C ii] due to C+–H2
collisions, with $R_{ul}$ = 3.8$\times 10^{-10}$ cm3 s-1. The value of $N$(C+)
estimated for the red-shifted shell is between (1–3)$\times 10^{18}$ cm-2,
while for the blue-shifted shell $N$(C+) ranges between (2–9)$\times
10^{17}$cm-2. Based on LWS observations with a beam size of 45″ Lefloch et al.
(2002) had estimated that $N$(C+) in the PDR of the globule is around
(4.5–5.7)$\times 10^{17}$ cm-2, after subtracting a 30% contribution to [C ii]
emission from the ionized gas in the H ii region. At TC2, we estimate a total
$N$(C+) of 1.2$\times 10^{18}$ cm-2 and if 30% of the [C ii] emission were to
arise from the ionized gas, the contribution of the PDR will be $\sim
8.4\times 10^{17}$ cm-2. Within the limits of the uncertainties this estimate
is consistent with the values determined by Lefloch et al. (2002).

In order to examine the properties of the PDRs created in the FUV irradiated
parts of the red- and blue-shifted molecular shells we derive the [C ii]/[O
i]63 µm intensity ratios for both components. The area over which the ratio
can be determined is restricted by the usable pixels of the [O i]63 µm map.
For the red and the blue-shells the [C ii]/[O i]63 µm intensity ratios (in
energy units) vary between (0.8 to 2.5) and (1 to 1.5), respectively. Lefloch
et al. (2002) estimated the FUV field intensity at the location of the globule
TC2 due to HD 164492A to be $G_{0}=1000$. Comparison of the observed [C ii]/[O
i]63 µm intensity ratios with the results of the Kaufman PDR model (Kaufman et
al., 2006) for an FUV intensity of $G_{0}$=1000 suggest densities of 4$\times
10^{3}$ and 500 cm-3 for the red and the blue-shifted shells. Based on the
detection of [O i]63 µm emission, it appears that these densities are lower
limits. The lower derived values of the density could be because the volume
and hence beam-filling factor of the [C ii] emission is likely to be
significantly larger than the [O i]63 µm emission that arise from high density
PDRs and this could lead to higher values of observed [C ii]/[O i]63 µm
intensity ratios, which imply lower densities. At the position corresponding
to TC2 (40″, 17″) we have fitted the [O i]63 µm spectrum using a single
velocity component Gaussian profile resulting in $v_{\rm LSR}$ = 9.5 km s-1
and $\Delta v$ = 1.6 km s-1. The [C ii] spectrum at this position between -8
to 17 km s-1 shows three velocity components, hence we have fitted it with
three Gaussians. While fitting the [C ii] profile we kept the center and the
width of one of the components fixed at the fitted values obtained for the [O
i]63 µm profile. Thus, for this velocity component, which is seen in both the
[C ii] and [O i]63 µm emission, we estimate a [C ii]/[O i]63 µm intensity
ratio (in energy units) of 0.2. For such a ratio and a G0 = 1000, the PDR
models indicate a density of 104 cm-3.

###  5.2 Kinematics of the PDR shells

Based on the [C ii] intensities, we estimate the total integrated column
densities ($N$(C+)) of the blue- and the red-shifted shells to be 5.9$\times
10^{20}$ and 2.4$\times 10^{21}$ cm-2, respectively. Considering a C/H2 ratio
of 1.5$\times 10^{-4}$ at the distance of M20 (1180 pc), the masses of the
blue- and red-shifted shells are estimated to be 100 and 416
$\mathrm{M}_{\odot}$, respectively. Based on the shells identified in the
position-velocity plots of [C ii] (Fig. 7), we estimate the median expansion
velocity of the shells to be $\sim 5$ km s-1. Thus the total kinetic energy of
the PDR shells is approximately 1.3$\times 10^{47}$ erg. Chaisson & Willson
(1975) derived a few parameters for the H ii region M20, created by the star
HD169942A: it extends over 5$\farcm$3$\times$6$\farcm$4, which at a distance
of 1180 pc corresponds to a size of 1.8$\times$2.2 pc, has an average electron
density ($n_{\rm e}$) of 100-150 cm-3 and has an electron temperature ($T_{\rm
e}$) of 8150 K. García-Rojas et al. (2006) estimated the electron number
density to be 270$\pm 60$ cm-3. The total thermal energy of the H ii region
written as ${\rm E_{th}=\frac{3}{2}n_{e}\frac{4\pi}{3}\,R^{3}k_{B}T_{\rm e}}$,
where ${R}$ is the radius of the H ii region and $k_{\rm B}$ is the Boltzmann
constant. Based on these parameters we estimate the total thermal energy of
the H ii region to be 6$\times 10^{46}$ erg. Thus, subject to uncertainties of
the electron density in the H ii region, the kinetic energy of the PDR shell
is about two times the thermal energy of the H ii region.

The photoionized gas in the interior of an expanding H ii region exerts a
pressure force and delivers outward radial momentum and kinetic energy to the
swept-up shell. For a spherical H ii region the momentum delivered to the
ambient medium is (Krumholz, 2017):

$p_{\rm sh}=1.5\times 10^{5}\left[\frac{n_{\rm
H}}{10^{2}}\right]^{-1/7}\left[\frac{T_{e}}{10^{4}}\right]^{-8/7}\left[\frac{N_{\rm
Lyc}}{10^{49}}\right]^{4/7}\left[\frac{t}{10^{6}}\right]^{9/7}\,M_{\odot}\mbox{
km s}^{-1}.$ (2)

where nH is the number density of hydrogen in the ambient medium, $T_{\rm e}$
is the electron temperature, $N_{\rm Lyc}$ is the Lyman continuum photon rate
due to HD164492A and $t$ is the age of the H ii region. Lefloch & Cernicharo
(2000) estimated the density of the shell to be 105 cm-3 and mean density of
the cloud to be 2$\times 10^{3}$ cm-3. We note that the assumption of the H ii
region expanding in a uniform density environment could lead to uncertainties
in the determination of its age and subsequently in the value of the density
as well. In order to include the effects of higher density material that the
expanding H ii region would necessarily encounter in an otherwise clumpy
molecular cloud, we assume the ambient density to be 104 cm-3. For HD 164492A,
an O7V type star the Lyman continuum photon rate is $5.6\times 10^{48}$ s-1
(Martins et al., 2005). Using Eq. (2) for the estimated age of the M20 nebula
of 0.5 Myr we estimate the total momentum of the PDR shell to be 2.8$\times
10^{4}$ $\mathrm{M}_{\odot}$ km s-1. From the observations we estimate the
momentum of the PDR shell with a mass of 516 $\mathrm{M}_{\odot}$ moving with
a velocity of 5 km s-1 to be $\sim 2.6\times 10^{3}$ $\mathrm{M}_{\odot}$ km
s-1, which is lower by a factor of 10 than the $p_{\rm sh}$ estimated from
Eq.(2). This could be due to the molecular (and photodissociated) material
which might have been dispersed, hence carrying away part of the momentum.

The kinetic energy of the swept up shell is given by (Krumholz, 2017):

$E_{\rm sh}=8.1\times 10^{47}\left[\frac{n_{\rm
H}}{10^{2}}\right]^{-10/7}\left[\frac{T_{e}}{10^{4}}\right]^{10/7}\left[\frac{N_{\rm
Lyc}}{10^{49}}\right]^{5/7}\left[\frac{t}{10^{6}}\right]^{6/7}\,\mbox{erg}$
(3)

The kinetic energy of the swept up shell estimated using Eq. (3) is 3.1$\times
10^{44}$ erg, which is much smaller than the observed value by a factor of
more than 320. It is clear that a simplistic model of an H ii region expanding
thermally in a uniform density environment cannot provide the observed kinetic
energy of the shell. on the other hand, the wind luminosity from HD169942A is
estimated to be 1.7$\times 10^{36}$ erg s-1 (Rho et al., 2008; Howarth &
Prinja, 1989; Prinja et al., 1990), indicating that the wind has pumped in a
total kinetic energy of 5.2$\times 10^{46}$ erg over the 0.5 Myr lifetime of
the Trifid nebula. This number matches quite well with the observed kinetic
energy of the PDR shells as well as with the thermal energy of the ionized gas
in the nebula.

##  6 Summary & Conclusions

We have analysed the distribution of the PDR surfaces created by the FUV
radiation from the O-type star HD 164492 that illuminates the H ii region
associated with the M20 nebula based on decomposition of the [C ii] emission
into velocity components. We disentangle the high density PDR material using
the [O i]63 µm emission. The structure of the PDRs is consistent with shells
created by the H ii region expanding approximately with a velocity of 5 km
s-1. Based on the [C ii]/[O i]63 µm intensity ratios we estimate the densities
of the red and the blue-shifted parts to be 4000 and 500 cm-3, which are
likely to be the lower limits considering the possible mismatch between the
beam-filling factors of the [C ii] and the [O i]63 µm emission. The ratio,
when estimated by considering the intensities of velocity components seen in
both the [C ii] and [O i]63 µm emission, suggest a density of 104 cm-3, which
is consistent with the ambient PDR density determined by Lefloch et al.
(2002). The total kinetic energy estimated for the shells, $\sim 10^{47}$ erg,
is consistent with both the thermal energy of the H ii region as well as the
total kinetic energy pumped in by the stellar wind during the lifetime (0.5
Myr) of the nebula. This is also an indirect confirmation of the age of the
nebula. The total momenta of the detected parts of the shell is about
2.6$\times 10^{3}$ $\mathrm{M}_{\odot}$ km s-1, which is about ten times less
than the momentum that a spherical H ii region with the physical parameters of
M20 can deliver. The presence of “holes” on either side of the dark lanes,
which essentially are remnants of the blue-shifted part of the shell, also
suggest that a significant part of this shell (molecular and/or PDR) has been
blown away by the H ii region and by the stellar wind. The material thus lost
would have carried with it significant momentum and hence explains the
discrepancy between the measured momentum of the shell and the momentum that
the H ii region is likely to deliver.

Radiative and mechanical feedback due to the massive stars both at the end of
their lives (in the form of supernovae) and in their youth (as expanding H ii
regions and stellar winds) influence the triggering and/or hindrance to the
formation of new generation of stars. The relative importance of the H ii
regions and the stellar winds in shaping the structure of the ambient
interstellar medium is of interest in the context of the evolution of the
Galactic interstellar medium and star formation scenario (cf. Pabst et al.,
2020; Tiwari et al., 2021, and references therein). This work presented
observational evidence of the role of the stellar winds in shaping the
structure of the putative sites of triggered star formation in the Trifid
nebula. The velocity information in the [C ii] data enabled us to
quantitatively compare the stellar wind inputs with the kinetic energy of the
PDR shells and confirm that in this case the stellar winds provide the energy
for expansion rather than the thermal expansion of the H ii region. For a more
comprehensive understanding of the kinematics of the M20 nebula, extending [C
ii] map to the north and obtaining a larger [O i]63 µm map would enable a
quantitative analysis of TC1 similar to the analysis done by Lefloch et al.
(2002) for TC2. Such studies are crucial to obtain an improved understanding
of the role of stellar feedback in determining the structure of the ambient
medium that possibly trigger the formation of a new generation of stars in the
region.

BM acknowledges the support of the Department of Atomic Energy, Government of
India, under Project Identification No. RTI 4002 and the software related
discussions with Akhil Lasrado. This research has made use of the NASA/IPAC
Infrared Science Archive, which is funded by the National Aeronautics and
Space Administration and operated by the California Institute of Technology.
This publication is based on data acquired with the Atacama Pathfinder
Experiment (APEX) under programmes 092.F-9315 and 193.C-0584. APEX is a
collaboration among the Max-Planck-Institut fur Radioastronomie, the European
Southern Observatory, and the Onsala Space Observatory. The processed data
products are available from the SEDIGISM survey database located at
https://sedigism.mpifr-bonn.mpg.de/index.html, which was constructed by James
Urquhart and hosted by the Max Planck Institute for Radio Astronomy. This
research used the facilities of the Canadian Astronomy Data Centre operated by
the National Research Council of Canada with the support of the Canadian Space
Agency. This work is based [in part] on observations made with the Spitzer
Space Telescope, which is operated by the Jet Propulsion Laboratory,
California Institute of Technology under a contract with NASA. This dataset or
service is made available by the Infrared Science Archive (IRSA) at IPAC,
which is operated by the California Institute of Technology under contract
with the National Aeronautics and Space Administration. Here we present
additional CO(3-2) channel maps and position-velocity diagrams.

##  Appendix A Channel map and Position-Velocity Plots for 12CO(3–2)

Figure 10: Map of 12CO(3–2) emission in 1.5 km s-1 wide channels shown as
contours overlaid on the Spitzer/IRAC 8 µm color image. The numbers in square
brackets in each panel indicate the velocity range over which the emission was
integrated. The bottom rightmost panel is an exception that shows intensity
integrated over a wider velocity range of 14 to 20 km s-1. The contours are
drawn at 0.5, 2.5, 4.5, 6 to 48 (K km s-1) in steps of 6 K km s-1. The color
bar to the right shows the 8 µm intensity scale in units of MJy sr-1. HD164492
is marked with a star symbol and the sources TC1 and TC2 are marked with ‘+’
signs.  Figure 11: Position-velocity diagrams of 12CO(3–2) emission along the
cuts shown in Fig. 5 shown as contours overplotted on the color image of the
[C ii] $p$-v diagram. The contour levels are for $T_{\rm mb}$ from 1 to 32 K
in steps of 3 K for the left panel and from 1 to 27 K in steps of 2 K for
right panel. The colorbar on the top of each panel shows the [C ii]
intensities in K.

IRSA, Spitzer, Herschel, SOFIA

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