# Sh 2-301: a blistered H ii region undergoing star formation

Rakesh Pandey  Aryabhatta Research Institute of Observational Sciences
(ARIES), Manora Peak, Nainital 263 002, India  School of Studies in Physics
and Astrophysics, Pt. Ravishankar Shukla University, Raipur, (C.G.), 492010,
India  Saurabh Sharma Aryabhatta Research Institute of Observational Sciences
(ARIES), Manora Peak, Nainital 263 002, India  Lokesh K. Dewangan  Physical
Research Laboratory, Navrangpura, Ahmedabad - 380 009, India  Devendra K. Ojha
Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Colaba,
Mumbai - 400 005, India  Neelam Panwar  Aryabhatta Research Institute of
Observational Sciences (ARIES), Manora Peak, Nainital 263 002, India  Swagat
Das  Indian Institute of Science Education and Research(IISER), Tirupati,
517507, India  D. P. Bisen  School of Studies in Physics and Astrophysics, Pt.
Ravishankar Shukla University, Raipur, (C.G.), 492010, India  Arpan Ghosh
Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak,
Nainital 263 002, India  School of Studies in Physics and Astrophysics, Pt.
Ravishankar Shukla University, Raipur, (C.G.), 492010, India  Tirthendu Sinha
Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak,
Nainital 263 002, India

(Received —-, 2021; Revised —-, 2021; Accepted —-, 2021)

###### Abstract

We present a multiwavelength study of the H ii region Sh 2-301 (S301) using
deep optical data, near-infrared data, radio continuum data and other archival
data at longer wavelengths. A cluster of young stellar objects (YSOs) is
identified in the north-east (NE) direction of S301. The H$\alpha$ and radio
continuum images trace the distribution of the ionized gas surrounding a
massive star ALS 207, and the S301 H ii region is bounded by an arc-like
structure of gas and dust emission in the south-eastern direction. The north-
western part of S301 seems to be devoid of gas and dust emission, while the
presence of molecular material between the NE cluster and the central massive
star ALS 207 is found. The distribution of warm dust emission, ionized gas,
and neutral hydrogen together suggests a blistered morphology of the S301 H ii
region powered by ALS 207, which appears to be located near the edge of the
cloud. The location of the NE cluster embedded in the cold molecular cloud is
found opposite to the blistered morphology. There is a noticeable age
difference investigated between the massive star and the NE cluster. This age
difference, pressure calculation, photodissociation regions (PDRs), and the
distribution of YSOs favour the positive feedback of the massive star ALS 207
in S301. On a wider scale of S301, the H ii region and the young stellar
cluster are depicted toward the central region of a hub-filamentary system,
which is evident in the infrared images.

stars: luminosity function, mass function – stars:formation – dust, extinction
– H ii regions

##  1 Introduction

Previous studies of bubbles associated with the H ii regions suggest that
their expansion probably triggers 14% to 30% of the star formation in our
Galaxy (e.g., Deharveng et al., 2010; Thompson et al., 2012; Kendrew et al.,
2012). Feedback from the massive stars ionizes the surrounding molecular cloud
through their immense UV radiation and powerful winds, resulting in an H ii
region, a glowing nebula of ionized gas. Lopez et al. (2014) have discussed
the various feedback processes in detail. The feedback from the massive star
may inhibit or terminate further star formation in the immediate vicinity
(termed as ‘negative feedback’) or it can also promote and accelerate the star
formation (known as ‘positive feedback’). Out of these two outcomes which one
will dominate depends not only on the process itself but also on the property
of clouds (Shima et al., 2017). Stars born out of these processes are
generally assembled in a group or cluster, an entity having a collection of
physically-related stars. The physical features such as shape, size, age, mass
distribution of these clusters vary with their hosting environment and seems
to show the imprints of star formation processes itself (Lada & Lada, 2003;
Allen et al., 2007; Grasha et al., 2017, 2018). Therefore, stellar clusters
constitute the nearest laboratories for direct astronomical investigation of
the physical processes of star formation and early evolution. The first
generation of massive stars in these regions also play a very important role
as they can give very crucial clues on the star formation through entirely
different physical processes. Thus, we can have a mix of stars in the same
star-forming region, giving hints of their origin. Dale et al. (2015) have
discussed other star formation processes like cloud-cloud collision,
filamentary interactions etc. In recent years many authors have pointed out
the active role of filamentary structures and their subsequent interaction in
star formation (Schneider et al., 2012; Dewangan et al., 2017b, 2020a).
Filamentary structures are often seen harboring young stellar clusters and
massive star-forming clumps, yet their precise role in star formation is not
very well understood.

With an aim to investigate the stellar clustering and their origin, star
formation, shape of the mass function (MF), and effects of the feedback from
massive stars on these processes, we have performed a multiwavelength study of
the H ii region ‘Sh 2-301’ (hereafter, S301). This is a southern sky
($\alpha$2000 =07h09m55s, $\delta$2000 = -18$\arcdeg$29$\arcmin$36$\arcsec$)
Galactic H ii region located in a very large molecular cloud complex ($\sim
6^{\circ}\times 3^{\circ}$) (Russeil et al., 1995). Avedisova & Palous (1989)
placed this region in a star-forming region ‘SFR 231.44-4.41’ along with
reflection nebulae Bran 6 and Bran 7. This region is thought to be ionized by
a massive O type star ALS 207 and also harbors two B type stars ALS 208 and
ALS 212 (Moffat et al., 1979; Garmany et al., 2015; Maíz Apellániz et al.,
2016). Despite being showing very interesting features at different
wavelengths, this region is one of the poorly studied H ii regions in our
Galaxy. In Figure 1, we show the color-composite image made by using the 3.4
$\mu$m (red), $K$-band (green), H$\alpha$ (blue) images. Clearly, the image
shows a heated environment in the central region near massive star ALS 207 as
indicated by the H$\alpha$ emission. The heated region is surrounded by gas
and dust indicated by the infrared (IR) emissions. All these morphologies
suggest a prominent feedback from the massive star which is influencing its
surroundings. Thus, S301 is an ideal site for our investigation of feedback of
massive star in the region.

We organize this work as follows. In Section 2, we describe the optical/IR
observations and data reduction along with the archival data sets used in our
analysis. In Section 3 we describe the schemes to study the stellar densities,
membership probability, distance, reddening, age, MF, identification of YSOs
etc. The main results of the present study are summarized and discussed in
Section 4 and we conclude in Section 5.

Figure 1:  Color-composite image of the $\sim 18^{\prime}.5\times
18^{\prime}.5$ FOV around S301 H ii region obtained using the WISE 3.4 $\mu$m
(red), $K$-band (green) and UK Schmidt telescope (UKST) H$\alpha$ (blue).The
locations of three massive stars ALS 207, ALS 208 and ALS 212 are also shown
in the figure by star symbols (Moffat et al., 1979). The white circle
represents the extent of the clustering identified in the present analysis
(cf. Section 3.2). The stellar surface density of the NIR sources generated
using nearest neighbor method (cf. Section 3.2) is shown with yellow contours.
The lowest contour is 1 $\sigma$ above the mean value of stellar density
(i.e., 2.6 stars/arcmin2) where the step size is 1$\sigma$=2 stars/arcmin2.
The cyan box represents the FOV covered in NIR (TIRSPEC) observation.

##  2 Observation and data reduction

###  2.1 Imaging data

We have observed S301 region in broad-band optical ($UBV{(RI)}_{c}$) filters
using the 1.3 m Devasthal fast optical telescope (DFOT) of Aryabhatta Research
Institute of Observational Sciences (ARIES), Nainital, India. The imaging
camera on the telescope is a 2K$\times$2K CCD covering $\sim
18^{\prime}.5\times 18^{\prime}.5$ field-of-view (FOV) of the sky. The readout
noise and gain of the CCD are 8.29 $e^{-}$ and 2.2 $e^{-}$/ADU, respectively.
The images of target field (S301) and standard field (SA98) ($\alpha_{J2000}$:
06h52m14s, $\delta_{J2000}$: -00$\arcdeg$18$\arcmin$59$\arcsec$, Landolt,
1992), along with flat and bias frames were taken during the observations. We
have also observed S301 in broad-band Near-IR (NIR, $JHK$) filters using the
TIFR Near Infrared Spectrometer and Imager
(TIRSPEC)111http://www.tifr.res.in/$\sim$daa/tirspec/ mounted on the 2 m
Himalayan $Chandra$ Telescope (HCT), Hanle, Ladakh, India. The specifications
of the instrument and the detector array are mentioned in Ninan et al. (2014).
The FOV of the instrument is $307^{\prime\prime}\times 307^{\prime\prime}$ in
the imaging mode. We took 9 pointing covering $15^{\prime}\times 15^{\prime}$
FOV around the central massive star (ALS 207). Five dithered positions with 7
frames of 20 secs in each position have been used to create average combined
images in the $J,H$ and $K$ filters. The complete log of observation is
provided in the Table 1. For image cleaning, photometry and astrometry, we
have used standard data reduction procedures as mentioned in Pandey et al.
(2020b).

We have followed the procedures outlined by Stetson (1992) for the calibration
of the optical instrumental magnitudes to the standard magnitudes (Vega) by
using following calibrations equations:

$\begin{split}u&=U+(5.328\pm 0.021)-(0.046\pm 0.01)(U-B)\\\ &+(0.490\pm
0.061)X_{U},\end{split}$ (1) $\begin{split}b&=B+(3.292\pm 0.007)-(0.127\pm
0.006)(B-V)\\\ &+(0.208\pm 0.006)X_{B},\end{split}$ (2)
$\begin{split}v&=V+(2.488\pm 0.007)+(0.072\pm 0.005)(V-I_{c})\\\ &+(0.131\pm
0.009)X_{V},\end{split}$ (3) $\begin{split}r_{c}&=R_{c}+(1.797\pm
0.006)+(0.102\pm 0.010)(V-R_{c})\\\ &+(0.074\pm 0.007)X_{R},\end{split}$ (4)
$\begin{split}i_{c}&=I_{c}+(2.529\pm 0.009)-(0.013\pm 0.006)(V-I_{c})\\\
&+(0.011\pm 0.007)X_{I}\end{split}$ (5)

where, $U,B,V,R_{c},I_{c}$ and $u,b,v,r_{c},i_{c}$ are the standard and
instrumental magnitudes of the standard stars in the SA98 field, respectively.
Instrumental magnitudes were normalized for the exposure time and X is the air
mass at the time of observation.

The instrumental magnitudes in the NIR bands were transferred to the standard
magnitudes (Vega) by using the following transformation equations:

$(J-K)=(0.99\pm 0.02)\times(j-k)+(0.61\pm 0.01)$ (6) $(H-K)=(0.94\pm
0.04)\times(h-k)+(0.66\pm 0.01)$ (7) $(K-k)=(-0.12\pm
0.05)\times(H-K)+(-4.95\pm 0.01)$ (8)

where, $JHK$ and $jhk$ are the standard and instrumental magnitudes of the
common stars from the 2MASS catalog and HCT observations, respectively. The
coefficients for the above equations were generated separately for each
pointing.

We have compared our derived standard magnitudes with the available $V$ and
$B$ band standard magnitudes in ‘APASS’222The AAVSO Photometric All-Sky
Survey, https://www.aavso.org/apass archive and the photometric agreement is
found to be reasonable.

Finally, we have made a photometric catalog by taking only those stars which
are having photometric errors $<$0.1 mag. Photometry of the stars that were
saturated in our deep observations, have been taken from the short exposures
and 2MASS catalog for the optical and NIR bands, respectively. The number of
sources detected in different bands and their detection limits are given in
Table 2.

####  2.1.1 Completeness of the photometric data

Due to nebulosity, crowding of the stars, detection limit, etc the photometric
data may be incomplete. It becomes very important to know the completeness
limit in terms of mass for MF analyses. We have used the procedure outlined in
Sharma et al. (2008) to determine the completeness factor. In this method we
artificially added stars of known magnitudes and position in the original
images using the $IRAF$ routine $ADDSTAR$. After getting images having
artificially added stars, we performed photometry of those images as we did
for the original images (keeping all the parameters same). Finally, the
completeness factor can be easily determined by taking the ratio of the number
of stars recovered to the added stars, in different magnitude bins as a
function of magnitude. In Figure 2, we have shown the completeness factor in
different bands as a function of magnitude. As expected, the completeness of
the data decreases with fainter magnitudes. The completeness limits in terms
of magnitudes and mass in different bands are given in Table 2.

Figure 2:  Completeness levels as a function of magnitude in $V,I,J,H,K$
filters derived from the artificial star experiments (ADDSTAR, see Section
2.1.1 for details).

###  2.2 Spectroscopic data

We have performed the spectroscopic observations of three bright sources ALS
207, ALS 208 and ALS 212 (see Figure 1) in the S301 region, using Hanle Faint
Object Spectrograph Camera (HFOSC) instrument mounted on the HCT, Hanle,
India. The observations were performed with GRISM 7 (3800-6840 Å ) with a
resolution of 1200. The spectroscopic standard star was also observed in the
same night for flux calibrations. The log of the spectroscopic observation is
provided in Table 1.

Spectroscopic data reduction is done with IRAF packages using standard
procedures (Jose et al., 2012). For wavelength calibration, FeAr and FeNe arc
lamps were observed during each night. For flux calibration, standard stars
such as Feige 34, Feige 110 and HZ 44 were observed. Aperture extraction,
identification of lines using lamps and dispersion correction are done by
tasks apall, identify and dispcor, respectively. standard, sensfunc and
calibrate tasks are used for flux calibration. Finally, the spectrum is
normalized using the task continuum in IRAF. The flux-calibrated normalized
spectra of the three sources are shown in the Figure 3.

Figure 3:  The flux-calibrated and normalized spectra of the sources ALS 207,
ALS 208 and ALS 212, shown with cyan, green and yellow color curves,
respectively. For comparison, we have also plotted the standard spectra from
the spectral library of Jacoby et al. (1984) in red color. The important
spectroscopic lines are also marked in the figure.

###  2.3 Radio continuum data

Radio continuum observation of the S301 region at 1280 MHz was carried out
using GMRT facility on 2018 December 22 (Proposal Code: $35\\_107$; PI: Rakesh
Pandey). We have observed the flux calibrators for $\sim$ 15 mins in the
beginning and end of the observation sequence. The phase calibrator is
observed periodically along with the target source. In each period, we
observed the target for $\sim$ 30 minutes and the phase calibrator for $\sim$
5 minutes. The data has been reduced using the Astronomical Image Processing
System (AIPS). We used VLA calibrators 3C147 and 3C286 as flux calibrators and
0744-064 as a phase calibrator. Tasks tvflg and Calib were used to remove the
bad data points and calibrations, respectively. After getting an image by
doing several rounds of self calibration, primary beam correction was done
using the task pbcor. In this way, we got our final reduced image at 1280 MHz
with a synthesized beam size of $\sim$ $7^{\prime\prime}.5\times
3^{\prime\prime}.4$.

###  2.4 Archival data

We have used the point source catalog from the 2MASS in NIR (Cutri et al.,
2003) and Wide-field Infrared Survey Explorer (WISE) (Wright et al., 2010) in
mid-IR (MIR) from the IRSA, the NASA/IPAC Infrared Science
Archive333https://irsa.ipac.caltech.edu/applications/Gator/. We have used the
recently released Gaia DR3444https://gea.esac.esa.int/archive/ data for the
membership determination (Gaia Collaboration et al., 2016, 2018). Apart from
that we have also used the multiband images from the surveys like
AAO/UKST555https://www.roe.ac.uk/ifa/wfau/halpha/,
2MASS666https://skyview.gsfc.nasa.gov/current/cgi/query.pl,
WISE777https://skyview.gsfc.nasa.gov/current/cgi/query.pl, AKARI
888https://irsa.ipac.caltech.edu/data/AKARI/ and Planck
999https://www.ipac.caltech.edu/project/planck (cf. Pandey et al., 2020b).

To trace the distribution of neutral hydrogen gas towards the S301 region, we
have also used the H i (21 cm) line data from the ‘HI Parkes All Sky Survey’
(HIPASS)101010https://www.atnf.csiro.au/research/multibeam/release/. The
angular resolution of the HIPASS data is 15′.5. The channel spacing is 13.2 km
s-1 and the velocity resolution is 18.0 km s-1.

##  3 Results and Analysis

###  3.1 Spectral analysis of the bright stars

The wavelength calibrated spectra of the bright stars in the S301 region are
shown in the Figure 3. To classify the spectral type of these sources, we have
used different spectral libraries and criteria available in the literature
(e.g,, Jacoby et al., 1984; Walborn & Fitzpatrick, 1990). In the case of star
ALS 207 (shown with the cyan color curve in Figure 3), we can see prominent
hydrogen lines (3970, 4101, 4340, 4861, 6563 Å) along with ionized Helium
lines, i.e., He II (4686, 4542 and 5411 Å) and He I (4144, 4387, 4471, 4713
Å). The line strength of He II usually gets weaker for late O type stars and
is last seen in B0.5 type stars (Walborn & Fitzpatrick, 1990). As this line
can be seen in the spectrum of ALS 207 star, we can constrain its spectral
type to be earlier than B0.5. The presence of He II lines (4686, 5411 Å) and
He II+I (4026 Å) limits the spectral type of ALS 207 to O type. In the case of
ALS 208 and ALS 212 (shown with green and yellow color curves in Figure 3) we
can see stronger He I lines at 4026, 4388 and 4417 Å, while the He II 4686 Å
line is absent. This implies that the ALS 208 and ALS 212 stars have spectral
type later than B0.5. The spectral types of these stars are further
constrained by visually comparing their spectra with the spectral library of
Jacoby et al. (1984) as shown in the Figure 3 by red color curves. Thus, ALS
207, ALS 208 and ALS 212 stars are classified as O6.5 V, B1 III and B1 V
spectral types, respectively. As we have assessed the spectral types of the
massive stars based on the low-resolution spectra, we may have an uncertainty
of $\pm$ 1 in the subclass identification. The massive star ALS 207 was
previously identified as O6.5 V ((f)) by Maíz Apellániz et al. (2016) which
matches with our spectral classification with an additional ((f)) feature in
the spectra. The ((f)) feature denotes the weak N III (4634-40-42 Å) emission
feature along with the strong He II (4686 Å) absorption feature. These two
features are also quite evident in the observed spectra of ALS 207 (Figure 3).

###  3.2 Search for stellar clustering/grouping in the S301 region

####  3.2.1 Isodensity contours : The North-East cluster

As most of the young star clusters are associated with the active star-forming
regions, we have determined stellar surface density distribution to identify
any kind of clustering/groupings present in the S301 region. We performed
nearest neighbor (NN) method on the 2MASS NIR catalog to generate surface
density maps in the $\sim 18^{\prime}.5\times 18^{\prime}.5$ FOV around S301 H
ii region. In a grid of 20′′$\times$20′′, the local surface density is
determined by varying the radial distance as it encompasses 20th nearest star
(for details, refer Pandey et al., 2020b). In Figure 1, the isodensity
contours are shown with the yellow color contours. The lowest contour is at
1$\sigma$ above the mean stellar density (2.6 stars/arcmin2), while the step
size is 1$\sigma$ (2 stars/arcmin2). The isodensity contours clearly reveal a
clustering with two different peaks in the north-east (NE) direction of S301.
We refer to this stellar clustering as a NE-cluster. This clustering also
includes one of the massive stars (ALS 212) whereas the other two massive
stars (ALS 207 and ALS 212) are located out of its boundary. The approximate
boundary of the NE-cluster is shown with a white circle in the Figure 1. The
radius of the NE-cluster is found to be 3′.3 centered at $\alpha_{2000}$:
07h10m17s.51, $\delta_{J2000}$: -18$\arcdeg$27$\arcmin$35$\arcsec$.

Figure 4:  PM vector-point diagrams (VPDs; top sub-panels) and $V$ vs. $(V-I)$
CMDs (bottom sub-panels) for the stars located inside the S301 region ($\sim
18^{\prime}.5\times 18^{\prime}.5$ FOV). The left sub-panels show all stars,
while the middle and right sub-panels show the probable cluster members and
field stars, respectively. Red circles in the bottom right panel are probable
member stars of the S301 region.  Figure 5:  Membership probability Pμ, PM
errors $\sigma_{PM}$ and parallax of stars as a function of $G$ magnitude for
stars in the NE-cluster region. The probable member stars (P${}_{\mu}>$80 %)
are shown by green circles while the 37 members of the S301 having parallax
values with good accuracy (i.e., error$<$ 0.1 mas) are shown by red triangles.

####  3.2.2 Membership probability of stars in the NE cluster

The advent of Gaia DR3 data with its precise measurements of proper motion and
parallax, has opened a new horizon in the study of star clusters. We have used
the $Gaia$ proper motion (PM) data with $\sigma_{PM}<$3 mas/yr to determine
the membership probability of the stars belonging to the NE-cluster found in
the S301 region using the method described in Balaguer-Núnez et al. (1998).
This method has been extensively used recently (for example, Sinha et al.,
2020; Sharma et al., 2020; Kaur et al., 2020; Pandey et al., 2020b). PMs in
$RA$ and $Dec$ are plotted as vector-point diagrams (VPDs) in the top panels
of Figure 4. The bottom panels show the corresponding $V$ versus $V-I$ color-
magnitude diagrams (CMDs). The dots in the top-left panel represent PM
distribution of all the stars in the S301 region ($\sim 18^{\prime}.5\times
18^{\prime}.5$ FOV) where a prominent clump within a radius of $\sim$0.8 mas
yr-1 centered at -1.84 mas yr-1 ($\mu_{\alpha}$cos($\delta$) and 2.74
($\mu_{\delta}$) mas yr-1 can be seen. This population of stars have almost
similar PMs and have high probability for cluster membership. Remaining stars
with scattered PM values are most probably a field population. This is more
clearer in the VPDs and CMDs of probable cluster and field populations as
shown in the middle and right panels, respectively. The probable cluster
members are showing a well defined MS in the CMD which is usually seen for the
similar population of stars. On the other hand, the probable field stars are
quite obvious by their broad distribution in the CMD. We first construct the
frequency distributions of cluster stars ($\phi^{\nu}_{c}$) and field stars
($\phi^{\nu}_{f}$) using the equations 3 and 4 of Balaguer-Núnez et al.
(1998). The input parameters such as PM center and its dispersion for the
cluster stars ($\mu_{\alpha}$cos($\delta$) = -1.84 mas yr-1, $\mu_{\delta}$ =
2.74 mas yr-1, $\sigma_{c}$, $\sim$0.06 mas yr-1) and field stars ($\mu_{xf}$
= -0.81 mas yr-1, $\mu_{yf}$ = 1.19 mas yr-1, $\sigma_{xf}$ = 3.52 mas yr-1
and $\sigma_{yf}$ = 3.62 mas yr-1) are estimated in the same manner as have
been discussed in our earlier work (Pandey et al., 2020b).

The membership probability (ratio of distribution of cluster stars with all
the stars) for the $i^{th}$ star is then estimated as:

$P_{\mu}(i)={{n_{c}\times\phi^{\nu}_{c}(i)}\over{n_{c}\times\phi^{\nu}_{c}(i)+n_{f}\times\phi^{\nu}_{f}(i)}}$
(9)

where $n_{c}$ (=0.26) and $n_{f}$ (=0.74) are the normalized numbers of stars
for the cluster and field regions ($n_{c}$+$n_{f}$ = 1).

We have plotted the estimated membership probability, errors in the PM and
parallax values as a function of $G$ magnitude in the Figure 5. It is clear
from the figure that the members are well separated from the field stars in
the brighter $G$ magnitude while in the fainter end uncertainty is larger. We
have only considered only those stars as the members of the NE-cluster which
have membership probability Pμ $>$80 %. Using this criterion, we have
identified 194 stars as the members of the NE-cluster. We further cross-
matched 136 of these sources to our optical catalog using a matching radius of
1 arc-second. The member stars are tabulated in Table 3.

Figure 6:  Left panel: $(U-B)$ vs. $(B-V)$ TCD for the sources in the NE-
cluster region (radius $<$ 3′.3, black dots). The identified member stars
using PM analysis are also plotted with red circles. The dotted blue curve
represents the intrinsic ZAMS for $Z=0.02$ by Schmidt-Kaler (1982). The red
continuous curve represents the ZAMS shifted along the reddening vector (see
text for details) by $E(B-V)_{cluster}$ = 0.50 mag for the stars associated
with the cluster. Right panel: $V$ vs. $(V-I)$ CMD for the same sources. The
ZAMS (blue continuous curve) and PMS isochrone for 0.5 Myr and 5 Myr (red and
black continuous curves) by Pastorelli et al. (2019), corrected for the
distance of 3.54 kpc and reddening $E(B-V)=0.50$ mag, are also shown. We have
also plotted the location of massive stars and YSOs (see section 3.3) by star
and triangle symbols in both the figures. Figure 7:  A plot of the MF for
member stars in the NE-cluster. Log $\phi$ represents log($N$/dlog $m$). The
error bars represent $\pm\sqrt{N}$ errors. The solid line shows the least
squares fit to the MF distribution (black filled circles). Open circle
represents the data below the 80% completeness limit.

Figure 8:  Histograms showing the distribution of the ages (left panel) and
mass of the member stars (right panel) of the NE-cluster derived using the CMD
(Figure 6).

####  3.2.3 Reddening, distance and age of the NE cluster

Two-color diagrams (TCDs) and CMDs of the member stars have been proven to be
very good tools to constrain reddening, distance and age of a cluster (for
example, Phelps & Janes, 1994; Sharma et al., 2006, 2020). In the left panel
of Figure 6, we have shown $(U-B)$ versus $(B-V)$ TCD of the stars located
within the boundary of the NE-cluster (radius $<$ 3′.3). The intrinsic zero-
age-main-sequence (ZAMS) from Schmidt-Kaler (1982, blue dotted curve), the
member stars (red circles) identified with the Gaia data and most massive
stars are also shown in the figure. We have shifted the ZAMS along the
reddening vector i.e., $E(U-B)/E(B-V)$ = 0.72 (corresponding to $R_{V}$ $\sim$
3.1, cf. Appendix A) so that it matches with the distribution of member stars
in the cluster region. The shifted ZAMS is shown with the red continuous
curve. By this method, we obtained a foreground reddening value $E(B-V)=0.50$
mag for the NE-cluster region.

Gaia data releases have opened up the possibility of an entirely new
perspective on the problem of distance estimation in cluster studies by
providing the new and precise parallax measurements upto very faint
limits111111 https://www.cosmos.esa.int/web/gaia/. But, the uncertainty of the
parallax is such that any parallax that comes with an uncertainty higher that
10% can not be simply converted into distance in parsec. Bailer-Jones et al.
(2018, 2021) have highlighted that for the vast majority of stars in the Gaia
data releases, reliable distances cannot be obtained by inverting the
parallaxes. Studies comparing the distances obtained using Gaia DR2 parallaxes
with those from other methods show the existence of a systematic offset. The
value of the offset ranges from $-0.082$ to $-0.029$ mas, depending on the
objects and the method used (for details, refer Arenou et al., 2017; Dias et
al., 2021). Recently, Dias et al. (2021) have found a systematic offset of
$-0.05\pm 0.04$ mas by comparing the open cluster distances obtained from
isochrone fitting with those obtained from a maximum likelihood estimate of
the individual member parallaxes. Using the Gaia data and maximum likelihood
procedure, Cantat-Gaudin et al. (2018) have estimated the distance of 1229
open clusters. However, they also highlighted that the distances to clusters
with mean parallaxes smaller than $\sim$0.2 mas would be better constrained by
a Bayesian approach using priors based on an assumed density distribution of
the Milky Way (Bailer-Jones et al., 2018) or photometric considerations (e.g.
Anderson et al., 2018), or simply with more classical isochrone fitting
methods (e.g., Phelps & Janes, 1994; Sharma et al., 2006, 2017; Pandey et al.,
2020a, b). Therefore, for the present study, we relied on the distance
estimates of Bailer-Jones et al. (2021). We have calculated the mean of the
distances (Bailer-Jones et al., 2021) of 37 members of the NE-cluster having
parallax values with good accuracy (i.e., error$<$ 0.1 mas, as shown in Figure
5 with red triangles) as 3.54$\pm$0.54 kpc. Moffat et al. (1979) have
mentioned the distance of S301 as 5.8 kpc based on the spectroscopy of massive
stars ALS 207, ALS 208 and ALS 212. Later on Avedisova & Palous (1989) have
estimated the distance of S301 kinematically as 5.1 kpc. With the availability
of the Gaia DR3 data, we can now constrain the distances of these stars with
better kinematical measurements. The distances of the massive stars ALS 207,
ALS 208 and ALS 212 are found to be $3.21\pm 0.14$ kpc, $2.99\pm 0.20$ kpc and
$2.93\pm 0.19$ kpc, respectively (Bailer-Jones et al., 2021). Similar
distances of both the NE-cluster and massive stars implies that the NE-cluster
is in fact associated with the S301 H ii region.

The difference between spectro-photometric and the kinematical distances can
be explained by the abnormal reddening law in this region. We have estimated
the distances of massive stars in the region by assuming a normal reddening
law, i.e, $R_{V}=3.1$ (as assumed by Moffat et al., 1979; Avedisova & Palous,
1989) and an abnormal reddening law, i.e., $R_{V}=3.7$ (a typical reddening
law observed in many Galactic star forming regions, for example, $R_{V}$ =
3.75 (Kaur et al., 2020, NGC 6910), 3.85 (Sharma et al., 2017, NGC 7538), 3.7
(Kumar et al., 2014, the Carina region), 3.3 (Pandey et al., 2013, NGC 1931),
3.5 (Sharma et al., 2012, NGC 281), and 3.7 (Pandey et al., 2008, Be 59). The
individual $E(B-V)$ color excesses of the massive stars ALS 207, ALS 208, and
ALS 212 are estimated as 0.80, 0.47, 0.57 mag (for $R_{V}=3.1$) and 0.84,
0.50, 0.60 mag (for $R_{V}=3.7$), respectively, based on the spectral types
using the intrinsic color-spectral type relation given in Schmidt-Kaler
(1982). Then, the individual spectro-photometric distances come out to be 5.9,
7.9, 5.9 kpc (for $R_{V}=3.1$) and 4.3, 6.5, 4.1 kpc (for $R_{V}=3.7$) for ALS
207, ALS 208, ALS 212, respectively, using again the corresponding
$M_{V}$-spectral type relation from Schmidt-Kaler (1982). Thus, assuming a
normal reddening law can overestimate the spectro-photomeric distance, e.g.,
in the above case, it was overestimated by $\sim$1.6 kpc, and hence, it can
explain the difference between the kinematical (Bailer-Jones et al., 2021) and
spectro-photometric (Moffat et al., 1979; Avedisova & Palous, 1989) distance
estimates. Also, from the above example, it is clear that the wrong $A_{V}$
correction in case of abnormal reddening law in the y-axis of the CMD is
mostly compensated by wrong distance estimate and the change in the individual
color excess value for the x-axis of the CMD is minimal, i.e., $E(B-V)\sim
0.035$ mag. Thus, a change in the reddening law might have marginal effect on
the derived physical parameters (for more details, see Sharma et al., 2017).

We have plotted the CMD for the stars located within the boundary of the NE-
cluster (radius $<$ 3′.3) and the member stars in the right-panel of Figure 6.
The most massive stars in the S301 region are also shown in the figure. The
blue continuous curve represents ZAMS from Pastorelli et al. (2019) corrected
for the extinction ($E(B-V)=0.50$ mag) and distance (3.54 kpc). The pre-main
sequence (PMS) isochrones of 0.5 and 5 Myr by Pastorelli et al. (2019) are
also shown in the right-panel of Figure 6 with continuous red and black
curves, respectively. The shifted ZAMS and PMS isochrones match well with the
distribution of member stars which further confirms the reddening and distance
of this cluster. An upper limit to the age of S301 region can be established
from the most massive member star. The location of the most massive star ALS
207 (O6.5V) in the $V$ versus $(V-I)$ CMD is traced back along the reddening
vector to the turn-off point in the main-sequence (MS), which is equivalent to
a 5 Myr old isochrone (cf. black curve in Figure 6 right panel). The ages and
masses of the member stars of the NE-cluster have also been derived by
applying the procedure described in our earlier work (Chauhan et al., 2009;
Sharma et al., 2017) using the $V$ versus $(V-I)$ CMD. Briefly, we created an
error box around each observed data point of a star in the CMD using the
errors associated with photometry as well as errors associated with the
estimation of reddening and distance. Five hundred random data points were
generated by using Monte-Carlo simulations in this box. The age and mass of
each generated point were then estimated from the nearest passing isochrone in
this CMD. For accuracy, the isochrones and evolutionary tracks were used in a
bin size of 0.1 Myr and were interpolated by 2000 points. At the end we have
taken the mean and standard deviation of age/mass of the above simulated 500
data points, as the final derived values and errors of each star, and are
provided in the Table 3. The distributions of estimated age and mass of the
member stars are shown in the Figure 8 which peak around 1.5 Myr and 0.8 M⊙,
respectively. The mean values of age and mass of the member stars are found to
be 3.9$\pm$3.2 Myr and 1.3$\pm$1.2 M⊙, respectively.

####  3.2.4 Mass function

The MF is usually expressed by the relation $N(\log m)\propto m^{\Gamma}$,
where the slope of the MF is given by $\Gamma=d\log N(\log m)/d\log m$ and
$N(\log m)$ is the number of stars per unit logarithmic mass interval. The MF
is generally used as a statistical tool to understand the formation process of
stars. It is basically the distribution of mass of stars formed in a star-
forming event. To calculate the MF in the NE-cluster of S301, we have used the
distribution of the masses of the member stars estimated in the previous
section. We have used only those stars for which the age was less than 5 Myr,
i.e., the upper age limit of this region is based on the age of the most
massive star. After applying the correction factor for the incompleteness of
the photometric data (Section 2.1.1) on the number of stars in different mass
bins, we have plotted the MF in Figure 7. The distribution seems to have no
turn-off point. The slope of the MF ($\Gamma$) in the mass range
$\sim$0.4$<$M/M⊙$<$7.0 comes out to be $-0.85\pm 0.07$ for the stars in the
NE-cluster region.

###  3.3 YSOs in the S301 region and their physical properties

The spatial distribution of YSOs and their physical properties can be used to
infer the star formation scenario and evolution of a star-forming region. In
this study, we have used NIR and MIR observations of the S301 region ($\sim$
$18^{\prime}.5\times 18^{\prime}.5$ FOV) to identify 37 candidate YSOs based
on their excess IR emission. The identification and classification schemes are
described in Appendix B. In Table 4, we provide the list of YSOs along with
their magnitudes in different bands and their classification.

Figure 9:  Sample SED for a Class ii YSO created by the SED fitting tools of
Robitaille et al. (2007). The black curve shows the best fit and the gray
curves show the subsequent well fits. The filled circles with error bars
denote the input flux values.

Figure 10:  Histograms showing the distribution of the ages (left panel) and
masses (right panel) of the YSOs (24) in the S301 as derived from the SED
fitting analysis (cf. Section 3.5).

We have derived physical properties like age and mass of the identified YSOs
using the SED fitting analysis. The grid models and fitting tools of Whitney
et al. (2003a, b, 2004) and Robitaille et al. (2006, 2007) have been used to
construct SEDs of the YSOs. This method has been used and described in our
previous publication (Pandey et al., 2020b). We have provided fluxes in
optical to MIR wavelengths (0.37, 0.44, 0.55, 0.65, 0.80, 1.2, 1.6, 2.2, 3.4,
4.6, 12 and 22 $\mu$m) as input data points. Taking extinction and distance as
free parameters, the SED fitting tool fits the models to the input data points
with a condition that minimum five data points should be there for fitting. We
gave input distance range from 3.0- 4.0 kpc keeping in mind the error
associated with the distance estimation (cf. Section 3.2.3). The $A_{V}$ range
is given as 1.6 mag (foreground reddening value) to 30 mag (to accommodate
deeply embedded YSOs). In Figure 9, a sample SED of a Class ii YSO is shown.
Black curve represents the best fit model while the Grey curve represents the
subsequent well fit models. The well-fit models for each source are defined by
$\chi^{2}-\chi^{2}_{min}\leq 2N_{data}$, where $\chi^{2}_{min}$ is the
goodness-of-fit parameter for the best-fit model and $N_{data}$ is the number
of input data points. From the well-fit models for each source derived from
the SED fitting tool, we calculated the $\chi^{2}$ weighted model parameters
such as the stellar mass and stellar age of each YSO and they are given in
Table 4. The histograms for age and mass of the YSOs are shown in Figure 10.
They peak around 2.5 Myr and 1 M⊙. The mean values of age and mass of the YSOs
are found to be 2.5$\pm$1.6 Myr and 2.3$\pm$0.8 M⊙, respectively. These values
are similar to those for the member stars of the NE-cluster.

###  3.4 Distribution of molecular cloud around S301 region

In the absence of CO data, we have used the extinction map as a proxy to trace
the molecular gas in the S301 region (Gutermuth et al., 2009, 2011; Sharma et
al., 2017). We derived extinction map using the $(H-K)$ colors of the MS stars
after removing the sources showing excess IR emission (cf. Gutermuth et al.,
2011). The method is described in Gutermuth et al. (2005, 2009) and
extensively used recently (Pandey et al., 2020b; Panwar et al., 2020). We have
used the NN method to derive the extinction value in a grid of
20′′$\times$20′′, by taking the mean extinction value of the 20 nearest stars.
To obtain the extinction value, we used the relation E =$(H-K)_{obs}$ \-
$(H-K)_{int}$, where $(H-K)_{obs}$ and $(H-K)_{int}$ are the observed and
intrinsic colors of the MS stars and then calculated the $A_{V}=15.87\times
E(H-K)$ (cf. Gutermuth et al., 2011). The sources deviating above 3$\sigma$
were excluded to calculate the final extinction value of each point. The
sources showing excess emission in IR can lead to overestimation of extinction
values in the derived maps. Therefore, the candidate YSOs and probable
contaminating sources are excluded for the calculation of extinction. Here, it
is also worthwhile to note that the derived $A_{V}$ values are the lower
limits of their values as the sources with higher extinction may not be
detected in our study and the spatial resolution of the map depends on the
local stellar density.

Figure 11:  (a) Small scale view ($\sim$18′.5$\times$18′.5) of the S301 shown
with a color-composite image made using WISE 22 $\mu$m (red), WISE 12 $\mu$m
(green) and H$\alpha$ (blue). (b) color-composite image of the S301 created
using AKARI 160 $\mu$m (red), AKARI 60 $\mu$m (green) and H$\alpha$ (blue)
images. (c) The NVSS (cyan) and Planck 870 $\mu$m (magenta) contours
superimposed on the AKARI 160 $\mu$m image. The lowest NVSS contour is at
0.006 Jy/beam and the step size is 0.006 Jy/beam. The lowest Planck contour is
at 0.021506 K and the step size is 0.004 K. (d) Cluster surface density
contours (red), extinction map contours (blue), NVSS radio contours (cyan) and
H i map contours (green) superimposed on the 2MASS $K$ band image. The lowest
extinction contour is 1 $\sigma$ above the mean value of extinction
($A_{V}$=4.6 mag) where step size is 1$\sigma$=0.2 mag. The lowest H i contour
is at 290 K and the step size is 1 K. The cluster surface density contours are
at same level as in Figure 1. The circle and star symbol represent the
location of YSOs and massive stars, respectively, in the S301 region.

###  3.5 Environmental conditions of S301

In Figure 11a, we have shown the color-composite image of the S301 region made
by using WISE $22$ $\mu$m (red), WISE $12$ $\mu$m (green) and H$\alpha$
(blue), images. The figure shows a stunning view of the gas and dust content
distributed in the southern and eastern part of this region. $WISE$ 12 $\mu$m
image covers the prominent polycyclic aromatic hydrocarbon (PAH) features at
11.3 $\mu$m, indicative of photo-dissociation regions (or photon-dominated
regions, or PDRs). The southern and eastern parts of the complex seem to
contain PDRs produced under the influence of massive stars. These features are
distributed as the arc-like structures which are probably swept-up gas and
dust under the influence of the nearby massive stars. In addition to above,
YSOs are also seen in and around these arc-like structures. The distribution
of warm dust at $22$ $\mu$m near the massive stars show that the most massive
star ALS 207 has created a warm dust envelope which seems to be bigger in size
as compared to that from the less massive star ALS 208. The other massive star
ALS 212 probably lacks a warm dust envelope. The H$\alpha$ emission seems to
trace the full extension of the S301 H ii region. Similar structures have been
found in several H ii regions excited by the massive stars (e.g., Deharveng et
al., 2010; Paladini et al., 2012). Though the H$\alpha$ emission is in general
spherically distributed, the south-eastern peripheries of the S301 region are
brighter with few dark lanes whereas the north-western part has low-intensity
extended emission. We notice that the most massive star, ALS 207, is at the
center of H$\alpha$ emission whereas the other two massive stars are at its
boundary.

We have also examined the distribution of colder dust (FIR emission) in the
S301 region using AKARI 160 $\mu$m (red), AKARI 60 $\mu$m (green) and
H$\alpha$ (blue) images shown in Figure 11(b). The cold dust emission at FIR
(traced by 160 and 60 $\mu$m) clearly surrounds the hot gas and dust in the
south and east direction and is slightly opened-up in the north-west
direction. Several YSOs are located at the peak of the distribution of cold
dust emission in the north-eastern part of this region. This distribution
almost mimics the arc-like structure seen at 12 $\mu$m, confining the heated
region in the south and east directions. The north-western part of S301 region
is found to be devoid of gas and dust.

In Figure 11c, we have also shown the NVSS 1.4 GHz radio contours that trace
the ionized emission (cyan contours) on the AKARI 160 $\mu$m image. Planck 353
MHz emission (magenta contours) which traces cold cloud-clumps is also shown
along with the distribution of YSOs and massive stars. Almost all the YSOs are
associated with cold gas and dust traced by the AKARI 160 $\mu$m and Planck
353 MHz emissions whereas the massive star ALS 207 is surrounded by ionized
gas. The peaks of 160 $\mu$m and 353 MHz emission are matching well with many
YSOs located around these peaks. Some of the YSOs are also located at the
boundary of the ionized region where the ionized emission seems to be
interacting with the cold or warm dust and gas. By looking at the morphology
of this region, it appears that the most massive star is at the edge or near
the outer surface of the molecular cloud where one can see the embedded YSOs
in it.

In Figure 11d, YSOs along with massive stars are superimposed on the $K$-band
image. We also show the radio emission at NVSS 1.4 GHz (cyan contours),
extinction contours (blue contours) and stellar surface density contours (red
contours) in the figure. The boundary of the newly identified NE-cluster
region (see Section 3.2.1) is shown by a yellow circle. Almost all the YSOs
are distributed in the north-eastern direction within the boundary of the
cluster region with few exceptions in the southern direction. The presence of
YSOs inside the cluster region implies that the cluster is very young and thus
might have formed via a recent star formation activity. The extinction map
shows that most of the molecular material is in front of the cluster at the
eastern boundary of the radio emission. The molecular cloud traced by the
extinction map is bow-shaped pointing towards the massive star ALS 207. The
peak of radio emission is near to the tip of the bow-shaped molecular cloud
which hints that the radio emission is density bounded in this direction. The
distribution of YSOs starts where the ionized emission interacts with the
surrounding gas and dust distribution, which could lead us to assume that the
formation of YSOs may be due to the interaction of the ionization front with
the surrounding material. We have also examined the H i (21cm) emission in the
direction of S301 and found a prominent velocity component ranging from 45 to
60 km-1 (see Figure 12). We fitted a Gaussian in the spectrum and found the
peak of the velocity component as 52.28 km-1. Fich et al. (1990) and
Wouterloot et al. (1988) have also reported a radial velocity of $\sim$ 56.1
kms-1 for both the ionized and the molecular gas in the direction of S301.
Quireza et al. (2006) have done a radio recombination lines survey in this
region and reported the line of site radial velocity as $\sim$55.0 kms-1 and
$\sim$53.7 km$s^{-1}$ for H and He lines, respectively. These velocities
suggest the association of ionised/molecular/neutral gases in the region. In
Figure 11d, we have also shown the distribution of H i gas integrated over a
velocity range of 45 to 60 km-1. The distribution of neutral gas represents
the low density matter in the south-western part of this region.

Figure 12:  H i (21cm) spectrum towards the S301 region. A Gaussian (shown
with the magenta color) is fitted to determine the peak of the velocity.

Figure 13:  (a) H$\alpha$ image of the S301 region (FOV$\sim 9^{\prime}\times
9^{\prime}$) superimposed with NVSS (1.4 GHz) contours. The lowest NVSS
contour is at 0.006 Jy/beam and the step size is 0.006 Jy/beam. (b) H$\alpha$
image of the S301 region superimposed with GMRT (1280 MHz) contours. The
lowest GMRT contour is at 0.001 Jy/beam and the step size is 0.001 Jy/beam.
The star symbols are the location of massive stars in the region. The beam
size of the radio map is shown by a black circle in the lower right corners of
the figures. The 12 $\mu$m intensity contours are shown with the black color
curves with lowest contour at 640 counts and the step size is 115 counts.

###  3.6 Distribution of the ionized gas

In Figure 13a, we have shown the distribution of ionized gas using the NVSS
1.4 GHz map, overplotted on the H$\alpha$ image of the S301 region. The
ionized gas is more or less circularly distributed with extended emission in
the west direction of the region. The most massive star ALS 208 is located
near the peak of the radio emission whereas the other two massive stars are
located at the boundary of the radio emission. The center of the ionized gas
is slightly toward the east direction of the region. The observed
distributions of the ionized gas, PDR and cold gas/dust indicate that most of
the ionized emission is due to a massive star ALS 207 and the radio emission
is density bounded towards the south and east peripheries. In the other
direction, the radio emission is more extended due to the absence of gas and
dust material.

Due to the coarse beam size of NVSS ($40^{\prime\prime}\times
40^{\prime\prime}$), small scale structures are absent in the image and we did
not get finer details of the ionized gas distribution. To investigate them, we
have used the GMRT 1280 MHz emission map (beam size $\sim
7^{\prime\prime}\times 7^{\prime\prime}$), over-plotted on the H$\alpha$ image
of S301 and as shown in Figure 13b. The GMRT 1280 MHz emission shows a
fragmented dense shell of the ionized gas which seems to be slightly opened up
towards the north-west direction. The ionized gas is density bounded by the
PDRs in the south-eastern direction as seen by the 12 $\mu$m emission (black
contours) in Figure 13b. The H$\alpha$ emission also shows similar signatures
as discussed in the previous section. It looks like the ionized gas might have
streamed out in the lower-density interstellar medium (ISM) through the north-
west direction which is having very less or no gas and dust distribution. We
have also estimated the value of Lyman continuum flux ‘log(NUV)’ as 48.41 (see
next Section) which corresponds to a O8.5V spectral type star (see Panagia,
1973). Since the main ionizing source ‘ALS 207’ of this H ii region has a
spectral type of O6.5V (see Section 3.1), the estimated value of Lyman
continuum flux is far less than the expected Lyman continuum flux of an O6.5V
star (i.e., log(NUV)=49.02). Even, if we consider the absorption by dust
grains prior to the ionization (Inoue, 2001; Binder & Povich, 2018), we could
not compensate for the remaining value of ionization flux. This also supports
our argument that the ionized gas has streamed out from the north-western
direction of this complex. This kind of H ii region morphology is already seen
in the literature (Panwar et al., 2020; Krumholz & Matzner, 2009; Henney et
al., 2005; Tenorio-Tagle, 1979), where the H ii regions open up in the
direction away from the cloud edge and is known as “blister-type” H ii region.
This mechanism of flowing ionized gas in the surrounding ISM is known as
Champagne-flow (Duronea et al., 2012; Panwar et al., 2020).

###  3.7 Lyman continuum emission and Dynamical age of the S301 H ii region

We have estimated the Lyman continuum flux associated with the ionized gas in
the S301 region by using the following equation by Schmiedeke et al. (2016):

$\begin{split}N_{\mathrm{UV}}(s^{-1})&=7.5\,\times\,10^{46}\,\left(\frac{F_{\mathrm{th}}(\nu)}{\mathrm{Jy}}\right)\left(\frac{D}{\mathrm{kpc}}\right)^{2}\\\
&\left(\frac{T_{e}}{10^{4}\mathrm{K}}\right)^{-0.45}\times\,\left(\frac{\nu}{\mathrm{GHz}}\right)^{0.1}\end{split}$
(10)

where, NUV is the Lyman continuum photons per second, Te is the electron
temperature, $\nu$ is the frequency, Sν is the integrated flux, D is the
distance of the region, and $\theta$ denotes the angular size of the region.
We have adopted the values of $T_{e}$ as 10 000 K assuming all ionizing flux
was generated by a single massive O type star. We have calculated the
integrated flux density S$\nu$=2.5 mJy by integrating flux down to the lowest
contour at 3 $\sigma$ level in the GMRT 1280 MHz map. The $\sigma$ value which
is the rms noise of the GMRT radio map was estimated to be 0.093 mJy/beam. The
size of the H ii region is found to be $\simeq$ 4′.7 (4.8 pc at a distance of
3.54 kpc). Using the distance ‘D’ of 3.54 kpc, we have estimated the log(NUV)
as 48.41.

The dynamical age of the S301 H ii region is estimated by using the following
equation (Dyson & Williams, 1980):

$t_{dyn}=\Big{(}\frac{4R_{s}}{7c_{s}}\Big{)}[\Big{(}\frac{R_{HII}}{R_{s}}\Big{)}^{7/4}-1]$
(11)

where, cs is the isothermal sound velocity in the ionized gas (cs = 11 km s-1)
(Stahler & Palla, 2005), RHII is the radius of the H ii region, and Rs is the
Strömgren radius of the H ii region which is given by:

$R_{s}=\Big{(}\frac{3S_{\nu}}{4\pi{{n_{0}}^{2}}{\beta_{2}}}\Big{)}^{1/3}$ (12)

where, n0 is the initial ambient density (in cm-3) and $\beta$2 is the total
recombination coefficient to the first excited state of hydrogen $\beta$2 =
$2.6\times 10^{-13}$ (Stahler & Palla, 2005). The dynamical age of the S301 H
ii region is estimated as 4.8 Myr corresponding to n0= 104 cm-3 (Beltrán et
al., 2006).

###  3.8 Feedback pressure from massive star

To quantitatively examine the effect of a massive star on its surrounding, we
have determined the feedback pressure from the massive star ALS 207. Total
feedback pressure from a massive star consists of three components, pressure
of a H ii region (P${}_{H\,{\sc II}}$), radiation pressure (Prad), and stellar
wind ram pressure (Pwind) (Bressert et al., 2012; Dewangan et al., 2017b).
These pressure components can be estimated by using the following equations
(see for details, Bressert et al., 2012):

$P_{HII}=\mu m_{H}c_{s}^{2}\,\left(\sqrt{3N_{UV}\over
4\pi\,\alpha_{B}\,D_{s}^{3}}\right);$ (13)

$P_{rad}=L_{bol}/4\pi cD_{s}^{2};$ (14)

$P_{wind}=\dot{M}_{w}V_{w}/4\pi D_{s}^{2};$ (15)

Where NUV is the Lyman continuum photons, cs is the sound speed in the
photoionized region (=11 km s-1; Stahler & Palla, 2005), “$\alpha_{B}$” is the
radiative recombination coefficient (= 2.6 $\times$ 10-13 $\times$ (104
K/Te)0.7 cm3 s-1; see Kwan, 1997), $\mu$ is the mean molecular weight in the
ionized gas (= 0.678; Bisbas et al., 2009), mH is the hydrogen atom mass,
$\dot{M}_{w}$ is the mass-loss rate, Vw is the wind velocity of the ionizing
source, and Lbol is the bolometric luminosity of the ionizing source. We
adopted $L_{bol}$ = 234422 L⊙ (Panagia, 1973), $\dot{M}_{w}$ $\approx$ 1.21
$\times$ 10-7 M⊙ yr-1 (Marcolino et al., 2009), Vw $\approx$ 5244 km s-1
(Martins & Palacios, 2017), NUV = 1.047 $\times$ 1049 (Panagia, 1973) for ALS
207 (O6.5V). We took $D_{s}$ =4.24 pc as the projected distance of the cluster
center from the massive star. We got the value of $P_{HII}$= 8.98 $\times$
10-11, $P_{rad}$ = 1.39 $\times$ 10-11 and $P_{wind}$ = 1.86 $\times$ 10-12
dynes cm-2. The total pressure ($P$= $P_{HII}$+$P_{rad}$\+ $P_{wind}$) comes
out to be 1.06 $\times$ 10-10 dynes cm-2.

##  4 Discussion

Broadly, the S301 cloud complex possesses gas and dust, massive stars, an H ii
region, a young embedded NE-cluster, and several YSOs. All of these features
hint towards a recent star forming activities in the region. As there are many
active star-forming regions in the literature where massive stars have
triggered the formation of YSOs (Zavagno et al., 2006; Pomarès et al., 2009;
Yadav et al., 2016; Sharma et al., 2017; Pandey et al., 2020b; Kaur et al.,
2020, and references therein), in the present work we also explored the impact
of massive stars on the star formation activities in the S301 region.

###  4.1 Feedback from massive star

The morphological features discussed in Section 3.5 and 3.6 point S301 towards
a blistered H ii region which is created when a massive star forms at the edge
of the molecular cloud. The ionization front soon reaches at the edge of the
cloud and H ii region opens away from the cloud edge devoid of gas and dust.
In the other directions, where the molecular material is distributed, feedback
from massive stars can trigger the formation of a new generation of stars.
Observational evidences of the triggered star formation have been discussed
for several H ii regions such as Sh 2-104, RCW 79, Sh 2-212, RCW 120, Sh 2-217
(Deharveng et al., 2003, 2008; Zavagno et al., 2006, 2010; Brand et al.,
2011). Recently, Panwar et al. (2020) studied a blistered H ii region Sh 2-112
and discussed the feedback effect of a massive star in triggering star
formation. We have also explored the possibility of triggered star formation
in the S301 region. Figure 11 and 13 shows the morphology of the S301 star
forming complex. The most massive star ALS 207 located in the region is
responsible for creating the H ii region (traced by radio and H$\alpha$
emission) and arc-like MIR shell/PDRs towards the south and eastern directions
of ALS 207. The H i (21cm) map confirms the presence of low density neutral
hydrogen material at the western border that is devoid of gas and dust. The
extinction map shows the presence of molecular clouds in the north-east
direction between the NE-cluster and the ionization layer of gas. We can also
see the presence of YSOs still embedded in the molecular cloud. These facts
point to a star formation process triggered by the massive central star ALS
207 in the north-east direction of the S301 region. Earlier in Section 3.8, we
have found that the pressure due to the H ii region ‘$P_{HII}$’ is relatively
higher than the other components, i.e, $P_{rad}$ and $P_{wind}$. Hence, we can
argue that the photoionized gas associated with the H ii region (against the
radiation and wind) can be a major contributor in the feedback process in the
S301 region. The total pressure due to a massive star ALS 207 at the center of
the NE-cluster was found to be higher than that of a typical cool molecular
cloud ($P_{MC}$$\sim$10-11–10-12 dynes cm-2 for a temperature $\sim$20 K and
particle density $\sim$103–104 cm-3) (see Table 7.3 of Dyson & Williams, 1980)
and hence can initiate the collapse of the molecular cloud.

Many authors (Pandey et al., 2020b; Kaur et al., 2020) have given age gradient
as an argument to support the triggered star formation scenario. To check
this, we have done a comparison between the age of the YSOs, age of the NE-
cluster, dynamical age of the H ii region and age of the massive star. The
peaks in the distributions of the ages of the YSOs and NE-cluster members were
found to be at $\sim$2.5 - 1.5 Myr (see Section 3.2.3 and 3.3). The MF slopes
for the cluster region was also found to be shallower ($\Gamma=-0.85\pm 0.07$)
than the Salpeter (1955) value i.e., $\Gamma=-1.35$. Usually, the higher mass
stars mostly follow the Salpeter MF (Salpeter, 1955). At lower masses, the MF
is less well constrained, but appears to flatten below 1 M⊙ and exhibits fewer
stars of the lowest masses (Kroupa, 2002; Chabrier, 2003; Lim et al., 2015;
Luhman et al., 2016). The MF distribution of the NE-cluster indicates that
there is a larger population of relatively massive stars suggesting the youth
of this cluster. The dynamical age of the H ii region S301 is found to be
$\sim$4.8 Myr (see Section 3.7).

Since ALS 207 has the spectral type of O6.5 and is still in the MS, hence the
age of the ALS 207 should be $\sim$5 Myr (Meynet et al., 1994) (see also
Section 3.2.3). The age of ALS 207 is more or less similar to the dynamical
age of the H ii region S301. Thus, H ii region S301 is old enough to initiate
the formation of a second generation of stars in its surroundings, which in
fact are younger in age. We have also calculated the time frame in which the
ionization front from the central massive star (ALS 207) with a speed of 9
kms-1 (Pismis & Moreno, 1976) will reach the NE-cluster (4.24 pc), as 1.4 Myr.
Hence, we can safely assume that the stars/YSOs in the NE-cluster formed after
the ionization front from the massive star reached there.

The distribution of MIR swept up shell ahead of the layer of ionized gas along
with the NE-cluster being on the opposite side embedded in the molecular gas
(Figure 11(b)) hint towards a ‘collect-and-collapse’ scenario of star
formation in this region (Dewangan et al., 2020b; Kirsanova et al., 2009;
Elmegreen & Lada, 1977). This scenario predicts the shell of MIR emission
around the H ii region and existence of young stellar clusters behind that.

Figure 14:  (a) Large-scale view ($0.7^{\circ}\times 0.7^{\circ}$) of the
region S301 shown with the color-composite image made by using the WISE 22
$\mu$m (red), WISE 12 $\mu$m (green) and optical V band (blue) images. (b)
Color-composite image of the S301 region made by using the Planck 353 MHz
(red), AKARI (160 $\mu$m, green) and WISE 22 $\mu$m (blue) images. The NVSS
1.4 GHz contours and stellar surface density contours are also over plotted
over the image in blue and green colors, respectively. Locations of the
massive stars and the YSOs are also shown by stars symbol and filled circles.
The region inside the box is the probable central hub part of the system.
Possible filamentary structures are shown by the magenta dashed lines. The
cross symbol in both the panels is showing reflection nebula Bran 7D (see
section 1)

###  4.2 Formation of massive stars: Possibility of a Hub-Filament System

MIR/FIR surveys like Herschel, AKARI etc. had revealed that the cold ISM,
known as the birthplace of stars, is organized in filamentary structures
(Inutsuka et al., 2015; Men’shchikov et al., 2010). There have been many
studies in which hub-filament system (HFS) is attributed as the possible
progenitors of high mass star formation (Schneider et al., 2012; Dewangan et
al., 2017b). Recently, Kumar et al. (2020) have given a detailed explanation
regarding the various stages of massive star formation in the HFS. In the HFS,
gas flows to the hub via filaments which are the objects of high aspect ratio
and low density. Hub is a place where filaments converge and are of low aspect
ratio and high density. Initially, flow-driven filaments approach each other
due to intra-molecular cloud velocity dispersion, which join to form a hub
system. Afterwards, a dense core can form at the junction followed by the
formation of massive star/s. These massive stars can further create H ii
region/s which can trigger formation of the second generation of stars. In
literature, there are multiple examples of H ii regions found in the HFSs
(Myers, 2009; Deharveng et al., 2015; Dewangan et al., 2017a, 2020a).

To explore the possible formation scenario of the massive stars in the S301
region, in Figure 14, we show the large scale ($0.7^{\circ}\times
0.7^{\circ}$) multi-wavelength view of the S301 region. In Figure 14a, we show
the color-composite image made by using the WISE 22 $\mu$m (red), WISE 12
$\mu$m (green) and optical $V$ band (blue) images. The Figure 14b shows the
color-composite image made by using the Planck 325 MHz (red), AKARI 160 $\mu$m
(green), and WISE 22 $\mu$m images. It is also superimposed with the NVSS
radio and stellar surface density contours. In both the figures, there is a
hint of a central hub of gas and dust along with small filamentary structures
(shown with magenta lines). The location of the main ionizing star ALS 207 at
the center of this hub suggests that it may be formed via the HFS related
physical processes. To further confirm the hypothesis of HFS at S301 getting
fed up by the flow driven filaments (traced in MIR/FIR wavelengths), we need a
high resolution molecular data. The P-V (position-velocity) diagrams along the
filaments showing velocity gradient would confirm this hypothesis.
Unfortunately, there is no high resolution data available currently. This will
be explored in further studies.

##  5 Conclusion

We have presented a multiwavelength study of the H ii region S301 using the
deep optical/NIR data, radio continuum data, spectroscopic data along with
MIR/FIR archival data taken from different telescopes. From the analyses
performed in the present work, we made the following conclusions.

  * •

Spectral classifications of the massive stars ALS 207 (O6.5V), ALS 208 (B1III)
and ALS 212 (B1V) have been done using the optical spectroscopic data taken
from the HCT.

  * •

We have identified a new NE-cluster in the north-east direction of the S301
region by doing stellar density distribution analysis. Gaia DR3 data have been
used to identify 194 members of this cluster. We constrained the distance and
reddening of this NE-cluster as 3.54 kpc and $E(B-V)$=0.5 mag, respectively.
The distance of the NE-cluster is similar to that of the H ii region S301.
Most of the members of the NE-cluster have age and mass around 1.5 Myr and 1.3
M⊙. The slope of the MF ‘$\Gamma$’ in the mass range $0.4<$M/M${}_{\odot}<7$
for the member stars of the NE-cluster is found to be $-0.85\pm 0.07$, which
is shallower than the Salpeter (1955) value -1.35.

  * •

We have identified 37 YSOs in the $\sim 18^{\prime}.5\times 18^{\prime}.5$ FOV
around the massive star ALS 207. Majority of YSOs were found to be spatially
associated with the NE-cluster. Out of them, four are Class i and the rest are
Class ii sources. The mean value of age and mass of the YSOs is found to be
2.5$\pm$1.6 Myr and 2.3$\pm$0.8 M⊙, respectively.

  * •

The morphology of the S301 H ii region has been analyzed using the H$\alpha$,
MIR to FIR and radio continuum images. The S301 region is showing distribution
of hot ionised gas near the massive star ALS 207 which is bounded by an arc-
like structure of gas and dust from the south-eastern direction. We have found
the dense molecular material between the massive star ALS 207 and the NE-
cluster using the extinction map. The north-western region seems to be devoid
of gas and dust material. The distributions of MIR dust, ionized gas and
neutral hydrogen suggest a blister morphology for the S301 with the massive
star ALS 207 being near the edge of the cloud and the NE-cluster is embedded
in the cold molecular cloud.

  * •

The pressure calculation, distribution of PDRs/YSOs and the age difference
between the massive star and the NE cluster provide supporting arguments for
the positive feedback of a massive star ALS 207 in the S301 H ii region. Our
analyses also suggest that the massive stars in the S301 region might have
formed in a HFS.

## Acknowledgments

We thank the anonymous referee for several constructive, useful comments and
suggestions, which greatly enhanced the quality of the paper. The observations
reported in this paper were obtained by using the 1.3m telescope, Nainital,
India and the 2 m HCT at IAO, Hanle, the High Altitude Station of Indian
Institute of Astrophysics, Bangalore, India. We also acknowledge the TIFR Near
Infrared Spectrometer and Imager mounted on 2 m HCT using which we have made
NIR observations. This publication makes use of data from the Two Micron All
Sky Survey, which is a joint project of the University of Massachusetts and
the Infrared Processing and Analysis Center/California Institute of
Technology, funded by the National Aeronautics and Space Administration and
the National Science Foundation. This work is based on observations made with
the $Spitzer$ Space Telescope, which is operated by the Jet Propulsion
Laboratory, California Institute of Technology under a contract with National
Aeronautics and Space Administration. This publication makes use of data
products from the Wide-field Infrared Survey Explorer, which is a joint
project of the University of California, Los Angeles, and the Jet Propulsion
Laboratory/California Institute of Technology, funded by the National
Aeronautics and Space Administration. DKO acknowledges the support of the
Department of Atomic Energy, Government of India, under project identification
No. RTI 4002.

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Table 1:  Log of observations. Telescope/Instrument | Comments | Exp. (sec)$\times$ No. of frames
---|---|---
$[$Date of observations$]$ |  $[$Filter$]$ |
1.3 m DFOT/2K CCD | Optical imaging of S301 |
$[$ 2019 Dec 25$]$ | $U$ | $300\times 3$
$"$ | $B$ | $300\times 3$
$"$ | $V$ | $180\times 3,60\times 3$
$"$ | $R_{c}$ | $180\times 3,60\times 3,10\times 3$
$"$ | $I_{c}$ | $180\times 3,60\times 3,10\times 3$
$[$2018 Jan 16 $]$ | $I_{c}$ | $1200\times 5$
$[$2018 Jan 16$]$ | $V$ | $1800\times 5$
| Optical imaging of Standard field (SA98) |
$[$2019 Dec 25$]$ | $U$ | $300\times 4$
$"$ | $B$ | $300\times 1,120\times 4,60\times 1$
$"$ | $V$ | $60\times 6,30\times 6$
$"$ | $R_{c}$ | $30\times 6,20\times 6$
$"$ | $I_{c}$ | $30\times 3,20\times 4$
2m HCT/TIRSPEC | NIR imaging of S301 (Nine pointings) |
$[$2018 Dec 25 & 2018 Dec 26$]$ | $J$ | $20\times 35$
$"$ | $H$ | $20\times 35$
$"$ | $K$ | $20\times 35$
2m HCT | Optical spectroscopy of ALS 207,ALS 208 ans ALS 212 |
$[$2019 Aug 20$]$ | GRISM 7 | $600\times 1,900\times 1,900\times 1$
Table 2:  Completeness of the photometric data

Band | Number of | Detection | Completeness limitd (upto 80 %)
---|---|---|---
| sourcesa | Limit (mag) | (mag) | Mass ($M_{\odot}$)
$U$ | 596 | 19.5 | $-$ | $-$
$B$ | 1156 | 20.2 | $-$ | $-$
$V$ | 5236 | 22.7 | 21.0 | 0.4
$R_{c}$ | 2965 | 20.7 | $-$ | $-$
$I_{c}$ | 5624 | 21.6 | 20.0 | 0.3
$J$ | 2277b\+ 1846c | 18.4 | 15.3 | 0.4
$H$ | 2811b\+ 1534c | 18.1 | 15.0 | 0.4
$K$ | 2809b\+ 970c | 17.8 | 15.0 | 0.3

a: for $18^{\prime}.5\times 18^{\prime}.5$ FOV;
b: data from $TIRSPEC$;
c: data from 2MASS for the bright stars;
d: for distance = 3.54 kpc and $E(B-V)$ = 0.5 mag.

Table 3:  Sample of 137 stars identified as a member of the NE-cluster (having optical counterparts). Magnitudes in different bands along with age and mass derived using the CMD analysis are also provided in the table. The complete table is available in the electronic form only. ID | $\alpha_{(2000)}$ | $\delta_{(2000)}$ | Parallax$\pm\sigma~{}~{}~{}~{}~{}$ | $\mu_{\alpha}\pm\sigma$ | $\mu_{\delta}\pm\sigma$ | $G$ | $G_{BP}-G_{RP}$ | Probability | Age$\pm\sigma$
---|---|---|---|---|---|---|---|---|---
| $(degrees)$ | $(degrees)$ | (mas) | (mas/yr) | (mas/yr) | (mag) | (mag) | (Percentage) | (Myr)
1 | 107.583885 | -18.488205 | $0.103\pm 0.130$ | $-1.691\pm 0.103$ | $2.728\pm 0.108$ | $18.124$ | 2.058 | 100 | $1.2\pm 0.32$
2 | 107.591286 | -18.478752 | $0.265\pm 0.523$ | $-2.020\pm 0.330$ | $3.343\pm 0.496$ | $19.305$ | 2.514 | 93 | $0.66\pm 0.08$
3 | 107.585960 | -18.478432 | $0.709\pm 0.374$ | $-2.003\pm 0.278$ | $2.309\pm 0.302$ | $19.388$ | 2.356 | 94 | $1.89\pm 0.47$
4 | 107.588402 | -18.483150 | $0.337\pm 0.085$ | $-1.758\pm 0.060$ | $2.652\pm 0.073$ | $16.939$ | 1.963 | 100 | $0.76\pm 0.26$
Mass$\pm\sigma$ | $U\pm\sigma$ | $B\pm\sigma$ | $V\pm\sigma$ | $R\pm\sigma$ | $I\pm\sigma$ | $J\pm\sigma$ | $H\pm\sigma$ | $K\pm\sigma$
---|---|---|---|---|---|---|---|---
($M_{\odot}$) | (mag) | (mag) | (mag) | (mag) | (mag) | (mag) | (mag) | (mag)
$0.99\pm 0.1$ | $-$ | $-$ | $18.922\pm 0.090$ | $17.771\pm 0.020$ | $16.940\pm 0.006$ | $15.519\pm 0.011$ | $14.735\pm 0.008$ | $14.466\pm 0.009$
$0.53\pm 0.04$ | $-$ | $-$ | $19.591\pm 0.031$ | $18.248\pm 0.028$ | $17.181\pm 0.017$ | $15.487\pm 0.009$ | $14.585\pm 0.006$ | $14.275\pm 0.008$
$0.81\pm 0.08$ | $-$ | $-$ | $19.769\pm 0.021$ | $18.641\pm 0.080$ | $17.652\pm 0.022$ | $16.073\pm 0.033$ | $15.258\pm 0.014$ | $14.928\pm 0.013$
$4.2\pm 0.72$ | $-$ | $13.993\pm 0.003$ | $13.186\pm 0.004$ | $12.638\pm 0.008$ | $12.109\pm 0.008$ | $11.214\pm 0.003$ | $10.738\pm 0.003$ | $10.427\pm 0.003$
Table 4:  A sample table containing information for 37 YSOs identified in the S301 region ($18^{\prime}.5\times 18^{\prime}.5$ FOV). Magnitudes in different bands along with age and mass derived using the SED analysis are also provided in the table. The complete table is available in an electronic form only. ID | $\alpha_{(2000)}$ | $\delta_{(2000)}$ | $U\pm\sigma$ | $B\pm\sigma$ | $V\pm\sigma$ | $R\pm\sigma$ | $I\pm\sigma$ | $J\pm\sigma$ | $H\pm\sigma$
---|---|---|---|---|---|---|---|---|---
| $(degrees)$ | $(degrees)$ | (mag) | (mag) | (mag) | (mag) | (mag) | (mag) | (mag)
1 | 107.493520 | -18.616960 | $-$ | $-$ | $21.842\pm 0.065$ | $-$ | $19.055\pm 0.014$ | $16.983\pm 0.045$ | $15.990\pm 0.026$
2 | 107.546871 | -18.493279 | $-$ | $-$ | $21.725\pm 0.078$ | $20.249\pm 0.096$ | $18.703\pm 0.012$ | $15.978\pm 0.013$ | $14.666\pm 0.007$
3 | 107.596327 | -18.488375 | $-$ | $-$ | $21.441\pm 0.050$ | $-$ | $18.772\pm 0.035$ | $16.859\pm 0.031$ | $15.612\pm 0.014$
4 | 107.546036 | -18.476207 | $-$ | $-$ | $22.420\pm 0.101$ | $-$ | $18.425\pm 0.009$ | $15.330\pm 0.012$ | $13.800\pm 0.009$
$K\pm\sigma$ | $[3.4]\pm\sigma$ | $[4.6]\pm\sigma$ | $[12]\pm\sigma$ | $[22]\pm\sigma$ | Class | $N_{data}$ | $\chi^{2}_{min}$ | Mass$\pm\sigma$ | Age$\pm\sigma$
---|---|---|---|---|---|---|---|---|---
(mag) | (mag) | (mag) | (mag) | (mag) |  |  |  | ($M_{\odot}$) | (Myr)
$15.330\pm 0.026$ | $14.5\pm 0.04$ | $14.116\pm 0.048$ | $9.33\pm 0.102$ | $7.653\pm 0.168$ | II | 9 | 11.3 | $2.01\pm 0.89$ | $4.16\pm 3.35$
$13.773\pm 0.008$ | $-$ | $-$ | $-$ | $-$ | II | 6 | 0.1 | $2.54\pm 1.68$ | $1.33\pm 2.21$
$14.853\pm 0.013$ | $-$ | $-$ | $-$ | $-$ | II | 5 | 0.1 | $1.51\pm 1.26$ | $2.26\pm 2.22$
$12.822\pm 0.009$ | $-$ | $-$ | $-$ | $-$ | II | $-$ | $-$ | $-$ | $-$

##  Appendix A Reddening Law

To study the nature of the diffuse interstellar medium (ISM) associated with
the S301 region we have used TCDs to derive the ratio of total-to-selective
extinction $R_{V}$ = $A_{V}$/$E(B-V)$ (Pandey et al., 2003). Although we have
a normal reddening law $R_{V}$ = 3.1$\pm$ 0.2 (Whittet, 2003; Guetter & Vrba,
1989; Lim et al., 2011) in solar neighborhood but some anomaly has been found
in case of few SFRs (see e.g., Pandey et al., 2000, 2008; Hur et al., 2012;
Pandey et al., 2013; Kumar et al., 2014). The $(V-\lambda)$ versus $(B-V)$
TCDs where $\lambda$ represents the wavelengths from one of the broad-band
filters ($R,I,J,H,K,L$), are shown in Figure 15. We used all member stars of
S301 detected in optical and near-infrared bands, except YSOs (source of
contamination due to excess in infra-red) to generate the TCDs. The method has
been discussed in the appendix of Pandey et al. (2020b). The slopes of the the
$(V-I_{c}),(V-J),(V-H)$ and $(V-K)$ versus $(B-V)$ TCDs fitted using the least
squares fit, are found to be $1.27\pm 0.15,2.19\pm 0.17,2.78\pm 0.19$ and
$2.92\pm 0.21$, respectively. These value are higher than those found for the
general ISM (1.10, 1.96, 2.42 and 2.60; cf., Pandey et al., 2003) by a factor
of $\sim$1.1, but considering the errors in the fitting values, we have
assumed normal reddening law ($R_{V}$ =3.1) for the S301 region.

Figure 15:  $(V-I_{c}),(V-J),(V-H),(V-K)$ vs. $(B-V)$ TCDs for the member
stars associated with the S301 region. Straight lines show the least-square
fit to the distribution of stars.

##  Appendix B YSOs identification and classification

To make a census of YSOs in the present study, we used the observed NIR and
archival MIR data of S301 in $\sim$ $18^{\prime}.5\times 18^{\prime}.5$ field
of view around the central massive star ALS 207. We used the following schemes
to identify and classify YSOs in the present study : We took the MIR data of
S301 from the ALLWISE catalog of WISE and used the procedure outlined in
Koenig & Leisawitz (2014) to identify YSOs. The procedure contains several
photometric quality criteria for different WISE bands as well as other
selection criteria to isolate candidates like extra galactic contaminants like
AGN, AGB stars and star-forming galaxies from the sample. In the Figure 16
(left panel), we have shown $([3.4]-[4.6])$ versus $([4.6]-[12])$ TCD for all
the sources belonging to this region. Using this procedure, we have identified
a single Class i and five Class ii sources shown with blue and green colors
respectively in Figure 16 (left panel). We also used the observed NIR data
from TIRSPEC along with the 2MASS data to identify YSOs in the region. For
that we made a combined catalog using the observed NIR data and 2MASS data as
the brighter sources are saturated in the TIRSPEC observation and were
replaced with the 2MASS data. We also removed those sources from the catalog
which have their counterparts in the ALLWISE catalog to cancel out any
possible redundancy in YSOs identification. The NIR TCD has been used to
identify YSOS using the final NIR catalog and the scheme is well described in
Ojha et al. (2004). The NIR TCD is plotted in Figure 16 (right panel) with all
the stars in our final NIR catalog. In Figure one can see the thick broken
curves representing the MS and giant branches (Bessell & Brett, 1988), the
locus of unreddened Classical T Tauri Stars (CTTS) (Meyer et al., 1997) is
shown by a dotted line. The parallel dashed lines drawn from the tip of the
giant branch, base of the MS branch and the tip of the intrinsic CTTS line,
are the reddening vectors. The sources belonging to the ‘F’ region in the
figure are recognized as either field stars or Class iii sources while the
sources in the ‘T’ region (between the middle and lower reddening lines) are
categorized as the CTTS or Class ii sources. The sources falling in the ‘P’
region are classified as Class i sources. Using the NIR data, we finally
identified 5 Class i and 26 Class ii sources marked with blue and green
colors, respectively in Figure 16 (right panel). In total, 37 YSOs were
identified in the $18^{\prime}.5\times 18^{\prime}.5$ FOV around the central
massive star ALS 207.

Figure 16:  Left panel: ${[4.6]-[12]]}$ vs. ${[[3.4]-[4.6]]}$ TCD for the
sources in the $\sim 18^{\prime}.5\times 18^{\prime}.5$ FOV of S301.The YSOs
classified as Class I and Class II, are based on the color criteria by Koenig
& Leisawitz (2014). Right panel: ${[H-K]}$ vs. ${[J-H]}$ TCD (Ojha et al.,
2004) for the sources in the same FOV. The continuous and thick magenta dashed
curves represent the reddened MS and giant branches (Bessell & Brett, 1988),
respectively. The dotted magenta line indicates the locus of dereddened CTTSs
(Meyer et al., 1997). The parallel magenta dashed lines are the reddening
lines drawn from the tip (spectral type M4) of the giant branch (left
reddening line), from the base (spectral type A0) of the MS branch (middle
reddening line) and from the tip of the intrinsic CTTS line (right reddening
line). The crosses on the reddening lines show an increment of $A_{V}$ = 5
mag. The YSOs classified as Class i and Class ii are shown with blue and green
circles, respectively.