# ANALYSIS OF THE INTERSTELLAR MATTER AT THE PERIPHERY OF THE SUPERSHELL
SURROUNDING THE CYG OB1 ASSOCIATION IN 2.12 MICRON MOLECULAR HYDROGEN LINE

D. S. Wiebe<EMAIL_ADDRESS>Institute of Astronomy (INASAN), Russian Academy
of Sciences, Pyatnitskaya str. 48, Moscow 119017, Russia  T. G. Sitnik
Lomonosov Moscow State University, Sternberg Astronomical Institute,
Universitetsky pr. 13, Moscow 119992, Russia  A. S. Rastorguev  Lomonosov
Moscow State University, Sternberg Astronomical Institute, Universitetsky pr.
13, Moscow 119992, Russia  Lomonosov Moscow State University, Faculty of
Physics, Leninskie Gory 1 b.2, Moscow 119992, Russia  T. A. Lozinskaya
Lomonosov Moscow State University, Sternberg Astronomical Institute,
Universitetsky pr. 13, Moscow 119992, Russia  A. M. Tatarnikov  Lomonosov
Moscow State University, Sternberg Astronomical Institute, Universitetsky pr.
13, Moscow 119992, Russia  Lomonosov Moscow State University, Faculty of
Physics, Leninskie Gory 1 b.2, Moscow 119992, Russia  A. A. Tatarnikova
Lomonosov Moscow State University, Sternberg Astronomical Institute,
Universitetsky pr. 13, Moscow 119992, Russia  A. P. Topchieva  Institute of
Astronomy (INASAN), Russian Academy of Sciences, Pyatnitskaya str. 48, Moscow
119017, Russia  M. V. Zabolotskikh  Lomonosov Moscow State University,
Sternberg Astronomical Institute, Universitetsky pr. 13, Moscow 119992, Russia
A. A. Fedoteva  Lomonosov Moscow State University, Sternberg Astronomical
Institute, Universitetsky pr. 13, Moscow 119992, Russia  A. A. Tatarnikov
Lomonosov Moscow State University, Sternberg Astronomical Institute,
Universitetsky pr. 13, Moscow 119992, Russia  Lomonosov Moscow State
University, Faculty of Physics, Leninskie Gory 1 b.2, Moscow 119992, Russia

###### Abstract

We present observations of the vdB 130 cluster vicinity in a narrow-band
filter centered at a $2.12\,\mu$m molecular hydrogen line performed at the
Caucasus Mountain Observatory of the Lomonosov Moscow State University. The
observations reveal an H2 emission shell around vdB 130, coincident with a
bright infrared shell, visible in all Spitzer bands. Also, numerous H2
emission features are detected around infrared Blobs E and W and in the
vicinity of a protocluster located to the east of the shell, in a tail of a
cometary molecular cloud. H2 emission in the vicinity of the vdB 130 cluster
is mostly generated in well-developed Hii regions and is of fluorescent
nature. In the protocluster area, isolated spots are observed, where H2
emission is collisionally excited and is probably related to shocks in
protostellar outflows. Obtained results are discussed in the context of
possible sequential star formation in the vicinity of the vdB 130 cluster,
triggered by the interaction of the expanding supershell surrounding the Cyg
OB1 association with the molecular cloud and an associated molecular filament.

ISM: kinematics and dynamics — ISM: clouds — ISM: lines and bands

##  1 Introduction

The study of the massive star formation complex in Cygnus has a long history
and goes back to Blaha and Humphreys (1989), who divided the sample of OB-
stars into five large groups based on their positions on the sky. One of these
groups is the well-known Cyg OB1 stellar association, which contains at least
50 OB stars and is surrounded by a large expanding supershell, having a size
of $3^{\circ}\times 4^{\circ}$ (Humphreys and McElroy, 1984). This work
concludes a series of our studies (Sitnik et al. 2015, 2019, 2020, hereinafter
referred to as Paper I, II, and III, and Tatarnikova et al. 2016) of the star
formation in a region ($\alpha=20^{\rm h}16^{\rm m}-20^{\rm h}18^{\rm m}$,
$\delta=39^{\circ}15^{\prime}-39^{\circ}35^{\prime}$), which is located in the
vicinity of the young embedded cluster vdB 130, next to a wall of the
supershell The overall outline of the region is shown in Fig. 1. Superimposed
Digitized Sky Survey (DSS2)111https://archive.stsci.edu/cgi-bin/dss_form red
and Spitzer222https://sha.ipac.caltech.edu/applications/Spitzer/SHA/ 8 $\mu$m
maps are overlaid with the integrated 13CO (1–0) emission contours outlining
the molecular cloud (Schneider et al., 2007).

A cometary molecular cloud, which has been designated as Cloud A in Schneider
et al. (2007), stands out in the considered part of the supershell (Fig. 1).
Using expressions from Schneider et al. (1998), we estimated its mass to be
$\sim 3000\,M_{\odot}$ within a 1 K $\cdot$ km s-1 level (observational data
have been kindly provided by N. Schneider). Also, Schneider et al. (2007) have
found a velocity gradient of 0.5 km s-1 pc-1 along a major axis of the cloud
and suggested that Cloud A is most likely shaped by UV-radiation and stellar
winds from nearby OB stars. The cloud is indeed extended toward an
illuminating source, that is, in the direction of the closest OB stars in Cyg
OB1. The size of the molecular cloud is $0.4^{\circ}$ or 12 pc at the adopted
distance of $\sim 1.67\pm 0.06$ kpc toward the cluster and the association
(Rastorguev et al., 2023).

The cloud is associated with two star-forming sites. A first one, young vdB
130 cluster, is located in the head of the cloud (a pillar), and another one,
a compact protocluster, is located in the cloud’s tail (Fig. 1, see also Fig.
11 in Paper II). A physical relation of the Cyg OB1 association, supershell,
the vdB 130 cluster, and the molecular cloud follows from estimated distances
and radial velocities as well as from some indirect factors (Paper I). An age
of vdB 130 does not exceed 10 Myr (Paper I and Paper III). Its youth is
consistent with the presence of Class I and Class II protostars (Kuhn et al.,
2021), which are visible both in the vdB 130 area and in the protocluster
area.

Analysis of interstellar extinction toward vdB 130 has shown that foreground
material is characterized by the normal extinction law with $R_{v}=3.1$
(Tatarnikova et al. 2016, Paper III). Absorption within the cluster itself is
very large and inhomogeneous. $R_{v}$ toward central stars in Blob E and Blob
W (Fig. 2, top) can be as high as 8 according to our data and data from Racine
(1974). From the west, the vdB 130 cluster is surrounded by an IR shell,
having a size of 3 pc and discernible in all the IRAC bands and in $24\,\mu$m
MIPS band of the Spitzer telescope (Fig. 2, top).

Figure 1: Northwestern part of the supershell surrounding the stellar
association Cyg OB1. Superimposed DSS2 red (red) and Spitzer 8 $\mu$m (green)
maps are overlaid with the integrated 13CO (1–0) emission contours outlining
the molecular cloud (Schneider et al., 2007). Contours run from 1 K $\cdot$ km
s-1 with a step of 2 K $\cdot$ km s-1. The circles mark the central part of
the vdB 130 cluster and the protocluster region. A small bar in the lower left
corner corresponds to 1 pc at a distance of 1.67 kpc.

Figure 2: Images of the vdB 130 vicinity in the Spitzer 8 $\mu$m band (top)
and in molecular hydrogen line ($\lambda=2.12\,\mu$m) (bottom). Infrared Blobs
E and W along with several stars from the original Racine (1974) list are
indicated on the top panel. Blobs’ approximate contours are also shown in the
bottom panel.

One of the major large-scale motions of the interstellar material in the
studied region is the expansion of the supershell under the influence of wind
and ionizing radiation of Cyg OB1 association stars. The expanding supershell
compresses pre-existing molecular clumps or clumps emerging in the swept-up
material (e.g., Deharveng et al., 2005). The expansion speed of the
supershell, derived from the analysis of interferometric observations in the
H$\alpha$ line, reaches 80 km/s in some directions (Lozinskaya et al., 1998,
Arkhipova et al., 2013).

It is tempting to suggest that a cometary shape of the molecular cloud (with
its head inside the supershell and its tail outside of it) as well as two
star-forming regions (the vdB 130 cluster and the protocluster), embedded in
the head and tail of the cloud, are all signatures of sequential star
formation triggered by the supershell expansion (Fig. 1, see also Paper II and
Paper III). This region is convenient for identifying the consequences of such
an expansion, since it is located at the periphery of the supershell, and the
suggested process of triggered star formation ‘‘unfolds’’ in the plane of the
sky.

Some arguments in favor of the sequential star formation in this region have
been given in our previous studies. Here we present results of our
observations of the vicinity of the vdB 130 cluster and the protocluster in
the infrared (IR) filter centered at 2.12-$\mu$m molecular hydrogen line,
installed at the 2.5-m telescope of the Caucasus Mountain Observatory of the
Sternberg Astronomical Institute, Lomonosov Moscow State University (CMO SAI
MSU).

This line is used quite often as a tool to probe physical conditions in
protostellar outflows and photodissociation regions (PDRs). Being among the
brightest H2 ro-vibrational lines, it can be excited in two competing ways.
Collisional excitation prevails in shocks, while radiative excitation is more
typical for PDRs, where material is irradiated by nearby massive stars. Due to
the lack of the line profile information, in this study we only consider H2
emission morphology and its relation to other tracers.

The structure of the paper is the following. In Section 2 the equipment and
methods for processing observations in the H2 line as well as the used
archived data are described. Section 3 presents an analysis of new
observational data in the vicinity of IR shell and protocluster. Possible
manifestations of sequential star formation, triggered by the expanding
supershell, are discussed in Section 4. In Conclusion the results of the study
are summarized.

##  2 Observations and data reduction

This study is based on IR observations performed with the ASTRONIRCAM infrared
camera (Nadjip et al., 2017) installed on the 2.5-m telescope (CMO SAI MSU),
from April 3rd to September 1st, 2020. FWHMs of star images varied from
$0.8^{\prime\prime}$ to $1.2^{\prime\prime}$. The ASTRONIRCAM frame size is
$1024\times 1024$ pixel with an angular scale of $0.27^{\prime\prime}$/pixel
and a field of view of $280^{\prime\prime}\times 280^{\prime\prime}$. We used
the H2 v=1–0 (S1) filter with $\lambda_{0}=2.132\,\mu$m and FWHM
$=0.046\,\mu$m and the Kcont filter with $\lambda_{0}=2.273\,\mu$m and FWHM
$=0.039\,\mu$m. The first one is centered on v=1–0 (S1) molecular hydrogen
line and the second one allows measuring the continuum level in the
neighboring spectrum region. Each final frame represents a sum of many
separate frames, obtained with short exposures ($\sim 9$ s). Between these
frames, the telescope was shifted by an angle of about $3-5^{\prime\prime}$. A
total exposure times for final frames is from 910 to 3940 s. A sky background
has been measured separately at the same air mass. Surface brightnesses and
standard deviations for final frames were estimated using 2MASS stars as flux
calibrators (Tatarnikov et al., 2023). Standard deviations are $1.5\cdot
10^{-17}$ erg/s/cm2/pixel for the shell area, $1.3\cdot 10^{-17}$
erg/s/cm2/pixel for the blobs’ area, and $8.8\cdot 10^{-18}$ erg/s/cm2/pixel
for the protocluster area. These numbers can be considered as upper estimates
for the sensitivity limit of our data. In the following we present data for
the H2 filter with the Kcont filter data subtracted under the simplified
assumption that the continuum is flat in the region of the spectrum around 2
$\mu$m.

Interstellar absorption is not taken into account. It can be important at
these wavelengths (Habart et al., 2022), but given the complicated structure
of extinction in the area, its taking into account is non-trivial. We believe
that its neglect would not change our conclusions, as we are mostly interested
in the emission morphology. All the considered data have been regridded to the
same astrometry, corresponding to H2 data. In some cases, described below, H2
images have been convolved to the resolution of the Spitzer $8\,\mu$m data
(FWHM $2^{\prime\prime}$, Fazio et al. 2004) using a Gaussian kernel provided
by Aniano et al. (2011).

We also use our H$\alpha$ observations of the region presented in Paper I and
archival data obtained with Spitzer.

##  3 Molecular hydrogen in the vicinity of the vdB 130 cluster and the
protocluster

Molecular hydrogen emission is clearly visible in several locations across the
studied region (Fig. 2, bottom). It neatly traces the infrared shell around
the vdB 130 cluster, surrounds infrared blobs, specifically Blob W and Blob E,
and appears as several compact spots in the protocluster region. In the top
panel of Fig. 3 we show the molecular hydrogen emission map for the IR shell
vicinity with Kcont continuum subtracted, highlighting four bright H2 emission
features, denoted S1, S2, S3, and S4, along with four cross-cuts (cs1, cs2,
cs3, cs4), which will be discussed below. The bottom panel of Fig. 3 shows
features (B1, B2, B3) and cuts (cb1, cb2, cb3) selected for the analysis in
the blobs’ area.

Figure 3: Molecular hydrogen emission in the vicinity of vdB 130
(erg/s/cm2/arcsec2, linear scale, Kcont continuum is subtracted). Top: IR
shell area. Bottom: blobs’ area. Green contours and red lines indicate regions
and cuts selected for detailed analysis. Also indicated are outflows OF360 and
OF361, identified previously by Makin and Froebrich (2018).

In order to relate observed quantities to each other and to suggested
morphological features of the region, we use the Cloudy code (Ferland et al.,
2017), which allows computing a structure of an HII region and an adjacent
photodissociation region with a very detailed account of most relevant
processes. Specifically, we calculated a number of models for a shell with a
central star having an effective temperature of 25000 K (B-star, $\log g=3.9$,
$\log L/L_{\odot}=3.7$) or 50000 K (O-star, $\log g=4.1$, $\log
L/L_{\odot}=6.1$). Corresponding stellar models have been downloaded from the
TLUSTY Web-site333http://tlusty.oca.eu/Tlusty2002/tlusty-frames-cloudy.html
(Lanz and Hubeny, 2007). The star is located at a distance $d$ from the inner
border of the modeled region, which is assumed to be uniform. Considered
density values are $10^{3}$ and $10^{4}$ cm-3.

Results are presented in Table 1. A first number in a model designation (a
first column of Table 1) is the density exponent, then comes the letter that
designates the ionizing star spectral type, and the last number is the
distance $d$ from the to the inner edge of the computational domain in
parsecs. So, for example, a model 3B0.1 corresponds to a region with a gas
number density of $10^{3}$ cm-3, and an inner edge located at a distance of
0.1 pc from a B star. An example of the theoretical shell structure for a
model 4O1 is shown in Fig. 4. In this case, an edge of the Hii region is
located at a distance of about $0.7^{\prime\prime}$ from an inner border of
the computational domain. As the geometry of the gas and dust distribution in
the vdB 130 vicinity is far from being spherical, we do not try to reproduce
emergent spectra, relying instead upon emissivities as functions of a distance
from an ionising source. This limits available options to considering mutual
locations of various ISM components and correlations between their respective
intensities. Stars have been masked both in the shell area and the blobs’
area.

Figure 4: Theoretical H$\alpha$ and H2 intensity profiles across the shell of
an ionized hydrogen region. The region is ionized by an O-type star located at
1 pc from the inner border of the computational domain (model 4O1). The gas
density in the shell is $10^{4}$ cm-3.

###  3.1 Infrared Shell

The H2 emission from the shell itself is clearly discernible, along with
diffuse foreground and/or background H2 emission projecting onto the cluster
area. The H2 shell spatial location coincides with the location of 8 $\mu$m
filaments. Pixel-by-pixel comparison of H2 emission and $8\,\mu$m emission for
features S1–S4 is presented in Fig. 5. The H2 data have been convolved to a
resolution of the Spitzer $8\,\mu$m data. We see that the molecular hydrogen
emission intensity correlates with 8 $\mu$m emission intensity in all the four
features, which hints that it is UV emission that excites both PAHs and
hydrogen molecules.

Figure 5: Relation between molecular hydrogen emission and 8 $\mu$m emission
in the selected regions of the IR shell.

Observed intensity profiles along the shell cuts marked in the top panel of
Fig. 3 are shown in four panels of Fig. 6. Here we again show H2 data that
have been convolved to a resolution of the Spitzer $8\,\mu$m data. It is seen
that peaks of H2 and PAH emission are nearly coincident and somewhat displaced
relative to the border of the Hii region, which we, somewhat arbitrarily,
identify with the location, where H$\alpha$ intensity starts decreasing. This
displacement is of the order of $5^{\prime\prime}-10^{\prime\prime}$. The
breadths of 8 $\mu$m and H2 emission shells are similar to each other and
equal to $3^{\prime\prime}-5^{\prime\prime}$.

Figure 6: H$\alpha$, 8 $\mu$m, and H2 intensity profiles along the four cuts through the shell indicated in the top panel of Fig. 3 (relative units). Red horizontal bars on the top panel indicate respective angular resolutions, which are adopted to be $1^{\prime\prime}$ for H2 data, $2^{\prime\prime}$ for $8\,\mu$m data, and $4^{\prime\prime}$ for H$\alpha$ data (Paper I). Table 1: Modeled parameters of the PDR structure. The model designation consists of a density exponent, a letter indicating a star spectral type, and a distance (in parsec) from the star to the inner border of the modeled region. Model |  $d$, | H2 $2.12\,\mu$m–IF | H2 shell
---|---|---|---
| pc | separation, ′′ | breadth, ′′
3B0.1 | 0.1 | 35 | 30
3B0.3 | 0.3 | 30 | 30
4B0.1 | 0.1 | 6 | 4
4B0.3 | 0.3 | 2 | 2
3O0.1 | 0.1 | 30 | 40
3O0.3 | 0.3 | 40 | 50
3O1 | 1 | 60 | 50
3O3 | 3 | 50 | 50
3O10 | 10 | 10 | 30
4O0.1 | 0.1 | 7 | 5
4O0.3 | 0.3 | 7 | 5
4O1 | 1 | 7 | 5
4O3 | 3 | 4 | 3

These values can be related with the results of theoretical modeling,
presented in Table 1. We see that observed geometrical constraints in the
shell are only satisfied in models with gas density of $10^{4}$ cm-3. At this
density, O-type stars produce breadths of H2 emission shells, which are
similar to observed ones, as are the separations between the shells and the
Hii region border, at all the considered distances. A B-type star located at a
distance greater than $\sim 0.3$ pc produces the shells that are too narrow
and too close to the Hii region border.

Overall, we conclude that despite the fact that the IR shell apparently
engulfs vdB 130, the cluster stars alone cannot be responsible for the shell
structure, and it also ‘‘feels’’ the influence of the Cyg OB1 stars.

###  3.2 Infrared Blobs

In the bottom panel of Fig. 3 we show H2 emission distribution to the east of
the IR shell, in the vicinity of Blob E and Blob W. Here the structure of the
H2 emission map is much more intricate. It consists of numerous curved
filaments with their convex parts being oriented both toward and from the
ionising stars 10r–14r (see Paper III and top panel of Fig. 2).

Figure 7: Relation between molecular hydrogen emission and 8 $\mu$m emission
in the selected regions of the blobs’ area.

In Fig. 7 we demonstrate a relation between H2 emission and 8 $\mu$m emission
for the features identified in Fig. 3 (bottom panel). The H2 data shown in
Fig. 7 and Fig. 8 have been convolved to a resolution of the Spitzer $8\,\mu$m
data. There is a positive correlation between the two tracers in the feature
B1 (red circles), but in the brightest feature B3, the situation is more
complicated, with the relation being clearly split into two branches (blue
squares). A further analysis shows that the descending branch, with $8\,\mu$m
emission decreasing as H2 emission becomes brighter, corresponds to the inner
part of the feature, while the ascending branch corresponds to its outer part.
Such a structure is indicative of some displacement between the H2 emission
peak and the $8\,\mu$m emission peak. A hint to a similar branch splitting is
also visible in the feature B2. This can also be a signature of a displacement
between the H2 and $8\,\mu$m emission peaks or be an indication of an even
more complex morphology.

Figure 8: H$\alpha$, 8 $\mu$m, and H2 intensity profiles along the three cuts
through the blobs indicated in the bottom panel of Fig. 3 (relative units).

In Fig. 8 we show profiles of various emission tracers along the cuts shown at
the bottom panel of Fig. 3. Small sizes of the blobs and associated H2
emission features along with insufficient resolution of available H$\alpha$
observational data only allow estimating upper limits to the separations
between ionisation and dissociation fronts (as we cannot reliably determine
the location of the ionisation front) and to the breadths of H2 emission
shells, which are of the order of few arcsec. In our modeling, this roughly
corresponds to a B1V star, located at a distance of about $0.1-0.3$ pc from
the shell with a density of $10^{3}-10^{4}$ cm-3, which is consistent with
small sizes of the blobs and with our hypothesis that they represent Hii
regions around the vdb 130 cluster stars 1r and 5r.

In Fig. 8 (bottom panel) we see a displacement between the $8\,\mu$m and H2
emission maxima in the cb3 feature, mentioned above. A hint to a similar
displacement is also seen in the cb2 feature (middle panel). It is tempting to
assume that different spatial locations of $8\,\mu$m emission shell and H2
emission shell indicate that H2 molecules, unlike PAH molecules, are mostly
collisionally excited in these features, so that we actually see a separation
between an ionisation front and a shock front.

###  3.3 Molecular hydrogen objects

A region to the west of the IR shell (where the protocluster is located) does
not host any extended H2 emission features, but it harbors several compact
fuzzy objects in our H2 filter image. As there are no early-type stars in this
area (Paper III), the H2 line emission requires collisional excitation,
hinting at the presence of shock waves. Thus, these compact objects may
correspond to molecular outflows formed by protostars, including massive ones
(see, for example, Wolf-Chase et al., 2013). (Possibility of massive star
formation in this region is supported by the presence of a feature reminiscent
of an infrared dark cloud visible on a large-scale 8 $\mu$m map.) And, indeed,
the compact H2 emission sources in the protocluster area have been marked as
outflow features by Makin and Froebrich (2018).

The entire Cygnus X complex has been mapped in the molecular hydrogen 2.12
$\mu$m line in the UWISH2 project (Froebrich et al., 2015). Makin and
Froebrich (2018) have used data from this survey to identify four potential
outflows, OF373, OF374, OF375, and OF376 in their notation. These outflows are
also associated with Molecular Hydrogen Emission-Line Objects (MHO) MHO 974,
MHO 973, MHO 976, and MHO 977 from the MHO catalog (Davis et al.,
2010)444http://astro.kent.ac.uk/~df/MHCat/index.html. The isolated features of
these outflows, shown in Fig. 9 (top), coincide with compact sources on our
map. Coordinates and approximates sizes of the molecular hydrogen objects,
identified by Makin and Froebrich (2018) with outflow features and also
observed in this work, are listed in Table 2.

Table 2: Molecular hydrogen objects, identified by Makin and Froebrich (2018) with outflow features and also observed in this work. In parentheses we indicate a panel number in Fig. 11 from Makin and Froebrich (2018) with a corresponding map. MHO ID | OF ID | $\alpha$ | $\delta$ | Size
---|---|---|---|---
MHO 973 (357) | OF374C | 20:17:01.6 | +39:22:21 | $4^{\prime\prime}$
MHO 974 (356) | OF373A | 20:17:03.6 | +39:22:01 | $7^{\prime\prime}$
| OF373B | 20:17:00.9 | +39:21:19 | $4^{\prime\prime}$
| OF373C | 20:16:59.0 | +39:20:55 | $5^{\prime\prime}$
| OF373DE | 20:16:56.2 | +39:20:15 | $5^{\prime\prime}$
MHO 976 (358) | OF375A | 20:17:04.7 | +39:21:21 | $2^{\prime\prime}$
| OF375B | 20:17:03.4 | +39:21:15 | $2^{\prime\prime}$
| OF375C | 20:16:56.6 | +39:20:34 | $4^{\prime\prime}$
| OF375D | 20:16:54.7 | +39:20:26 | $2^{\prime\prime}$
| OF375E | 20:16:52.7 | +39:20:03 | $3^{\prime\prime}$
MHO 977 (359) | OF376A | 20:17:03.2 | +39:20:53 | $4^{\prime\prime}$
| OF376B | 20:17:03.7 | +39:20:01 | $4^{\prime\prime}$
MHO 981 (345) | OF361A | 20:17:40.5 | +39:20:32 | $10^{\prime\prime}$
| OF361B | 20:17:40.4 | +39:20:47 | $5^{\prime\prime}$
| OF361C | 20:17:40.4 | +39:20:59 | $5^{\prime\prime}$
MHO 982 (344) | OF360A | 20:17:46.7 | +39:20:46 | $3^{\prime\prime}$
| OF360B | 20:17:44.9 | +39:20:33 | $3^{\prime\prime}$

Figure 9: Top: a region of the protocluster core in 2.12 micron molecular
hydrogen line after continuum subtraction. Shown are outflows features,
identified by Makin and Froebrich (2018). Bottom: a color image of the same
region composed from $3.6\,\mu$m (red), $4.5\,\mu$m (green), and $8\,\mu$m
(blue) Spitzer bands.

The most prominent detail OF373C as well as another feature, OF376A, coincide
with diffuse emission in near-IR Spitzer bands, including $4.5\,\mu$m (Fig. 9,
bottom), which supposedly contains emission from shocked molecular hydrogen
(Rathborne et al., 2005, Beuther et al., 2005). Diffuse $4.5\,\mu$m emission
objects are known as extended green objects (EGOs) and are believed to be
signposts of outflow activity. Their association with the H2 emission features
is shown in Fig. 9 (bottom), which is a three-color representation of the
protocluster area using a conventional color-coding based on $3.6\,\mu$m
(red), $4.5\,\mu$m (green), and $8\,\mu$m (blue) Spitzer bands. It is seen
that of all the suggested outflow features and associated MHOs only two,
OF373C and OF376A, are clearly associated with EGOs.

Another way to demonstrate this is to consider a pixel-by-pixel relation
between molecular hydrogen emission and 4.5 $\mu$m emission in the selected
MHOs, shown in Fig. 10. Features OF373A and OF373DE (as well as OF376B and
OF374C, which are not shown to avoid cluttering) show 4.5 $\mu$m emission at a
background level, while being quite bright in 2.12 $\mu$m molecular hydrogen
emission. Emission at 4.5 $\mu$m in OF373B feature (green filled triangles)
slightly exceeds a background level, but also does not show any correlation
with 2.12 $\mu$m emission. Features OF373C (blue filled squares) and OF376A
(red filled circles), on the contrary, show 4.5 $\mu$m and 2.12 $\mu$m
intensities, which seem to be correlated, but with a significant scatter and a
complicated shape of the point distribution on the plot. This complicated
shape may indicate the presence of some small-scale irregularities in the
distribution of 4.5 $\mu$m and 2.12 $\mu$m intensities, like displacements
mentioned previously in relation to the infrared blobs. Still, we believe that
the features OF373C and OF376A contain possible outflows from massive
protostellar clumps.

Figure 10: Relation between molecular hydrogen emission and 4.5 $\mu$m
emission in the selected regions of the protocluster area.

It is interesting that OF373A–OF373DE are designated as features of the same
outflow by Makin and Froebrich (2018), but in Fig. 10 they follow different
trends, and the same is true for OF376A and OF376B as well. We also note that
if OF373A–OF373DE features are indeed part of the same jet, at a distance of
vdB 130 that would result in a projected end-to-end jet length well over 1 pc
and place it among the longest young stellar object (YSO) jets seen in H2
emission (Hirano et al., 2006, Makin and Froebrich, 2018).

While in the vicinity of the infrared blobs, discussed in the previous
subsection, we were mostly concerned with extended H2 emission, we also note a
presence of compact H2 emission sources between the shell and the blobs,
indicated on Fig. 3. These sources are associated with two outflows, which are
listed in the catalog by Makin and Froebrich (2018) as OF360 and OF361.

##  4 Sequential star formation in the vicinity of the vdB 130 cluster?

The vdB 130 cluster and the surrounding area host a multitude of objects at
various stages of prestellar and early stellar evolution (Rastorguev et al.,
2023). In 13CO emission, the entire region is dominated by a molecular cloud,
having a length of about 12 pc, which is mostly located outside of the Cyg OB1
supershell, but also protrudes into the ionized cavity as a molecular pillar
having a cometary shape and facing east (Cloud A, Schneider et al., 2007). The
outline of this cloud is shown in Fig. 11 (top) both with CO emission contours
and as a gray-scale map of 160 $\mu$m emission, based on data obtained with
the PACS instrument (Poglitsch et al., 2010) on-board of the Herschel Space
Observatory (Pilbratt et al., 2010). This pillar is directed toward nearby Cyg
OB1 stars, but this is most probably a mere coincidence as the pillar seems to
be an eastern tip of a long molecular filament, visible in Herschel maps as an
emission feature and in Spitzer IRAC and MIPS maps as an absorption feature
(an infrared dark cloud; Fig. 11, bottom). The whole filament extends far
beyond the supershell and, thus, could not have been shaped by ionizing
radiation from Cyg OB1 stars. It should be noted that at least some non-
ionizing UV radiation leaks into the unperturbed region as revealed by diffuse
$8\,\mu$m emission visible beyond the shell (Fig. 1). While this radiation can
alter the thermal state of the filament to a certain degree, causing its
compression, it is hardly responsible for its formation and shaping, as such
filaments are now believed to be a common feature of the molecular cloud
evolution and the star formation process (Pineda et al., 2022).

The star formation might have been accelerated and orchestrated in the eastern
part of the filament by its interaction with the expanding supershell.
Specifically, the shock wave associated with the expanding shell might had
compressed a pre-existing clump within the molecular filament, speeding up its
collapse and the birth of the vdB 130 cluster. Hottest cluster stars have
carved their own cavity, which probably resides at the observer’s side of the
filament as we see a shell encircling vdB 130, some emission, including
diffuse H2 emission, associated with the back wall, but no extinction features
in front of the core of the vdB 130 cluster, which would indicate the presence
of some foreground material. The filament itself and the associated molecular
cloud have probably been partially disrupted by the infrared shell formation.

The eastern wall of the shell swept-up by the vdB 130 cluster stars might have
compressed other dense clumps within the eastern part of the filament in its
own turn, giving rise to Blob E and Blob W, which are currently still embedded
in this side wall, which partially shows up as an extinction feature that
engulfs Blob E and extends to the south of it (see Fig. 9b in Paper I). Our H2
observations indicate that the infrared blobs are ultra-compact or maybe even
hyper-compact Hii regions, so they are apparently younger than the half-opened
bubble around the vdB 130 cluster.

Some other dense clumps are visible along the filament in its unperturbed
part, to the west of the IR shell, which stand out as patches of enhanced IR
($24-160\,\mu$m) emission. These clumps host IRAS 20149+3913 and IRAS
20151+3911 sources identified as ultra-compact Hii regions by Bronfman et al.
(1996). The Hii regions are visible in the CS $(2-1)$ line at radial
velocities $V_{\rm LSR}$ equal to 3.1 and 4.2 km/sec typical of the cometary
cloud (Schneider et al. 2007, Paper I and Paper II). In Fig. 11 (top), we also
show protostars identified by Kryukova et al. (2014) (yellow circles). Two of
these protostars are apparently associated with the IRAS sources; the
protostar associated with IRAS 20151+3911 probably drives the outflow OF373.
Some other outflow driving sources, marked with magenta color in Fig. 11
(top), also coincide with protostars from the work of Kryukova et al. (2014).

Red and green diamonds show, respectively, Class I and Flat Spectrum YSOs and
Class II/III YSOs, probably, associated with vdB 130 as described in
Rastorguev et al. (2023). They are mostly concentrated within the infrared
shell with only a few of them being located outside. Our estimates show that
the YSOs in the protocluster area are not gravitationally bound to vdb 130.
They probably represent more scattered population of this star-forming region.

Figure 11: Variety of star-forming objects in the vicinity of vdB 130. Top:
Background shows a map of $160\,\mu$m emission. YSOs identified in Rastorguev
et al. (2023) as vdB 130 cluster members are shown with red (Class I and Flat
Spectrum) and green (Class II/III) diamonds. Yellow circles are YSOs found by
Kryukova et al. (2014). White circles indicate ultra-compact Hii regions.
Bottom: The same region in the 24 $\mu$m Spitzer band. Green contours indicate
regions of bright H2 emission studied in this work. Blue circles show dense
clumps associated with enhanced 24 $\mu$m emission. Outflow features (Makin
and Froebrich, 2018) are shown with magenta circles.

In the bottom panel of Fig. 11 we show the same region mapped in the 24 $\mu$m
Spitzer band. On this panel we mark H2 emission regions studied in this work
along with four dense clumps associated with enhanced 24 $\mu$m emission. As
this emission is most probably generated by stochastically heated small grains
absorbing UV photons, it also hints at the presence of young massive stars.
Two of these regions, Clump 1 and Clump 2, located in the molecular cloud
tail, coincide with regions of bright $160\,\mu$m emission (Fig. 11, top) and
are also visible as enhancements on a column density map (see Fig. 13 in Paper
II, ). Clump 2, coincident with IRAS 20151+3911, is located in the direction
of the compact protocluster core and of the peak in the CO emission. It is
also associated with the possible EGOs OF373 and OF376 (denoted by green
contours in Fig. 11, bottom). A part of the filament connecting the infrared
shell, IRAS 20149+3913, and IRAS 20151+3911 is seen as a infrared dark cloud
both on the 8 $\mu$m and 24 $\mu$m emission maps. There is no obvious
protostar concentration toward Clump 1. To the south of vdB 130 there are two
other infrared nebulosities, Clump 3 and Clump 4. Location of Clump 4 (which
is also indicated in Fig. 2, top) coincides with the cluster star 4r, so it
may represent yet another small-scale star-forming site within the greater
complex. Without the distance information the relation of Clump 3 to the
studied region is unclear.

Two outflows with their associated MHOs along with several YSOs are located in
dense parts of the filament in the interior of the infrared shell (in
projection), but in general star formation seems to be at a more advanced
stage in the vicinity of the vdB 130 cluster, which also shows up in the
different properties of H2 emission. In a close vicinity of vdB 130 we observe
several extended diffuse H2 emission features, which correlate with bright PAH
emission features and are clearly associated with Hii regions, denoted as Blob
E, Blob W, and H$\alpha$ Blob in our previous works. The shell around the vdB
130 cluster (H$\alpha$ Blob) appears to be quite developed and shows signs of
being affected by the large-scale Cyg OB1 irradiation in its H2 line emission.
Blob E and Blob W seem to be younger (despite of being closer to Cyg OB1), and
their H2 emission is similar to that from a more compact Hii region. To the
west of the IR shell, H2 emission is only observed in several compact spots,
with some of them being probably associated with jets. No signs of Hii regions
similar to Blob E and Blob W are visible there.

In Paper II we proposed an evolutionary scenario, which relates the star
formation activity in the vicinity of vdB 130 cluster to the expansion of the
Cyg OB1 supershell and its interaction with the cometary CO cloud, which
represents a front end of the long molecular filamentary structure. According
to this scenario, the cluster itself represent a most advanced star formation
site, which lies in the center of the smaller scale infrared shell,
encompassing the cluster from the west. To the east of the cluster, the
expansion of the Hii region around the cluster might have stimulated star
formation and subsequent Hii region formation in Blob W and Blob E (and,
maybe, in Clump 4 around the 4r star). The infrared shell seemingly has not
yet triggered any noticeable star formation activity (except for, maybe, Clump
3).

The general structure of H2 emission is broadly consistent with this scenario,
but few cautionary notes are in order. First, if the vdb 130 cluster age is
indeed 10 Myr, its formation could not have been triggered by the expansion of
the Cyg OB1 supershell. A projected distance from the cluster to the
supershell is of the order of at most a few parsec. This is much smaller than
the distance that the supershell have traversed in 10 Myr. In this case, the
supershell might have just cleaned up the vicinity of the cloud head from the
less dense molecular gas and revealed the pre-existing star-forming cluster.
However, as we argued above, 10 Myr can only be considered as an upper limit
of the cluster age. The presence of numerous YSOs, including those of Class I
and Class II, that are probable cluster members (Rastorguev et al., 2023),
indicates that the actual age of vdB 130 can be much smaller, being comparable
to the time since the passage of the supershell.

Star formation in the quiescent (undisturbed) part of the filament, including
the protocluster formation, is probably caused by some other events unrelated
directly to the supershell expansion. One possibility is that the protocluster
formation has been somehow triggered or accelerated by non-ionizing UV
radiation of the association O-stars, for example, by compressing pre-existing
dense clumps (Shematovich et al., 1999). This radiation may indeed penetrate
the supershell, which is emphasized by the presence of diffuse emission in
near-IR Spitzer bands beyond the supershell (see Fig. 1). Such an option may
be explored further with observations of molecular lines (e.g. Sun et al.,
2008).

Finally, it is possible that the region has undergone several episodes of star
formation triggering, driven by mutual interactions of accretion flows,
ionized regions, and (somewhat later) supernova explosions from the Cyg OB1
stars. At earlier stages of the star-forming complex evolution this may indeed
produce ‘‘oscillating’’ Hii regions that grow and shrink repeatedly (Peters et
al., 2010) leading to advance and retreat of the associated ionizing
radiation. In such a case, the vdB 130 formation and the protocluster
formation might have been caused by two independent triggering events. But at
later evolutionary stages after the supershell formation such a complex
behavior hardly can be expected.

Generally speaking, all the listed evidences do not contradict the scenario of
the sequential star formation in the vdB 130 area. However, we must admit that
they do not prove it with certainty. Much more detailed data on the region are
needed to make more sound conclusions.

##  5 Conclusions

We present results of new observations of the vbB 130 cluster vicinity
performed with a filter centered on the H2 line at 2.12 $\mu$m installed on
the 2.5-metre telescope of the CMO MSU. It is shown that in the vicinity of
the vdB 130 cluster molecular hydrogen ro-vibrational emission has low
contrast but is clearly spatially coincident with 8 $\mu$ emission related to
PAH particles both in the infrared shell and in the regions around infrared
blobs. There is a positive correlation between the H2 line and 8 $\mu$m band
intensity almost everywhere except for the inner part of the brightest H2
emission feature surrounding Blob E. This indicates that H2 emission in the
vdB 130 cluster vicinity mostly represents fluorescent emission, excited by
the UV radiation of the cluster stars in numerous PDRs, revealing complex gas
distribution in the area. Compact patches of H2 emission in the protocluster
area are probably collisionally excited and trace several protostellar
outflows.

The observational results do not contradict the scenario of sequential star
formation stimulated by an expanding supershell around the Cyg OB1
association, but on the other hand are not sound enough to claim it with
certainty.

###### Acknowledgments.

This work is partially based on the data from the Spitzer Space Telescope
operated by the Jet Propulsion Laboratory, California Institute of Technology,
under NASA contract 1407, and the data from the 2MASS catalogue (University of
Massachusetts, California Institute of Technology, NASA and NSF). The Spitzer
GLIMPSE360 images are shown courtesy of NASA (National Aeronautics and Space
Administration)/JPL-Caltech. CO observational data have been kindly provided
by N. Schneider (Schneider et al., 2007, 2011). The MHO catalog is hosted by
the University of Kent. Herschel is an ESA space observatory with science
instruments provided by European-led Principal Investigator consortia and with
important participation from NASA.

## FUNDING

This study was carried out using the equipment bought with the funds of the
Program of the Development of M.V. Lomonosov Moscow State University and
supported by the RFBR grants (18-02-00976, 18-02-00890, 19-02-00611). D.
Wiebe, A.M. Tatarnikov, and A.A. Tatarnikov were supported by the RFBR grant
20-02-00643. Authors acknowledge the support from the Program of Development
of M.V. Lomonosov Moscow State University (Leading Scientific School ‘‘Physics
of stars, relativistic objects and galaxies’’). The work of A. Topchieva was
supported by the Foundation for the Advancement of Theoretical Physics and
Mathematics ‘‘BASIS’’.

## CONFLICT OF INTERESTS

The authors declare the absence of the conflict of interests.

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North Carolina. (A) Weekly COVID-19 attributable deaths reported by the CDC
stacked by age groups (color). (B) Posterior mean of the weekly COVID-19
attributable deaths stacked by age, estimated with a regularised B-splines
projected GP prior. Three weeks in the observation period are selected and
designated by a shape. The black line shows the all-age weekly COVID-19
attributable deaths reported by JHU, that were used to adjust the CDC data for
reporting delays (3.4). (C) Posterior median (black line) and 95% credible
interval (ribbon) of the age-specific contribution to COVID-19 attributable
deaths in three weeks. Figure S33: State summary for New York. (A) Weekly
COVID-19 attributable deaths reported by the CDC stacked by age groups
(color). (B) Posterior mean of the weekly COVID-19 attributable deaths stacked
by age, estimated with a regularised B-splines projected GP prior. Three weeks
in the observation period are selected and designated by a shape. The black
line shows the all-age weekly COVID-19 attributable deaths reported by JHU,
that were used to adjust the CDC data for reporting delays (3.4). (C)
Posterior median (black line) and 95% credible interval (ribbon) of the age-
specific contribution to COVID-19 attributable deaths in three weeks. Figure
S34: State summary for Ohio. (A) Weekly COVID-19 attributable deaths reported
by the CDC stacked by age groups (color). (B) Posterior mean of the weekly
COVID-19 attributable deaths stacked by age, estimated with a regularised
B-splines projected GP prior. Three weeks in the observation period are
selected and designated by a shape. The black line shows the all-age weekly
COVID-19 attributable deaths reported by JHU, that were used to adjust the CDC
data for reporting delays (3.4). (C) Posterior median (black line) and 95%
credible interval (ribbon) of the age-specific contribution to COVID-19
attributable deaths in three weeks. Figure S35: State summary for
Pennsylvania. (A) Weekly COVID-19 attributable deaths reported by the CDC
stacked by age groups (color). (B) Posterior mean of the weekly COVID-19
attributable deaths stacked by age, estimated with a regularised B-splines
projected GP prior. Three weeks in the observation period are selected and
designated by a shape. The black line shows the all-age weekly COVID-19
attributable deaths reported by JHU, that were used to adjust the CDC data for
reporting delays (3.4). (C) Posterior median (black line) and 95% credible
interval (ribbon) of the age-specific contribution to COVID-19 attributable
deaths in three weeks. Figure S36: State summary for Texas. (A) Weekly
COVID-19 attributable deaths reported by the CDC stacked by age groups
(color). (B) Posterior mean of the weekly COVID-19 attributable deaths stacked
by age, estimated with a regularised B-splines projected GP prior. Three weeks
in the observation period are selected and designated by a shape. The black
line shows the all-age weekly COVID-19 attributable deaths reported by JHU,
that were used to adjust the CDC data for reporting delays (3.4). (C)
Posterior median (black line) and 95% credible interval (ribbon) of the age-
specific contribution to COVID-19 attributable deaths in three weeks.