# Structure of IRAS 05168+3634 star-forming region

Nikoghosyan E. H  Azatyan N. M  Andreasyan D. H  Baghdasaryan D. S
<EMAIL_ADDRESS>
###### Abstract

This study aims to determine the main physical parameters (N(H2) hydrogen
column density and Td dust temperature) of the Interstellar medium, and their
distribution in the extended star-forming region, which includes IRAS
05156+3643, 05162+3639, 05168+3634, 05177+3636, and 05184+3635 sources. We
also provide a comparative analysis of the properties of the Interstellar
medium and young stellar objects. Analysis of the results revealed that
Interstellar medium forms relativity dense condensations around IRAS sources,
which are interconnected by a filament structure. In general, in sub-regions
Td varies from 11 to 24 K, and N(H2) - from 1.0 to 4.0 $\times$ 1023 cm-2. The
masses of the ISM vary from 1.7 $\times$ 104 to 2.1 $\times$ 105 M⊙. All
BGPSv2 objects identified in this star-forming region are located at the N(H2)
maximum. The direction of the outflows, which were found in two sub-regions,
IRAS 05168+3634 and 05184+3635, correlates well with the isodenses’ direction.
The sub-regions with the highest N(H2) and Interstellar medium mass have the
largest percentage of young stellar objects with Class I evolutionary stage.
The wide spread of the evolutionary ages of stars in all sub-regions (from 105
to 107 years) suggests that the process of star formation in the considered
region is sequential. In those sub-regions where the mass of the initial,
parent molecular cloud is larger, this process is likely to proceed more
actively. On the Gaia EDR3 database, it can be assumed that all sub-regions
are embedded in the single molecular cloud and belong to the same star-forming
region, which is located at a distance of $\sim$ 1.9 kpc.

00footnotetext: Byurakan Astrophysical Observatory, 0213, Aragatsotn prov.,
Armenia

Keywords ISM: clouds, dust – radiative transfer – infrared: stars –
individual: IRAS 05156+3643, IRAS 05162+3639, IRAS 05168+3634, IRAS
05177+3636, IRAS 05184+3635

##  1 Introduction

It is known that star formation is an intrinsically complex process involving
the collapse and accretion of matter onto protostellar objects, but also the
mass loss from the star-forming system in the form of bipolar outflows or
stellar wind (Lada and Lada, 2003). Therefore, there is a genetic connection
between young stellar objects (YSOs) and surrounding gas-dust matter (e.g.
González-Samaniego and Vazquez-Semadeni, 2020). This necessitates an
integrated approach to study star-forming regions. The integrated approach
implies a detailed study and determination of the main properties of already
formed young stellar clusters (density, mass function, evolutionary age
distribution, etc.) and the environment (density, temperature, chemical
composition, maser emission, etc.). Moreover, embedded stellar clusters can
many tell us about the initial star formation scenarios. For example, if the
star formation is triggered, the age spread of new generation stars should be
small, while in self-initiated condensations the age spread of young stellar
clusters is large (e.g. Zinnecker et al., 1993; Preibisch, 2012).
Consequently, an integrated approach is needed to understand the holistic
picture of the star formation process, which includes the study of both ISM
and YSOs.

This work aims to study the Interstellar medium (ISM) of the molecular cloud
and search for evidence of a relation between the properties of YSOs and their
parent molecular cloud in the extended star-forming region associated with
IRAS 05168+3634 source, which is also known as Mol 9 (Molinari et al., 1996).
There are different manifestations of star formation activity in the region,
associated with this IRAS source: H2O, NH3, 44 GHz CH3OH, and OH maser
emission (Zhang et al., 2005; Molinari et al., 1996; Ruiz-Velasco et al.,
2016; Fontani et al., 2010), CS emission (Bronfman et al., 1996), SiO (J = 2
$-$ 1) line emission (Harju et al., 1998), 13CO cores (Guan et al., 2008),
etc. Zhang et al. (2005) have discovered a molecular outflow in this region,
and Wolf-Chase et al. (2017) have detected collimated outflows of near-
infrared Molecular Hydrogen emission-line Objects (MHOs).

According to Sakai et al. (2012) the trigonometric parallax of IRAS 05168+3634
is 0.532 $\pm$ 0.053 mas, which corresponds to a distance of
1.88${}^{+0.21}_{-0.17}$ kpc. However, a kinematic distance was estimated at
6.08 kpc (Molinari et al., 1996). Taking into account the high accuracy of
direct parallax measurement, in our work all calculations were performed for
this distance estimate.

The embedded stellar cluster, associated with IRAS 05168+3634 star-forming
region has been investigated by various authors in the near-infrared (NIR) and
middle-infrared (MIR) wavelengths (Kumar et al., 2006; Faustini et al., 2009).
Wang et al. (2009) suggested that in the vicinity of IRAS 05168+3634 source
there is a high-mass star-forming region in the pre-UC HII phase. Azatyan et
al. (2016) have shown that in the star-forming region a bimodal cluster is
located. Moreover, a more extensive and detailed study shown that IRAS
05168+3634 source is embedded in the extended molecular cloud, which includes
four additional star-forming sub-regions, associated with IRAS 05156+3643,
05162+3639, 05177+3636, and 05184+3635 sources (Azatyan, 2019, hereafter A19).
Totally, in the molecular cloud, it was identified 240 YSOs, with evolutionary
ages from 0.1 to 3 Myr, and masses from 0.2 to 7 M⊙ (A19). It should be noted
that, excluding IRAS 05168+3634, there is very little information about other
IRAS sources and their surrounding ISM.

The paper presents the results of a study of ISM properties, including
hydrogen column density and dust temperature in IRAS 05168+3634 extended star-
forming region. We also provide a comparative analysis of the properties of
the ISM and YSOs. We have organized the paper in the following way: Section 2
describes used data; in Section 3.1 we discuss the properties of the gas–dust
matter in the region; in Section 3.2 it is analyzed the properties of each
sub-region individually. The obtained results are discussed and summarized in
Sections 4 and 5, respectively.

##  2 Used data

For study gas-dust matter, we used far-infrared (FIR) wavelengths images in
the range of 160–500 $\mu$m, obtained by using the Photodetector Array Camera
and Spectrometer (Poglitsch et al., 2010), as well as the Spectral and
Photometric Imaging Receiver (Griffin et al., 2010) at the 3.5 m Herschel
Space Observatory. We used the 2.5 Level PACS and SPIRE maps available in the
Herschel Science Archive. The observations for PACS (160 $\mu$m band) and
SPIRE (250$-$500 $\mu$m bands) maps were carried out in parallel mode with
fast scanning speed (60 arcsec/sec). The FWHM beam sizes of the original
Herschel maps are approximately 12.3, 17.6, 23.9, and 35.2 arcsec/pixel at
160, 250, 350, and 500 $\mu$m, respectively.

We also used the images obtained by the Wide-field Infrared Survey Explorer
(WISE, Wright et al., 2010) in four MIR bands (3.4, 4.6, 12, and 22 $\mu$m).
The angular resolutions of WISE maps are respectively 6.1, 6.4, 6.5, and 12.0
arcsec/pixel at corresponding WISE bands and have an astrometric precision
better than 0.15 sources detected with good S/N. The processed WISE images
were acquired via the NASA/IPAC Infrared Science Archive (IRSA).

For checking the distance and proper motion of several objects in the star-
forming region, we used the archival data of Gaia Data Release 3 (EDR3,
https://gea.esac.esa.int/archive/, Gaia Collaboration, 2021), including
coordinates, parallaxes, and proper motions. Gaia EDR3 contains data of
astrometry and photometry for 1.8 billion sources brighter than magnitude 21,
complemented with the list of radial velocities from Gaia DR2. Gaia EDR3
represents a significant advance over Gaia DR2, with parallax precisions
increased by 30 %, proper motion precisions increased by a factor of 2, and
the systematic errors in the astrometry suppressed by 30 $-$ 40 % for the
parallaxes and by a factor $\sim$ 2.5 for the proper motions.

##  3 Results

###  3.1 Dust emission

As already noted in the Introduction, A19 found that IRAS 05168+3634 source
belongs to an extended star-forming region, which includes 4 more IRAS
sources. Around each of them, a group of YSOs with intermediate- and low-mass
is formed. The star-forming region is embedded in a molecular cloud that is
reflected in the top panel of Figure 1, where a colour-composite image
composed through the WISE 22 $\mu$m and Herschel 500 $\mu$m bands is
presented. Figure 1 shows that stellar objects are well distinguishable in the
MIR range, while the continuum emission of the ISM in this range is weak but
intense in the FIR. This indicates a relatively low temperature of the ISM.
The bottom panel of Figure 1 shows the colour-composite image composed through
the Herschel 160, 350 and 500 $\mu$m bands. The gas-dust matter has an
inhomogeneous structure, forming well-defined condensations around IRAS
sources. Condensations are most pronounced in the vicinity of IRAS 05168+3634,
05177+3636, and 05184+3635. It should be noted that the intensity of radiation
of the ISM in all three ranges is almost the same. The condensations are
connected by a filament structure, which is better expressed in the longer
wavelength range, which indicates its lower temperature.

To obtain the hydrogen column density and dust temperature, the thermal
emission from cold dust lying in the Herschel FIR optically thin bands
(160–500 $\mu$m) can be used (Hildebrand, 1983; Battersby et al., 2011).
Following the discussion in the previous studies (e.g. Battersby et al., 2011)
this wavelength range well applicable for those cases where the dust
temperature is in the range of 5-50 K, while there are no clear restrictions
for the column density. Because we excluded the emission of 70 $\mu$m band,
which would have a significant contribution from the warm dust component, and,
therefore, modelling with a single-temperature blackbody would over-estimate
the derived temperatures. Besides, in this range the optically thin assumption
would not hold.

For our task, Level 2.5 processed Herschel images were downloaded through the
Herschel Interactive Processing Environment (HIPE; Ott, 2010). To eliminate
the effect of bad pixels on the final result, we gave them the median value of
eight immediate-neighbour pixels. For the estimation of hydrogen column
density and dust temperature, using the convolution kernels of Aniano et al.
(2011), we convolved the 160–350 $\mu$m images to the resolution of the 500
$\mu$m image (the lowest among all), and regridded to a pixel scale of 14
arcsec (same as at 500 $\mu$m). This procedure were carried out using the HIPE
software.

Modified single-temperature blackbody fitting was subsequently carried out on
a pixel-by-pixel basis using the following formula:

$S_{\nu}=B_{\nu}(\nu,T_{d})\Omega(1-e^{-\tau(\nu)}),\\\ $ (1)

with

$\tau(\nu)=\mu_{H_{2}}m_{H}k_{\nu}N(H_{2}),$ (2)

where $\nu$ is the frequency, $S_{\nu}(\nu$) is the observed flux density,
$B_{\nu}(\nu,T_{d})$ is the Planck function, $T_{d}$ is the dust temperature,
$\Omega$ is the solid angle in steradians from where the flux is obtained (in
this case for all bands the solid angle subtended by a 14 arcsec $\times$ 14
arcsec pixel), $\tau(\nu)$ is the optical depth, $\mu_{H_{2}}$ is the mean
molecular weight (adopted as 2.8 here), $m_{H}$ is the mass of hydrogen,
$k_{\nu}$ is the dust opacity, and $N(H_{2})$ is the hydrogen column density.
For opacity, we adopted a functional form of
$k_{\nu}=0.1(\nu/1000\,GHz)^{\beta}\,cm^{2}g^{-1}$, with $\beta=2$ (see
Hildebrand, 1983; André et al., 2010). For each pixel, equation (1) was fitted
using the four data points (160, 250, 350, and 500 $\mu$m) keeping Td and
N(H2) as free parameters. Launhardt et al. (2013) used a conservative 15$\%$
uncertainty in the flux densities of the Herschel bands. We adopted the same
value here for all bands. The uncertainties of the parameters were derived
using Pearson’s $\chi^{2}$ statistics:

$\chi^{2}=\sum_{i=1}^{N}\frac{(D_{i}-F_{i})^{2}}{F_{i}},$ (3)

where Di is the observed flux and Fi is the flux predicted by the model, N is
the number of bands.

From the derived column density values, we estimate the mass of the dusty
clumps using the following expression:

$M_{clump}=\mu_{H_{2}}m_{H}Area_{pix}\sum N(H_{2}),$ (4)

where Areapix is the area of a pixel in cm2.

Using the modified blackbody fitting on the Herschel images obtained at 160,
250, and 500 $\mu$m bands, we constructed maps of hydrogen column density and
dust temperature in the considered star-forming region. The maps are presented
in Figure 2. On the N(H2) and Td maps, the outer contours correspond to 0.9
$\times$1023 cm-2 (that exceeds the average N(H2) of the surrounding molecular
cloud by $\sigma$) and 11 K, respectively. The maps show that the star-forming
region, which includes five IRAS objects, clearly stands out against the
background of the surrounding molecular cloud both with a higher density and
temperature. The relatively hotter gas-dusty matter forms dense condensations
around IRAS objects. An exception is the IRAS 05162+3639 sub-region, near
which on the N(H2) map there is practically no region with a relatively higher
density. In this regard, it should be noted that in A19, around IRAS
05162+3639 source, in essence, no group of YSOs has been identified, but only
5 stars. In addition, in the vicinity of IRAS 05162+3639 and IRAS 05156+3643
sources, in contrast to the other three IRAS sources, no increase in
temperature is observed. In general, in whole star-forming region Td varies
from 11 to 24 K, and N(H2) - from $\sim$ 1.0 to 4.0 $\times$ 1023cm-2.
Detailed analysis of each sub-region is given below (see Section 3.2 and Table
1).

Fig. 1 : Colour-composite images of IRAS 05168+3634 star-forming region. Top
panel: W4 22 $\mu$m (green) and Herschel 500 $\mu$m (red), bottom panel:
Herschel 160 $\mu$m (blue), 350 $\mu$m (green), and 500 $\mu$m (red). The
positions of IRAS sources are marked by black crosses. Fig. 2 : Maps of N(H2)
column density (top panel), Td dust temperature (middle panel), and $\chi^{2}$
uncertainty of the parameters (bottom panel) of IRAS 05168+3634 star-forming
region. On the N(H2) map the outer isodense corresponds to 0.9 $\times$ 1023
cm-2, and the interval between isodenses is $\sim\,$0.4 $\times$ 1023 cm-2. On
the Td map, the outer isotherm corresponds to 11 K, and the interval between
isotherms is 1 K. The arrow on the bottom panel indicates the positions of the
region with a relatively high level of $\chi^{2}$. The positions of IRAS are
marked by crosses. Fig. 3 : Herschel image at 250 $\mu$m (2.5 Level). The red
circle marks the location of bad pixels in the vicinity of IRAS 05168+3634
source.

It should be noted that the parameters obtained with the modified blackbody
model do not deviate from certain errors, which are related to a number of
factors, including the quality of the images used and the uncertainty of
calibration of Herschel images. Besides, the errors are a strong function of
the parameters’ values. To a large extent, the final result is also affected
by the fact that star-forming regions show the structure of many scales, and
so fitting a single column density and temperature to any point does not
adequately represent the whole region (Battersby et al., 2011). Nevertheless,
the $\chi^{2}$ map, presented in the bottom panel of Figure 2, indicates that
the fitting uncertainties within the star-forming region are relatively small.
In general, the $\chi^{2}$ is about 0.01 $-$ 0.02. However, directly in the
center of IRAS 05168+3634 sub-region, the $\chi^{2}$ value increases up to
$\sim$ 1.5. This area is marked with an arrow in the bottom panel of Figure 2.
In this case, the increase of $\chi^{2}$ is most likely explained by the
quality of the images. Unfortunately, in all available Herschel images, this
star-forming region is located at the edge, where the image quality,
especially in the 250 $\mu$m channel, is not good enough. Figure 3 clearly
shows that in the area of interest, in the image of Level 2.5, there are many
bad pixels. Moreover, a whole strip of bad pixels passes through the bright
condensation in IRAS 05168+3634 sub-region, which whatever filtering cannot
completely remove. This is most likely the reason for the increase in the
$\chi^{2}$ value in this region.

Fig. 4 : Colour-composite image of IRAS 05156+3643 and IRAS 05162+3639 sub-
regions: W3 $-$ green, W4 $-$ red, white curves $-$ N(H2) isodenses. In this
and subsequent figures the outer isodense corresponds to 1.0 $\times$ 1023
cm-2 and interval between isodenses is 0.1 $\times$ 1023 cm-2. The positions
of IRAS 05156+3643 and 05162+3639 are marked by black crosses, BGPSv2
G170.384-00.407 $-$ by blue diamond.

###  3.2 Individual Regions

IRAS 05156+3643. Figure 4 shows the superposition of the N(H2) enlarged
contours of this sub-region with images W3 and W4. In these ranges, the key,
brightest stellar members are clearly distinguishable. The maximal density and
temperature in this sub-region are 1.6 $\times$1023 cm-2 and 12 K,
respectively. The masses of ISM within the isodence corresponding to 1.1
$\times$1023 cm-2, which exceeds the average surrounding medium hydrogen
column density by 2$\sigma$, for this sub-region and all subsequent are given
in Table 1. In the region with a maximum density, there is an object
identified in 1.1ṁm Bolocam Galactic Plane Survey (BGPS) of dust emission. It
is BGPSv2 G170.384-00.407, marked by a blue diamond in Figure 4. It should be
noted that observations in the optically thin, minimally affected by
temperature millimeter continuum provide one of the best methods to identify
star-forming clumps (Ginsburg et al., 2013). It is quite expected that the
BGPS object is localized precisely in the region of the N(H2) maximum.
According to A19, a young star with ID 239, 4.4 $\pm\,$0.3 M⊙ mass and
evolutionary age of $\sim$ 106 years (here and after ID, mass, and
evolutionary age are borrowed from Tables A.1 and A.3 in A19) is associated
with it. The dust temperature reaches the maximum for this sub-region (12 K)
precisely at the location of BGPSv2 G170.384-00.407 and IRAS 05156+364
sources. IRAS 05156+3643, also known as MSX G170.3964-00.3827, some shifted
from the density maximum. The YSO with ID 234 in the list in A19 is identified
with it. It is the star with $\sim$ 3.8 $\pm$ 3.6 M⊙ mass and evolutionary age
of $\sim$ 106 years. According to coordinates and 2MASS JHK photometric data,
this stellar object corresponds to the high proper-motion star LSPM
1267-0099892 from the LSPM-NORTH catalog (Lépine and Shara, 2005). Taking into
account the high accuracy of parallax measurements and proper motion in Gaia
EDR3, we tried to identify the stellar object associated with IRAS 05156+3643
in this database. According to the coordinates, it is the object with ID
187051701258568960 in Gaia EDR3, which parameters are presented in Table 2
(see Section 4.2). Unfortunately, the parallax measurement accuracy is not
high enough and is $\varpi$/$\sigma_{\varpi}$ = 1.7, while for high quality
parallaxes $\varpi$/$\sigma_{\varpi}$ $>$ 5 (Gaia Collaboration, 2021).
However, according to the transformations of Bailer-Jones et al. (2021), which
can infer meaningful distances even if the parallaxes are negative and/or the
parallax accuracy is not enough, the low distance estimation is $\sim$ 2.2
kpc, what is close to the parallax estimation for the star-forming region.
Using the Galactocentric transformations of Johnson and Soderblom (1987) (the
similar transformation is also available in
Astropy.coordinates111https://astropy-
cjhang.readthedocs.io/en/latest/coordinates/index.html package), we compute
peculiar velocity of this star for 2.2 kpc distance: Vpec = 57 km/s.

The two most common scenarios to explain the existence of runaway stars and
their high velocity are: (1) the supernova of the companion star in a massive
binary (Blaauw, 1961) and (2) the dynamical ejection from a young cluster in
the early stages of evolution (Poveda et al., 1967). Apparently, the high
peculiar velocity of the stellar object associated with IRAS 05156+3643 is the
result of (2) scenario. In this scenario, the runaway gains its velocity in
the range 35 $-$ 185 km/s through a dynamical interaction with one or more
other stars (Poveda et al., 1967).

IRAS 05162+3639. In the vicinity of this IRAS source, we were unable to
identify a region with a relatively high column density against the background
of the surrounding molecular cloud. The dust temperature at the location of
the IRAS source is 12 K. Two stellar objects are associated with the IRAS
source (A19). It is the object with ID 190 (1.0 $\pm$ 0.4 M⊙ mass and
evolutionary age of $\sim$ 105 years), as well as the object with ID 191 (0.6
$\pm$ 0.1 M⊙ mass and evolutionary age of $\sim$ 106 years). Recall that A19
identified only five YSOs, which can be considered as potential members of
this star-forming sub-region. In general, regarding their position on the
colour-magnitude diagram and the results of the SED fitting tool in A19, there
is no reason to assert that these stars are at an earlier evolutionary stage
than the YSOs in other sub-regions, i.e. to suggest that the process of star
formation is at an earlier stage here. Most likely, the initially not so
significant concentration of ISM in this region contributed to the formation
of only a few stellar objects.

IRAS 05168+3634. The contours of the hydrogen column density and the dust
temperature, superimposed on the W3 and W4 images, are shown in Figure 5. The
maximum values of density and temperature are 3.8 $\times$ 1023cm-2 and 24 K,
respectively (see Table 1). It should be noted that in this region the values
of these parameters are noticeably higher than in the others.

Figure 5 clearly shows that the localization of density and temperature maxima
coincides. As in the previous region, the BGPSv2 object is located at the
density and temperature maximum (blue diamond on Figure 5). It is a dense core
G170.661-00.249. According to the data in Zhang et al. (2005), the CO outflow
is centered near this dense core. Moreover, in Wolf-Chase et al. (2017) at
least two collimated outflows of near-infrared MHOs have been detected, which
follow the large-scale morphology of CO outflow. Wolf-Chase et al. (2017)
suggest that their source may be one YSO, identified in the WISE database as
J052022.03+363756.5, which is only 6 arcsec away from the BGPS object. It
should be noted that the direction of the isodences (PA is about 35∘) is
almost identical to that of CO outflow (PA is about 40∘). According to the
data in A19, the closest to G170.661-00.249 and J052022.03+363756.5 (on a
distance 6 $\arcsec$ $-$ 14 $\arcsec$) two objects are located: ID 156 and ID
175. Unfortunately, their parameters were not determined by the SED fitting
tool. According to photometric data, these stars are objects of the II and I
evolutionary classes, respectively. It can be assumed that each of them can be
the source of one of two near-infrared MHOs outflows.

Fig. 5 : Colour-composite images of IRAS 05168+3634 and IRAS 05177+3636 sub-
regions: W3 - green, W4 - red. Top panel: white curves are N(H2) isodenses.
The outer isodense and interval between them are the same as in the previous
figure. Bottom panel: white curves are Td isotherms. The outer isotherm
corresponds to 12 K. The interval between isotherms is 1 K. The positions of
IRAS 05156+3643 and 05162+3639 sources marked by black crosses, BGPS objects
$-$ by diamonds, MSX objects $-$ by red diamonds, and pre-UC HII region $-$ by
blue X.

According to previous studies (Molinari et al., 1996; Wang et al., 2009),
northeast of BGPSv2 G170.661-00.249, at a distance of about 30 $\arcsec$ it
was detected a high-mass star-forming region in pre-UC HII phase, which
appears to be associated with the infrared source (blue X on Figure 5). The
radius of the pre-UC HII region is more than 8 $\arcsec$ (Wang et al., 2009).
In A19, the stellar object with ID 186 is the closest to the pre-UC HII region
($\sim$ 11 $\arcsec$). Unfortunately, its parameters (mass and evolutionary
age) have not been determined. However, according to photometric data, this
YSO has a very significant infrared excess (J - K = 4.7), which may indicate
its very early evolutionary stage. Although the distance is slightly larger
than the radius, this object can presumably be considered as an infrared
source of the pre-UC HII region. Of course, it is possible that the central
protostellar object of pre-UC HII formation is at a very early evolutionary
stage and is so embedded that it has not been identified in A19 in NIR and MIR
bands. The closest MSX object G170.6589-00.2334, which is associated with
intermediate-mass YSO (ID 177, 1.9 $\pm$ 0.7 M⊙) with an evolutionary age of
$\sim$ 105 years, is located much further, at a distance of about 24 $\arcsec$
to the northeast (red diamond on Figure 5).

IRAS 05168+3634 source (MSX G170.6575-00.2685), with which the intermediate-
mass YSO (5.1 $\pm$ 0.3 M⊙) with an evolutionary age of about 106 years is
associated (ID 183 in A19), is displaced from the position of the N(H2) and Td
maxima (black cross on Figure 5). Directly in its vicinity, the value of N(H2)
is 1.4 $\times$ 1023 cm-2, which exceeds the density estimate from 13CO data J
= 2 $-$ 1 observations (2.1 $\times\,10^{22}$ cm-2, Wang et al., 2009).
Another MSX object, G170.6758-00.2691, marked by a red diamond, is located
almost on the periphery of the cluster, southeast of IRAS 05168+3634. It is
also associated with an intermediate-mass YSO (4.9 $\pm$ 0.5 M⊙) with an
evolutionary age of $\sim$ 106 years (ID 182 in A19).

IRAS 05168+3634 sub-region is entirely embedded in the ”non-sourceless” (there
is at least one MSX source within on the beam size from the core centre) 13CO
core (Guan et al., 2008). According to the results presented in Guan et al.
(2008), two more 13CO cores, but already ”sourceless” (the core is sourceless
by MSX and IRAS catalogs), are located between IRAS 05168+3634 and IRAS
05177+3636 sub-regions, where no stellar objects belonging to the star-forming
region were detected (A19). Another ”non-sourceless” 13CO core was identified
in the IRAS 05177+3636 sub-region, which will be discussed below.

IRAS 05177+3636. The contours of the hydrogen column density and dust
temperature superimposed on W3 and W4 images of this star-forming region are
also presented in Figure 5. The maximum density here is 2.3 $\times$ 1023
cm-2. The isotherm corresponds to Td = 12 K. Inside it, the Td reaches its
maximum (13 K) in the vicinity of the brightest stellar objects. The IRAS
object (also known as MSX G170.7268-00.1012) is associated with the
intermediate-mass young star with 2.6 $\pm$ 1.3 M⊙ mass and an evolutionary
age of less than 106 years (ID 97 in A19). The second MSX object (red diamond
in Figure 5), G170.7196-00.1118, is also associated with intermediate-mass YSO
(4.5 $\pm$ 0.7 M⊙) and an evolutionary age also less than 106 years (ID 128 in
A19).

In contrast to the previous case, in this star-forming region, the IRAS source
is located practically in the center of the cluster, in the zone of maximum
density. There are two BGPSv2 objects in its immediate vicinity (blue diamonds
in Figure 5): G170.724-00.105 and G170.733-00.103. They are associated with
three objects of the submillimeter continuum detected by SCUBA (450 $\mu$m,
850 $\mu$m): J052109.2+363934, J052106.7+363946, and J052112.2+ 363916 (Di
Francesco et al., 2008). Besides the IRAS object, two more YSOs are located
closest to the BGPSv2 objects: the intermediate-mass star (3.7 $\pm$ 0.7 M⊙)
with an evolutionary age of less than 106 years (ID 85 in A19) and the low-
mass star (0.3 $\pm$ 0.2 M⊙), apparently at a very early evolutionary stage
(ID 96 in A19). It has an evolutionary age of less than 105 years.

We would also like to note that the search for molecular outflows based on
12CO observations in Casoli et al. (1986) did not yield a positive result.

IRAS 05184+3635. Lundquist et al. (2014), on the basis of extended emission at
12 $\mu$m (W3) and 22 $\mu$m (W4), characterized this star-forming region as a
region with blobs/shells or circular/elliptical morphology. This conclusion is
consistent with the almost spherical isodenses and isoterms of the star-
forming region, which are shown in Figure 6. The maximum values of density and
temperature are 1.5 $\times$ 1023 cm-2 and 15 K. Unlike other sub-regions,
there is a significant shift between the density and temperature maxima.
Moreover, all key, bright stellar objects, namely the IRAS (black cross) and
two MSX sources (red diamonds), are localized mainly in the region of the
temperature maximum. A young star of intermediate-mass (3.1 $\pm$ 0.6 M⊙) and
evolutionary age of $\sim$ 106 years (ID 31 in A19) is associated with the
IRAS source (or MSX G170.8276+00). There are two more MSX sources in its
vicinity: G170.8321+00.0045 and G170.8319+00.0086. According to the lists in
A19, in the vicinity of these sources, in addition to the ID 31 stellar
object, within a radius of 11 $\arcsec$ $-$ 14 $\arcsec$, only one stellar
object of the II evolutionary class has been identified (ID 11 in A19), which
mass and evolutionary age have not been determined. We would also like to note
that no BGPS objects have been identified here. In addition, no stellar
objects were identified in A19 in the vicinity of the second, northeastern
maximum of density.

There is no information in previous studies on the presence of collimated
outflow in this sub-region. In addition, the search for water maser emission
did not return a positive result (Wouterloot et al., 1993). Nevertheless, by
comparing W1, W2, and W3 images in the vicinity of IRAS 05184+3635 source, we
were able to identify a diffuse structure with excess emissions in the W2 band
(see Figure 7, left panel). In the diffuse structure, two concentrations are
clearly distinguishable, which are indicated by arrows. It is known that
stellar outflows have strong molecular hydrogen emission, which is detectable
in the visible, NIR, and MIR. In MIR shocked H2 emission appears particularly
strong in the 4.5 $\mu$m Spitzer Infrared Array Camera (IRAC) band (e. g.
Noriega-Crespo et al., 2004; Ybarra and Lada, 2009; Teixeira et al., 2008).
Unfortunately, for this star-forming region, IRAC images were obtained in only
two bands: 3.6 µm and 4.5 µm, and there are no longer wavelength images for
comparison. Therefore, we used the colour-composite WISE images, obtained in
3.4, 4.6, and 12.0 $\mu$m bands. Of course, the resolution of WISE images is
inferior to IRAC data. Nevertheless, the excellent sensitivity of the WISE
data enables the detection of many faint features. Undoubtedly, a shocked
nature of the obtained emission objects requires a spectroscopic follow-up.

For comparison, we also present the W1, W2, and W3 colour-composite image for
IRAS 05168+3634 sub-region (see Figure 7, right panel). As noted above in
Wolf-Chase et al. (2017), there were presented the results of a narrow-band
NIR imaging survey for MHOs toward IRAS 05168+3634 sub-region. It was revealed
that there is collimated chains of MHOs. We detect that several MHOs, marked
in Figure 7, also have excess emission in W2 band. This fact increases the
confidence that the concentrations of excess emissions in the W2 band in IRAS
05184+3635 sub-region are indeed traces of outflow. Their source is presumably
a stellar object 31 in A19. Moreover, we want to draw attention to the fact
that, as in IRAS 05168+3634, the direction of the alleged outflow coincides
with that of the external isodenses.

It should also be noted that the search for MIR tracers of outflow carried out
in other sub-regions did not yield a positive result.

Fig. 6 : Colour-composite images of IRAS 05184+3635 sub-region: W3 - green, W4
- red. Left panel: white curves are N(H2) isodenses. Right panel: white curves
are Td isotherms. The outer isodense and isotherm, as well as the intervals
between them, are the same as in the previous figure. The position of IRAS
05184+3635 source is marked by a black cross, of the MSX objects - by red
diamonds. Fig. 7 : Colour-composite images of IRAS 05184+3635 (left panel) and
IRAS 05168+3634 (right panel) sub-regions: W1 (blue), W2 (green), and W3
(red). The mid-infrared tracers of outflow in IRAS 05184+3635 sub-region are
marked by arrows. The near-infrared MHOs in the IRAS 05168+3634 sub-region are
labeled.

##  4 Discussion

###  4.1 ISM and stellar content

Table 1 shows the main parameters of ISM in the IRAS sub-regions, including
N(H2), Td, and ISM masses. For comparison, we also included in Table 1 some
parameters of the stellar content (sub-region’s radius, YSOs number, surface
stellar density, range of stellar masses) borrowed from A19. Taking into
account the number of identified stellar objects, the average value of their
masses, as well as the ISM mass in the surrounding cloud, we find that star
formation efficiency (SFE = Mstars/(Mstars \+ MISM)) in all sub-regions is
less than 0.1%. Even if we take into account that not all stellar objects were
detected, the SFE value still indicates an active star formation process in
the entire region.

Table 1 : Parameters of the stellar content and ISM in IRAS sub-regions

Parameter / IRAS | 05156+3643 | 05162+3639 | 05168+3634 | 05177+3636 | 05184+3635
---|---|---|---|---|---
Radius (arcmin) | 2.8 | 0.25 | 3.0 | 3.5 | 2.5
YSO’s number | 47 | 5 | 57 | 79 | 52
nstar (arcmin-2) | 1.9 | 25.5 | 2.0 | 2.1 | 2.6
Class I (%) | 20 | - | 43 | 28 | 21
Stellar masses (M⊙) | 0.2 $-$ 1.6 | - | 0.5 $-$ 2.5 | 0.2 $-$ 2.2 | 0.3 $-$ 1.5
N(H2) ($\times$1023 cm-2) | 1.1 $-$ 1.6 | 0.9 $-$ 1.0 | 1.1 $-$ 3.8 | 1.1 $-$ 2.3 | 1.1 $-$ 1.5
Td (K) | 11 $-$ 12 | 11 $-$ 12 | 11 $-$ 24 | 12 $-$ 13 | 12 $-$ 15
∗ISM Mass (M⊙) | (1.7 $\pm$ 0.05) $\times$ 104 | - | (2.1 $\pm$ 1.6) $\times$ 105 | (9.2 $\pm$ 0.3) $\times$ 104 | (4.0 $\pm$ 0.1) $\times$ 104

††footnotetext: ∗ISM masses are determined in the area limited by the isodence
corresponding to 1.1 $\times$ 1023 cm-2 value, which exceeds the average
hydrogen column density in surrounding medium by 2$\sigma$. The error bars are
due to the average value of the uncertainty of parameters ($\chi^{2}$) in the
sub-regions and the uncertainty in estimating the distance to the star-forming
region.

The BGPSv2 objects identified in the three sub-regions (IRAS 05156+3643,
05168+3634, and 05177+3636) are located at maximum density and temperature. On
the one hand, this is an expected fact, since these 1.1 mm sources are good
indicators of dust emission and, therefore, star-forming clumps. On the other
hand, this confirms the reliability of our results. However, another key,
brightest stellar objects, including associated with the IRAS and MSX sources,
localized as regions of N(H2) and Td maxima, as well as significantly
displaced from them. We also want to note that in most cases, that the
evolutionary age of associated with the BGPSv2, IRAS and MSX sources
intermediate-mass stellar objects is about 106 years.

The dust temperature, excluding IRAS 05168+3634 sub-region, is only a few
degrees higher than that in the surrounding molecular cloud. According to the
results in A19, no high-mass stars were found in the cluster, which is
probably the reason for the relatively low temperature in this region. A small
increase in Td to 15 K in the IRAS 05184+3635 sub-region, most likely, can be
explained by the highest stellar density. Since only 5 stellar objects were
detected in IRAS 05162+3639 sub-region, we do not take it into account in the
comparison. As mentioned above, in IRAS 05168+3634 sub-region, it was
identified a high-mass star-forming region in pre-UC HII phase (Molinari et
al., 1996), which could be the reason why the dust temperature in this region
is significantly, almost 10 K, higher.

Regarding the distribution of N(H2), we would like to draw attention to one
more fact. In the two sub-regions, IRAS 05168+3634 and 05184+3635, where the
outflow was detected, its orientation is well correlated with the isodenses’
directions. This is in good agreement with the findings in Arce et al. (2010);
Mottram and Brunt (2012) that outflows from newly formed stars inject momentum
and energy into the surrounding molecular cloud, which may affect the
morphology of the ISM, including the distribution of the hydrogen column
density in the local environment.

###  4.2 Distance of the star-forming region

As noted above, in Section 3.1, IRAS 05156+3643 or the 234 star in A19 was
identified in the Gaia EDR3 database. The parallax measurement accuracy was
not high enough ($\varpi$/$\sigma_{\varpi}$ = 1.7), however, according to the
transformations of Bailer-Jones et al. (2021) the low distance estimation is
$\sim$ 2.2 kpc, what is close to the parallax estimation with VERA ($\sim$ 1.9
kpc, Sakai et al., 2012). This led to an attempt to identify the remaining
stars from the list in A19 in the Gaia EDR3 database. In total, we were able
to identify 65 objects, but only for 11 of them the parallax measurement
accuracy is high enough ($\varpi$/$\sigma_{\varpi}$ $>$ 5). The data of these
11 objects are shown in Table 2. Such a small number of objects identified
with sufficient accuracy is quite expected. The embedded in a dense ISM
cluster contains YSOs, which are surrounded by optically thick circumstellar
disks and envelopes. In combination, this makes cluster members difficult to
access optically. The number of identified objects is not sufficient for a
detailed statistical study, but nevertheless allows us to draw some
conclusions about the distance of the star-forming region as a whole.
Distances of 6 out of 11 stars in Table 2 are close to the parallax estimation
for IRAS 05168+3634. Their positions and ID numbers in Table 2 are shown on
Figure 8. We can see that these stellar objects are scattered in almost all
sub-regions, with the exception of only IRAS 05162+3639. Moreover, one of
them, the 97th, is associated with IRAS 05177+3636. We would like to remind
that direct measurements of distances were made only for IRAS 05168+3634. The
conclusion that all sub-regions belong to a single star-forming region in A19
was made mainly on the statistical analysis of the distribution of YSOs of the
I evolutionary class. The result obtained from Gaia EDR3 data can be
considered as one more argument in favor of the fact that all sub-regions are
embedded in the single molecular cloud and belong to the same star-forming
region, which is located at a distance of $\sim$ 1.9 kpc.

Unlike IRAS 05156+3643, the Vpec values for these 11 stars (see Table 2) do
not give grounds for considering them as runaway objects. Although it should
be noted that the peculiar velocities of stellar objects are incomplete, since
their radial velocities are not available.

Table 2 : Gaia EDR3 data

N | Gaia EDR3 |  $\varpi$ (mas) |  $\sigma_{\varpi}$ (mas) | PM (mas/yr) | LD (pc) | HD (pc) | Vpec (km/sec)
---|---|---|---|---|---|---|---
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8)
1 | 184011624623256704 | 0.520 | 0.025 | 3.054 | 1780 | 1963 | 26 $\pm$ 1.5
14 | 184013166513879808 | 7.795 | 1.234 | 18.875 | 116 | 162 | 16 $\pm$ 1.0
60 | 184053612223579136 | 0.558 | 0.052 | 2.846 | 1597 | 1936 | 20 $\pm$ 2.0
82 | 184059620881914880 | 2.443 | 0.390 | 1.875 | 376 | 516 | 10 $\pm$ 0.5
91 | 184053436128920448 | 1.683 | 0.261 | 2.755 | 532 | 758 | 11 $\pm$ 0.3
97 | 184059350299876608 | 0.541 | 0.087 | 2.525 | 1608 | 2290 | 17 $\pm$ 4.0
101 | 184053401769173888 | 0.965 | 0.130 | 2.864 | 897 | 1139 | 12 $\pm$ 0.5
132 | 184055536368961792 | 1.279 | 0.019 | 11.533 | 744 | 769 | 36 $\pm$ 1.0
143 | 184054746094985984 | 0.505 | 0.053 | 2.819 | 1749 | 2170 | 22 $\pm$ 3.0
219 | 187052182295036672 | 0.648 | 0.092 | 2.854 | 1319 | 1820 | 17 $\pm$ 3.0
227 | 187051877354793984 | 0.583 | 0.109 | 3.428 | 1534 | 2662 | 22 $-$ 40
234 | 187051701258568960 | 0.401 | 0.236 | 3.428 | 2224 | 4780 | 51$-$110

††footnotetext: (1) $-$ a serial number of stars in A(19), (2) $-$ a source
designation in Gaia EDR3, (3) - (5) $-$ parallax, parallax error and proper
motion obtained from Gaia EDR3 database, (6), (7) $-$ low and high distances
obtained through Bailer-Jones et al. (2021), (8) $-$ peculiar velocity
obtained by using the Galactocentric transformations of Johnson and Soderblom
(1987).

Fig. 8 : W4 image of the star-forming region. The positions (red circles) and
ID numbers of 6 stars, which, according to the Gaia EDR3 data, are located at
a distance of $\sim$ 2 kpc (see Table 2) are marked.

###  4.3 Origin of the star-forming region

If we classify the evolutionary stage of sub-regions in relation to the
percentage of Class I and Class II YSOs, then IRAS 05168+3634 sub-region can
be considered the “youngest”. The next “youngest” sub-region by the percentage
of Class I stellar objects is IRAS 05177+3636. At the same time, IRAS
05168+3634 and IRAS 05177+3636 sub-regions have the highest hydrogen column
density and ISM mass. The sub-regions around IRAS 05184+3635 and IRAS
05156+3643 in the outer part of the molecular cloud, according to the same
criterion, are the “oldest”. It should be noted once again that since there is
no real concentration around IRAS 05162+3639, we do not consider this region.
On the other hand, if we consider the position of stellar objects on the
colour-magnitude diagram in A19, it is clearly seen that stars from all sub-
regions have a wide spread relative to the colour index (J-K)0 and, therefore,
isochrones. For clarity, we also built a histogram of the evolutionary ages of
sub-regions’ members, defined using the SED fitting tool and borrowed from
A19. Unfortunately, this information is available only for 50% of all
identified members of the cluster. A histogram for a distance of 1.9 kpc is
shown in Figure 9. The histogram also clearly shows that the evolutionary age
of stars in all sub-regions has very wide spread. The overwhelming majority of
them are located in the range from 105 to 107 years.

Fig. 9 : Histogram of the evolutionary ages determined by the SED fitting tool
for members of the IRAS sub-regions. The bin size corresponds to Log (Age) =
0.25.

On the basis of the above, it can be assumed that, in general, the star
formation process in the considered region is sequential. Moreover, in those
sub-regions where the mass of the initial, parent molecular cloud is greater,
this process is more active and presumably has a longer duration. As a result,
at the moment, the percentage of younger stars could be more. Based on the
results obtained from different regions of star formation, it seems that some
clusters owe their origin to an external triggering shock and other clusters
can be formed as independent condensations in molecular clouds (e.g. Elmegreen
and Lada, 1977; Zinnecker et al., 1993). If the star formation is triggered,
the age spread of new generation stars should be small, while in self-
initiated condensations the age spread of young stellar clusters is large
(Zinnecker et al., 1993). Indeed, many careful studies of star-forming regions
find no clear evidence or at most very moderate age spreads of several Myr
(Lee and Chen, 2007). Moreover, in some cases, the measured age spread is less
than the crossing time, which is well consistent with the scenario of the fast
trigger nature of the star formation process (Preibisch, 2012, and ref.
therein). However, the age spread of the considered star-forming region is
much larger, and, therefore, it can be concluded that the stellar population
is formed as a result of independent condensations in the parent molecular
cloud.

##  5 Conclusion

The main research tasks in the star-forming region, which includes IRAS
05156+3643, 05162+3639, 05168+3634, 05177+3636, and 05184+3635 sources, are
the determination of ISM physical parameters (N(H2) hydrogen column density
and Td dust temperature), as well as their distribution. We also provide a
comparative analysis of the properties of the ISM and YSOs. To determine the
distribution of N(H2) and Td, we used the Modified blackbody fitting on
Herschel images obtained in four bands: 160, 250, 350, and 500 $\mu$m. An
analysis of the results revealed the following:

  * •

The gas-dust matter has an inhomogeneous structure, forming relativity dense
condensations around the majority of IRAS sources, which are interconnected by
a filament structure.

  * •

In general, in sub-regions Td varies from 11 to 24 K, and N(H2) - from 1.0 to
4.0 $\times$ 1023 cm-2. The masses of the ISM vary from 1.7 $\times$ 104 to
2.1 $\times$ 105 M⊙.

  * •

The SFE in the sub-regions is less than 0.1%.

  * •

All identified in this star-forming region BGPSv2 objects are located at the
density maximum.

  * •

In two sub-regions, IRAS 05168+3634 and 05184+3635, where the outflows were
detected, their orientations correlate well with isodenses directions.

  * •

The intermediate-mass YSO associated with IRAS 05156 +3643 source is a runaway
star with Vpec equal to $\sim$ 50 km/s.

  * •

On the Gaia EDR3 database, it can be assumed that all sub-regions are embedded
in the single molecular cloud and belong to the same star-forming region,
which is located at a distance of $\sim$ 1.9 kpc.

  * •

The sub-regions with the highest hydrogen column density and ISM mass have the
largest percentage of YSOs with Class I evolutionary stage.

Based on the fact that the evolutionary ages of stars in all sub-regions have
a very wide spread (from 105 to 107 years), we assumed that in the considered
region the stellar population is formed as a result of independent
condensations in the parent molecular cloud, and, therefore, the star
formation process is sequential. In sub-regions where the mass of the initial,
parent molecular cloud is greater, this process is most likely more active.

Acknowledgements We are very grateful to the anonymous referee for the helpful
comments and suggestions. This work was made possible by a research grant
number № 21AG-1C044 from Science Committee of Ministry of Education, Science,
Culture and Sports RA. This work also was made possible in part by a research
grant astroex-5525 from the Yervant Terzian Armenian National Science and
Education Fund (ANSEF) based in New York, USA. This work is based on
observations made with Herschel, a European Space Agency Cornerstone Mission
with significant participation by NASA. Support for this work was provided by
NASA through an award issued by JPL/Caltech.This publication also 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.We gratefully acknowledge the use of data
from the NASA/IPAC Infrared Science Archive, which is operated by the Jet
Propulsion Laboratory, California Institute of Technology, under contract with
the National Aeronautics and Space Administration.This work also presents
results from the European Space Agency (ESA) space mission Gaia. Gaia data are
being processed by the Gaia Data Processing and Analysis Consortium (DPAC).
Funding for the DPAC is provided by national institutions, in particular the
institutions participating in the Gaia MultiLateral Agreement (MLA).

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