Star Formation Activity in the molecular cloud G35.20$-$0.74: onset of cloud-cloud collision

4.4 MB·

by  L. K. Dewangan

PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
STAR FORMATION ACTIVITY IN THE MOLECULAR CLOUD G35.20−0.74: ONSET OF CLOUD-CLOUD
COLLISION
L. K. Dewangan1
ABSTRACT
To probe the star-formation (SF) processes, we present results of an analysis of the molecular
cloud G35.20−0.74 (hereafter MCG35.2) using multi-frequency observations. The MCG35.2 is de-
picted in a velocity range of 30–40 km s−1. An almost horseshoe-like structure embedded within the
MCG35.2 is evident in the infrared and millimeter images and harbors the previously known sites,
7
ultra-compact/hyper-compact G35.20−0.74N Hii region, Ap2-1, and Mercer 14 at its base. The site,
1
Ap2-1 is found to be excited by a radio spectral type of B0.5V star where the distribution of 20 cm
0
and Hα emission is surrounded by the extended molecular hydrogen emission. Using the Herschel
2
160–500 µm and photometric 1–24 µm data analysis, several embedded clumps and clusters of young

n stellar objects (YSOs) are investigated within the MCG35.2, revealing the SF activities. Majority of
a the YSOs clusters and massive clumps (500–4250 M ) are seen toward the horseshoe-like structure.
J (cid:12)
The position-velocity analysis of 13CO emission shows a blue-shifted peak (at 33 km s−1) and a red-
1 shiftedpeak(at37kms−1)interconnectedbylowerintensityintermediatedvelocityemission,tracing
3
a broad bridge feature. The presence of such broad bridge feature suggests the onset of a collision

between molecular components in the MCG35.2. A noticeable change in the H-band starlight mean
]
R polarization angles has also been observed in the MCG35.2, probably tracing the interaction between
molecular components. Taken together, it seems that the cloud-cloud collision process has influenced
S
the birth of massive stars and YSOs clusters in the MCG35.2.
.
h Subjectheadings: dust,extinction–HIIregions–ISM:clouds–ISM:individualobjects(G35.20−0.74)
p
– stars: formation – stars: pre-main sequence
-
o
r
t 1. INTRODUCTION als.
s
a The birth mechanisms of massive stars ((cid:38) 8 M ) and Situated at a distance of 2.0 kpc (Birks et al.
[ clustersofyoungstarsarestillbeingdebated(Zin(cid:12)necker 2006; Zhang et al. 2009; Paron & Weidmann 2010),
& Yorke 2007; Tan et al. 2014). Using the effort of var- G35.20−0.74(Vlsr=33kms−1;Setaetal.1998;Paron&
1 ious Galactic plane surveys spanning from near-infrared Weidmann 2010) is a nearby star-forming complex that
v
(NIR)toradioregime,ithasbeenobservedthatmolecu- contains at least two active star-forming condensations,
7
lar clouds often host dark clouds, infrared shells or bub- G35.20−0.74N (hereafter G35.20N) and G35.20−0.74S
0
0 bles associated with Hii regions, and young star clus- (hereafter G35.20S) (also see Figure 4 in Mooney et al.
0 ters. Hence, observationally, the understanding of phys- 1995). Thesecondensationsareactivesitesofstarforma-
0 icalprocessesinsuchsitesisverychallenging, indicating tion and contain some well known sources, such as EGO
. the onset of numerous complex phenomena involved in G35.20−0.74 (Cyganowski et al. 2008), Ap2-1 (Paron &
2 star formation. At the same time, these sites are ex- Weidmann 2010), and Mercer 14 (Froebrich & Ioanni-
0 tremely promising to probe important observational ev- dis 2011b). In the complex, the G35.20N is very well
7 idences which can directly allow us to validate the ex- studied site using various data-sets (including Atacama
1
isting theoretical scenarios concerning the formation of Large Millimeter/submillimeter Array (ALMA)) and is
:
v massivestarsandstellarclusters. Moredetailsaboutthe alsoconsideredasmainsiteofMSFactivity(seeS´anchez
i theoreticalmechanismsofmassivestarformation(MSF) et al. 2014, and references therein). In the G35.20N,
X
can be found in Zinnecker & Yorke (2007) and Tan et Qiu et al. (2013) found the presence of a parsec-sized
r al. (2014). Interestingly, the collisions between molec- andwide-anglemolecularoutflowdrivenfromastrongly
a
ular clouds can also provide the appropriate conditions rotating region whose magnetic field is largely toroidal.
needed for MSF and birth of star clusters (e.g., Habe & Using the ALMA 870 µm data (350 GHz; resolution
Ohta 1992; Furukawa et al. 2009; Ohama et al. 2010; In- ∼0(cid:48).(cid:48)4), an elongated dust structure in the G35.20N was
oue & Fukui 2013; Fukui et al. 2014, 2016; Torii et al. investigated and has been found to be fragmented into a
2015,2016;Haworthetal.2015a,b). However,theobser- number of dense cores (S´anchez et al. 2014). They fur-
vational evidences of star formation (including massive thersuggestedthatthesedensecoresmightformmassive
stars) through the cloud-cloud collisions are still very stars. Using the Very Large Array (VLA) and ALMA
scarce. Therefore, it demands a systematic investiga- observational data-sets, Beltr´an et al. (2016) also inves-
tion of a given molecular cloud using the observational tigatedabinarysystemofultra-compact/hyper-compact
data having both large field-of-view and high resolution Hii regions at the geometrical center of the radio jet in
at wavelengths which probe obscuring molecular materi- G35.20N.Paron&Weidmann(2010)foundthatAp2-1is
anHiiregionexcitedbyanearlyB-typestarandisasso-
[email protected] ciated with the extended 8.0 µm emission. They further
1Physical Research Laboratory, Navrangpura, Ahmedabad - reported a radial velocity of V ∼31 km s−1 for Ap2-1
380009,India. lsr
2 L. K. Dewangan
using the Hα and [Nii] lines detected in its optical low- employed. The GRS line data have a velocity resolution
resolutionspectrum(seeFigure9inParon&Weidmann of 0.21 kms−1, an angular resolution of 45(cid:48)(cid:48) with 22(cid:48)(cid:48)
2010). Froebrich & Ioannidis (2011b) reported several sampling, a main beam efficiency (η ) of ∼0.48, a ve-
mb
molecularhydrogenoutflowsinMercer14andsuggested locity coverage of −5 to 135 km s−1, and a typical rms
ongoing star formation in this site (see Figure 1 in Froe- sensitivity (1σ) of ≈0.13 K (Jackson et al. 2006).
brich & Ioannidis 2011b). Several Spitzer dark clouds
were reported in the G35.20−0.74 complex (Peretto & 2.3. Herschel and ATLASGAL Data
Fuller 2009). Together, these previous studies indicate
To infer the far-infrared (FIR) and sub-millimeter
the ongoing star formation activity in the G35.20−0.74
(mm)emission, Herschelcontinuummapsat70µm, 160
region as well as the formation of young massive stars.
µm, 250 µm, 350 µm, and 500 µm were retrieved from
Note that the previous works were mainly concentrated
the Herschel data archive. The beam sizes of these im-
towardthesites,G35.20NandG35.20S(includingAp2-1
ages are 5(cid:48).(cid:48)8, 12(cid:48)(cid:48), 18(cid:48)(cid:48), 25(cid:48)(cid:48), and 37(cid:48)(cid:48) (Poglitsch et al.
and Mercer 14).
2010; Griffin et al. 2010), respectively. The processed
The study of star formation processes in the entire
level2 5 products were selected using the Herschel In-
molecular cloud associated with the star-forming com- −
teractive Processing Environment (HIPE, Ott 2010).
plex, G35.20−0.74 (hereafter MCG35.2) is yet to be
The sub-mm continuum map at 870 µm (beam size
probed. Despite the presence of various observational
∼19(cid:48).(cid:48)2) was also obtained from the ATLASGAL.
investigations,thephysicalprocessesresponsibleforstar
formationintheMCG35.2arenotwellexplored. Acare-
2.4. Spitzer and WISE Data
ful investigation of velocity structure of molecular gas
in the MCG35.2 is still missing in the literature. The The photometric images and magnitudes of point
large scale magnetic field structure projected in the sky sources at 3.6–8.0 µm were obtained from the Spitzer-
plane toward the MCG35.2 is yet to be examined. The GLIMPSE survey (resolution ∼2(cid:48)(cid:48)). In this work, we
objective of this paper is to study the physical condi- downloaded the GLIMPSE-I Spring ’07 highly reliable
tions in the MCG35.2 and to infer the physical mech- photometric catalog. We also used MIPSGAL 24 µm
anisms operating within the molecular cloud. In this image and the photometric magnitudes of point sources
paper, we present the results of the MCG35.2 extracted at MIPSGAL 24 µm (from Gutermuth & Heyer 2015).
using a more detailed and extensive analysis of observa- We also utilized the publicly available archival WISE1
tional data from centimeter, NIR, and optical Hα wave- image at mid-infrared (MIR) 12 µm (spatial resolution
lengths. These multi-frequency data-sets are retrieved ∼6(cid:48)(cid:48)).
from various Galactic plane surveys (see Table 1). Us-
ing these multi-frequency data-sets, we probe the distri- 2.5. H Narrow-band Image
2
bution of dust temperature, column density, extinction,
Continuum-subtracted narrow-band H (v = 1 − 0
2
ionizedemission,molecularhydrogenemission,magnetic
S(1)) image at 2.12 µm was obtained from the UWISH2
fieldlines, kinematicsof13COgas, andyoungstellarob-
database. The UWISH2 is an unbiased survey of the
jects (YSOs) in the MCG35.2.
inner Galactic plane in the H 1-0 S(1) line using the
2
This paper is organized as follows. In Section 2, we
UKIRT Wide Field Camera (WFCAM; Casali et al.
describe the data selection. In Section 3, we present the
2007). The v= 1-0 S(1) line of H at 2.12 µm is an
2
outcomesofourextensivemulti-frequencydataanalysis.
excellent tracer of shocked regions.
In Section 4, we discuss the implications of our results
concerningtheformationofmassivestarsandyoungstel-
2.6. NIR Data
lar clusters. Finally, the findings are summarized and
We retrieved NIR photometric JHK magnitudes of
concluded in Section 5.
point sources from the UKIDSS GPS 6th archival data
2. DATAANDANALYSIS release (UKIDSSDR6plus). The UKIDSS observations
(resolution ∼0(cid:48).(cid:48)8) were carried out using the WFCAM
In this work, we have chosen a region of ∼42.(cid:48)5 × 30.(cid:48)3
at UKIRT. The final fluxes were calibrated using the
(central coordinates: l = 35◦.144; b = −0◦.845) around
2MASS data. Several bright sources were found to be
the MCG35.2. In the following, we give a brief descrip-
saturated in the catalog. In this work, we obtained
tion of the adopted multi-frequency data-sets (also see
only reliable NIR photometric catalog. More informa-
Table 1).
tion about the selection procedure of the GPS photom-
etry can be obtained in Dewangan et al. (2015). Our
2.1. Radio Centimeter Continuum Maps
selected GPS catalog contains point-sources fainter than
WeusedradiocontinuummapsatMAGPIS20cmand J = 12.5, H = 11.7, and K = 10.6 mag to avoid satura-
NVSS 1.4 GHz. The MAGPIS 20 cm map has a 6(cid:48).(cid:48)2 × tion. 2MASS data were adopted for bright sources that
5(cid:48).(cid:48)4 beam size and a pixel scale of 2(cid:48)(cid:48)/pixel. The beam were saturated in the GPS catalog.
size of the NVSS 1.4 GHz map is ∼45(cid:48)(cid:48). These maps
wereobtainedwithatypicalrmsof0.3–0.45mJy/beam. 2.7. H-band Polarimetry
The centimeter radio continuum map traces the ionized
NIR H-band (1.6 µm) polarimetric data (resolution
emission in the complex.
∼1(cid:48).(cid:48)5) toward the MCG35.2 were obtained from the
GPIPS. The region probed in this paper was covered
2.2. 13CO (J=1−0) Line Data
In order to trace molecular gas associated with the se- 1 WISEisajointprojectoftheUniversityofCaliforniaandthe
lected target, the GRS 13CO (J=1−0) line data were JPL,Caltech,fundedbytheNASA
Star Formation Activity in the molecular cloud G35.20−0.74 3
in twelve GPIPS fields (i.e., GP1407, GP1408, GP1421, G35.20−0.74andAp2-1. InFigure3b,themolecularhy-
GP1422, GP1423, GP1435, GP1436, GP1437, GP1450, drogenoutflowsarefoundtowardtheEGOG35.20−0.74
GP1451, GP1452, and GP1464). The GPIPS data were and Mercer 14, which were previously known in the lit-
observed with the Mimir instrument, mounted on the erature (e.g. Froebrich & Ioannidis 2011b; S´anchez et al.
1.8 m Perkins telescope, in H-band linear imaging po- 2014). An arc-like feature is seen in the Spitzer 3.6–8.0
larimetry mode (see Clemens et al. 2012, for more de- µm, which is designated as Ap2-1 nebula (e.g. Paron &
tails). To obtain the reliable polarimetric results, we se- Weidmann 2010). The Spitzer 8.0 µm image of Ap2-1
lected sources having Usage Flag (UF) = 1 and P/σ ≥ nebula is shown in Figure 3c and is also overlaid with
p
2. These conditions yielded a total of 610 stars. The the MAGPIS 20 cm emission. In Figure 3d, we present
polarization vectors of background stars trace the mag- Hα image overlaid with the MAGPIS 20 cm emission
neticfielddirectionintheplaneoftheskyparalleltothe and find the similarity in the spatial distribution of ra-
direction of polarization (Davis & Greenstein 1951). dio continuum and Hα emission. The 20 cm and Hα
emissions are coincident with the 8.0 µm feature. The
2.8. Hα Narrow-band Image
8.0 µm feature is surrounded by the extended molecular
We downloaded narrow-band Hα image at 0.6563 µm H emission, suggesting the origin of the H emission is
2 2
from the IPHAS survey database. The survey was made likely caused by ultraviolet (UV) fluorescence (see Fig-
with the Wide-Field Camera (WFC) at the 2.5-m INT, ure 3b).
located at La Palma. The WFC consists of four 4k × In Figure 3d, thedistribution ofionized emission isal-
2kCCDs,inanL-shapeconfiguration. Thepixelscaleis most spherical. Using the MAGPIS 20 cm map and the
0(cid:48).(cid:48)33 (see Drew et al. (2005) for more details). The Hα “CLUMPFIND”IDLprogram(Williamsetal.1994),the
map depicts the distribution of ionized emission. integrated flux density and the radius (R ) of the Hii
HII
3. RESULTS region are estimated to be 230.3 mJy and 0.17 pc, re-
spectively. Using this integrated flux density value, the
3.1. Multi-wavelength picture of MCG35.2
number of Lyman continuum photon (N ) is estimated
uv
An extensive analysis of multi-wavelength data is es- following the equation given in Matsakis et al. (1976)
sential to study the physical environment of a given (see Dewangan et al. 2016, for more details). This anal-
star-forming complex. In Figure 1, we show the molec- ysis is carried out for a distance of 2.0 kpc and for the
ular 13CO (J = 1–0) gas emission in the direction of electron temperature of 10000 K. We compute N (or
uv
the MCG35.2. The 13CO profile depicts the MCG35.2 logN )tobe∼7.3×1046 s−1 (46.86)fortheHiiregion
uv
in a velocity range of 30–40 km s−1. The molecular associatedwithAp2-1,correspondingtoasingleionizing
map traces previously known condensations G35.20N star of spectral type B0.5V-B0V (see Table II in Pana-
and G35.20S that are also seen in the ATLASGAL 870 gia (1973)). Furthermore, the dynamical age of the Hii
µm map. The NVSS 1.4 GHz emission is detected to- region associated with Ap2-1 is defined below at a given
ward the G35.20S (see Figure 1). Considering the previ- radius R (e.g., Dyson & Williams 1980):
HII
ousresults(suchasV )onG35.20NandAp2-1sources
(Sa´nchezetal.2014;Plsarron&Weidmann2010), weinfer (cid:18)4R (cid:19) (cid:34)(cid:18)R (cid:19)7/4 (cid:35)
that the condensations G35.20N and G35.20S are phys- tdyn = 7cs RHII −1 (1)
ically associated with the MCG35.2. In Figure 2a, we s s
present a three-color composite image made using the
where c is the isothermal sound velocity in the ionized
s
SpitzerandWISEimages(24µminred,12µmingreen, gas (c = 11 km s−1; Bisbas et al. (2009)), R is
s HII
and 8.0 µm in blue). The image traces several dark
defined earlier, and R is the radius of the Str¨omgren
s
cloudswithintheMCG35.2andthedistributionofthese
sphere (= (3 N /4πn2α )1/3, where the radiative re-
dark clouds appears as an almost horse-shoe like struc- uv 0 B
combination coefficient α = 2.6 × 10−13 (104 K/T)0.7
ture. The horseshoe-like structure of the dark clouds is B
cm3 s−1 (Kwan 1997), N is defined earlier, and “n ”
identified based on the absorption features against the uv 0
istheinitialparticlenumberdensityoftheambientneu-
Galactic background in the 8–24 µm images. This par-
ticular structure is more obvious in the 13CO emission tralgas. Adoptingtypicalvalueofn0(as103(104)cm−3),
from35to36kms−1 (seeFigure2b). Severalembedded we estimated the dynamical age of the Hii region to be
∼4100(32800) yr.
stellar sources are also visually seen in the dark clouds
Together, Figures 1–3 help us to infer the embedded
at 8–24 µm. Figure 2b is a three-color composite image
structures in the MCG35.2 and its physical association
madeusingtheHerschel350µminred,250µmingreen,
and 160 µm in blue. The 13CO emission from 35 to 36 with the cloud.
km s−1 is also superimposed on the color composite im-
3.2. Herschel temperature and column density maps
age. The dark clouds appear as bright emission regions
at wavelengths longer than 70 µm. The distribution of TheHerscheltemperatureandcolumndensitymapsof
13CO emission is used to depict column density in the MCG35.2 are presented in this section. These maps are
outer regions of dark clouds, tracing the horseshoe-like generatedinamannersimilartothatoutlinedinMallick
structure. The condensations G35.20N and G35.20S ap- et al. (2015). Here, we also provide a brief step-by-step
peartobesituatedatthebaseofthehorseshoe-likestruc- description of the procedures.
ture. Figure 3a shows the zoomed-in view toward the We produced the Herschel temperature and column
G35.20N and G35.20S (a color composite image: 70 µm density maps from a pixel-by-pixel spectral energy dis-
(red), 8.0 µm (green), and 5.8 µm (blue) images). The tribution (SED) fit with a modified blackbody curve to
MAGPIS 20 cm emission is also overlaid on the compos- thecolddustemissionintheHerschel160–500µmwave-
ite image, revealing radio continuum detections in EGO lengths. TheHerschel70µmdataarenotincludedinthe
4 L. K. Dewangan
analysis,becausethe70µmemissionisdominatedbythe 3.3. Kinematics of molecular gas
UV-heated warm dust. The Herschel 160 µm image is
A kinematic analysis of the molecular gas in the
calibratedintheunitsofJypixel−1, whiletheimagesat
MCG35.2 is presented in this section. As mentioned be-
250–500 µm are calibrated in the surface brightness unit fore, the 13CO profile traces the MCG35.2 in a veloc-
of MJy sr−1. Plate scales are 3.2 arcsec/pixel for the ity range of 30–40 km s−1 (see Figure 1). In Figure 5,
160 µm image, 6 arcsec/pixel for the 250 µm image, 10
we present the integrated GRS 13CO (J=1−0) velocity
arcsec/pixel for the 350 µm image, and 14 arcsec/pixel
channel maps (at intervals of 1 km s−1), showing two
for the 500 µm image.
molecular components along the line of sight. The chan-
Prior to the SED fit, we convolved the images at 160–
nel maps show a molecular component associated with
350 µm to the lowest angular resolution of image at 500
the sites, G35.20N and G35.20S in a velocity range of
µm(∼37(cid:48)(cid:48))andconvertedtheseimagesintothesameflux
30–36 km s−1 (having a velocity peak at ∼33 km s−1),
unit (i.e. Jy pixel−1). Furthermore, the images at 160–
whiletheothermolecularcomponentisseenat35–40km
350 µm were regridded to the pixel size of image at 500
s−1 (having a velocity peak at ∼37 km s−1). Hence, we
µm(∼14(cid:48)(cid:48)). Theseprocedureswerecarriedoutusingthe
infer the red-shifted and blue-shifted molecular compo-
convolutionkernelsavailableintheHIPEsoftware. Next,
nents in the MCG35.2. The sites, G35.20N and G35.20S
we determined the sky background flux level be 0.211,
appear to be seen at the intersection of these molecular
0.566, 1.023, and 0.636 Jy pixel−1 for the 500, 350, 250,
components. Paron&Weidmann(2010)alsoutilizedthe
and 160 µm images (size of the selected region ∼4.(cid:48)9 ×
GRS13COdataandpresentedtheintegratedGRS13CO
6.(cid:48)4; centered at: l = 35◦.208; b = −1◦.47), respectively.
intensity map and velocity channel maps mainly toward
To avoid diffuse emission linked with the MCG35.2, the
the sites, G35.20N and G35.20S (see Figures 3 and 4
featureless dark field away from the selected target was
in Paron & Weidmann 2010). However, the position-
carefully selected for the background estimation.
velocity analysis of the MCG35.2 is still lacking.
Finally, to produce the maps, a modified blackbody
In Figure 6, we show the integrated 13CO intensity
wasfittedtotheobservedfluxesonapixel-by-pixelbasis
map and the position-velocity maps. In Figure 6a, the
(seeequations8and9inMallicketal.2015). Thefitting
integrated GRS 13CO intensity map is similar to that
wascarriedoutusingthefourdatapointsforeachpixel,
shown in Figure 1. The galactic position-velocity di-
maintaining the dust temperature (T ) and the column
d agrams of the 13CO emission trace a noticeable veloc-
density (N(H )) as free parameters. In the analysis, we
2 ity gradient (see Figures 6b and 6d). In Figure 6c, the
adoptedameanmolecularweightperhydrogenmolecule
molecular emission integrated over 30 to 36 km s−1 is
(µ =) 2.8 (Kauffmann et al. 2008) and an absorption
H2
also overlaid on the molecular map. In the background
coefficient(κ =)0.1(ν/1000GHz)β cm2 g−1,including
ν CO map, the molecular emission is shown from 35 to
a gas-to-dust ratio (R =) of 100, with a dust spectral
t 40 km s−1. Figure 6c shows the spatial distribution of
index of β=2 (see Hildebrand 1983). In Figure 4, the
moleculargaslinkedwiththered-shiftedandblue-shifted
final temperature and column density maps (resolution
molecular components in the MCG35.2. Figure 6d also
∼37(cid:48)(cid:48)) are presented.
shows a red-shifted peak (at ∼37 km s−1) and a blue-
In the Herschel temperature map, the sites, G35.02N
shiftedpeak(at∼33kms−1)thatareinterconnectedby
and G35.02S are seen with the warmer gas (T ∼21-35
d lower intensity intermediated velocity emission, indicat-
K) (see Figure 4a), while the Spitzer dark clouds are
ing the presence of a broad bridge feature. The broad
traced in a temperature range of about 14–18 K. The
bridge feature in the position-velocity diagram has been
horseshoe-like structure is also evident in the Herschel
reported as an evidence of collisions between molecular
column density map and several condensations are seen
clouds (Haworth et al. 2015a,b). Furthermore, Torii et
toward this structure.
al. (2016) reported that the complementary distribution
Inthecolumndensitymap,weemployedtheclumpfind
is an expected output of cloud-cloud collisions, and the
algorithm to identify the clumps and their total column
bridge feature at the intermediate velocity range can be
densities. Fourteen clumps are detected above a three
explained as the turbulent gas excited at the interface
sigma noise level in the MCG35.2. These clumps are
of the collision. In the extended MCG35.2 complex, a
labeledinFigure4bandtheirboundariesarealsoshown
possible complementary pair is 32-33 km s−1 and 37-38
in Figure 4c. The mass of each clump is estimated using
kms−1 (seeFigure5). Wediscusstheimplicationofour
itstotalcolumndensityandcanbedeterminedusingthe
findings in more detail in the discussion Section 4.
equation:
3.4. GPIPS H-band Polarization
M =µ m Area ΣN(H ) (2)
clump H2 H pix 2 In this section, we present the large scale morphol-
whereµ isassumedtobe2.8, Area istheareasub- ogy of the plane-of-the-sky projection of the magnetic
H2 pix
tended by one pixel, and ΣN(H ) is the total column field toward the MCG35.2. Using the GPIPS data, we
2
density. The mass of each Herschel clump is listed in study the polarization vectors of background stars that
Table 2. The table also provides an effective radius and are selected based on the conditions (i.e. UF = 1 and
apeakcolumndensityofeachclump. Theclumpmasses P/σ ≥ 2). These conditions are very useful to select
p
vary between ∼110 M and ∼4250 M . The peak col- the reliable background stars and the confirmation of
(cid:12) (cid:12)
umn densities of these clumps also vary between ∼1.1 × these background sources can also be obtained using the
1022 cm−2 (A ∼12 mag) and ∼14 × 1022 cm−2 (A NIRcolor-colordiagram(e.g.Dewanganetal.2015). As
V V
∼150 mag) (see Table 2 and also Figure 4b). There are mentionedbefore,thepolarizationvectorsofbackground
at least ten massive clumps (M ∼500 – 4250 M ) stars give the field direction in the plane of the sky par-
clump (cid:12)
distributed toward the horseshoe-like structure. alleltothedirectionofpolarization(Davis&Greenstein
Star Formation Activity in the molecular cloud G35.20−0.74 5
1951). In Figure 7a, we display the H-band polarization Anandarao (2011). This scheme gives 99 YSOs (37
vectorsoverlaidonthemolecularmap. Tostudythedis- Class I; 62 Class II), 2 Class III, and 589 contaminants.
tribution of H-band polarization, we present mean po-
larizationvectorsinFigure7b. Ourselectedspatialfiled 3. In this scheme, we considered sources having
is divided into 14 × 10 equal divisions and a mean po- detections in the first three GLIMPSE bands (except
larization value is computed using the Q and U Stokes 8.0 µm band) (i.e. three GLIMPSE scheme). The
parameters of H-band sources located inside each divi- color-color space ([4.5]−[5.8] vs [3.6]−[4.5]) is used
sion. In Figure 7, the degree of polarization is traced to depict the infrared excess sources. We use color
by the length of a vector, whereas the angle of a vector conditions, [4.5]−[5.8] ≥ 0.7 and [3.6]−[4.5] ≥ 0.7, to
indicates the polarization galactic position angle. select protostars. One can find more details about the
The mean distribution of H-band vectors does not ap- YSO classifications based on the three GLIMPSE bands
pear uniform. Our analysis reveals a noticeable change in Hartmann et al. (2005) and Getman et al. (2007).
intheH-bandstarlightmeanpolarizationanglesbetween This scheme yields 17 protostars in our selected region
the ZoneI and ZoneII (see Figure 7b). (see Figure 8c).
3.5. Young stellar objects in the MCG35.2
4. In this scheme, we use the NIR color-magnitude
The knowledge of young populations helps to trace space (H−K/K) to depict additional YSOs. We used
the ongoing star formation activity in a given embedded a color H−K value (i.e. ∼2.35) that separates H−K
molecular cloud. In this section, the young populations excess sources from the rest of the population. This
are investigated using their infrared excess for a wider color condition is chosen based on the color-magnitude
field of view around the MCG35.2. Various infrared plot of sources from the nearby control field (size of the
photometric surveys (e.g., MIPSGAL, GLIMPSE, selected region 12.(cid:48)1 × 8.(cid:48)7; centered at: l = 34◦.898; b =
UKIDSS-GPS, and 2MASS) have been employed to −1◦.023). Using this color H−K cut-off condition, 246
explore the YSOs in the MCG35.2. In the following, a embedded YSOs are selected in the region probed in
brief description of the selection of YSOs is provided. this work (see Figure 8d).
1. In this scheme, we considered sources having Usingtheanalysisofthephotometricdataat1–24µm,
detections in both the MIPSGAL 24 µm and GLIMPSE we obtain a total of 499 YSOs in our selected region
3.6 µm bands (i.e. GLIMPSE-MIPSGAL scheme). around the MCG35.2 (as shown in Figure 1). The posi-
The GLIMPSE-MIPSGAL color-magnitude space tions of all these YSOs are shown in Figure 9a. These
([3.6]−[24]/[3.6]) allows to depict the different stages of YSOs are seen within the entire molecular cloud.
YSOs (Guieu et al. 2010; Rebull et al. 2011; Dewangan
et al. 2015). Following the conditions provided in Guieu
3.5.1. Study of distribution of YSOs
et al. (2010), the boundary of different stages of YSOs
is marked in Figure 8a. The color-magnitude space also Inthissection,westudythespatialdistributionofour
exhibits the boundary of possible contaminants (i.e. selected YSOs using the standard surface density util-
galaxies and disk-less stars) (see Figure 10 in Rebull ity (see Gutermuth et al. 2009; Bressert et al. 2010; De-
et al. 2011). In Figure 8a, a total of 467 sources are wangan & Anandarao 2011; Dewangan et al. 2015, for
presented in the color-magnitude space. We find 137 moredetails),whichcanallowustodepicttheindividual
YSOs (26 Class I; 38 Flat-spectrum; 73 Class II) and groupsorclustersofYSOs. Usingthenearest-neighbour
328 Class III sources. Additionally, two Flat-spectrum (NN) technique, the surface density map of YSOs was
sources are seen in the boundary of possible contam- obtained in a manner similar to that outlined in Dewan-
inants and are not included in our selected young gan et al. (2015) (also see equation in Dewangan et al.
populations. Together, the selected YSO populations 2015). The surface density map of all the selected 499
are free from the contaminants. YSOs was constructed using a 5(cid:48)(cid:48) grid and 6 NN at a
distance of 2.0 kpc. In Figure 9b, the surface density
2. In this scheme, we considered sources having detec- contours of YSOs are presented and are drawn at 5, 10,
tions in all four GLIMPSE 3.6–8.0 µm bands (i.e. four and 20 YSOs/pc2, increasing from the outer to the in-
GLIMPSE scheme). The GLIMPSE color-color space ner regions. The positions of Herschel clumps are also
([3.6]−[4.5]vs[5.8]−[8.0])isusedtoidentifytheinfrared marked in Figure 9b. We find noticeable YSOs clusters
excess sources. In the selected infrared excess sources, toward all the Herschel clumps in the MCG35.2. The
various possible contaminants (e.g. broad-line active HerschelclumpsareverywellcorrelatedwiththeCOgas
galactic nuclei (AGNs), PAH-emitting galaxies, shocked and YSOs. Furthermore, majority of the YSOs clusters
emission blobs/knots, and PAH-emission-contaminated arefoundtowardthehorseshoe-likestructure. Thesites,
apertures) are removed using the Gutermuth et al. G35.20N and G35.20S (including Ap2-1 and Mercer 14)
(2009) schemes. Furthermore, the final selected YSOs areseentowardtheHerschelclumps#1–3,wherethein-
are classified into different evolutionary stages based on tense star formation activities are traced. Additionally,
their slopes of the GLIMPSE SED (α ) computed radio continuum emission is also detected in at least two
3.6−8.0
from 3.6 to 8.0 µm (i.e. Class I (α > −0.3), out of these three clumps (see Figure 3a). Krumholz &
3.6−8.0
Class II (−0.3 > α > −1.6), and Class III McKee (2008) suggested a threshold value of 1 gm cm−2
3.6−8.0
(−1.6>α >−2.56)) (e.g., Lada et al. 2006). The (orcorrespondingcolumndensities ∼3 × 1023 cm−2)for
3.6−8.0
GLIMPSE color-color space is presented in Figure 8b. thebirthofmassivestars. Hence,theoretically,thebirth
One can find more details about the YSO classifications of massive stars in these Herschel clumps is likely (see
based on the four GLIMPSE bands in Dewangan & column densities in Table 2). Previously, S´anchez et al.
6 L. K. Dewangan
(2014) referred the G35.20N as the main site of MSF ∼4 km s−1. We also investigate a broad bridge feature
(also see Beltr´an et al. 2016). In Section 3.1, we find inthevelocityspaceandthecomplementarydistribution
that the Hii region associated with Ap2-1 is very young of the two molecular components in the velocity channel
(i.e., ∼4100–32800 yr) and is excited by a radio spectral maps. Allthesecharacteristicfeaturesofcloud-cloudcol-
type of B0.5V star. In addition to the YSOs clusters, lision are evident in the MCG35.2, favouring the onset
there is also ongoing MSF in the Herschel clumps #1–3 of the cloud-cloud collision process. Using the physical
located at the base of the horseshoe-like structure. It is separation(i.e. ∼4.5pc)andthevelocityseparation(i.e.
also worth mentioning that these three Herschel clumps ∼4kms−1)ofthetwocollidingcloudcomponentsinthe
havemassesmorethan1000M(cid:12) andpeakcolumndensi- MCG35.2, we estimate the typical collision timescale to
ties between ∼4.2 × 1022 cm−2 (AV ∼45 mag) and ∼14 be ∼1.1 Myr. The dynamical age of the Hii region as-
× 1022 cm−2 (A ∼150 mag) (see Table 2). The radio sociated with Ap2-1 is found to be ∼4100–32800 yr. An
V
continuum emission is not observed toward the remain- average age of the Class I and Class II YSOs is reported
ing Herschel clumps (i.e. #4-14), indicating the absence to be ∼0.44 Myr and ∼1–3 Myr, respectively (Evans et
of the Hii regions in these clumps. Hence, it implies al. 2009). Considering these timescales, it appears that
that these remaining clumps appear to harbor the clus- the birth of massive stars and the youngest populations
tersoflow-massstars. ThestudyofdistributionofYSOs is influenced by the cloud-cloud collision. The observed
convincingly illustrates the star formation activities (in- highcolumndensitiestowardtheclumpsfurthersupport
cluding MSF) in the MCG35.2. this interpretation (see Table 2), which contain massive
stars. However, the stellar populations older than the
4. DISCUSSION
collision timescale may have formed prior to the colli-
A recent demonstration of the tool of a broad-bridge sion. Hence, we cannot also rule out the star formation
feature concerning the cloud-cloud collision is given by before the collision in the MC35.20.
thepositionvelocityanalysisofmoleculargas. Recently, Another result presented in this study is that a no-
Haworthetal.(2015a)andHaworthetal.(2015b)carried ticeable change in the distribution of the starlight H-
out simulations in favor of this understanding (also see band polarization vectors in the MC35.20. It is known
Toriietal.2011,2015,2016;Fukuietal.2014,2016;De- that the large scale magnetic field structure projected
wanganetal.2015;Baugetal.2016). Inadditiontothe in the sky plane can be inferred using the starlight H-
broad bridge feature, one can also expect the spatially band polarization data and the aligned dust is utilized
complementary distribution between the two clouds in toprobemagneticfields. Takenthisfact,theinteraction
the sites of the cloud-cloud collision (e.g., Torii et al. of molecular components could influence the dust distri-
2016). Fukui et al. (2014) and Torii et al. (2015) dis- butionandappearstoberesponsibleforthevariationin
cussedthatthecloud-cloudcollisionprocesscanalsotrig- the polarization angles.
ger the birth of massive stars (also see Fukui et al. 2016;
5. SUMMARYANDCONCLUSIONS
Torii et al. 2016). Fukui et al. (2016) pointed out that
the H column density is the critical parameter in the In this paper, we have utilized the multi-wavelength
2
cloud-cloudcollisionscenarioandalsoreportedathresh- data covering from optical-Hα, NIR, and radio wave-
old value of 1023 cm−2 for the birth of the massive star lengths and have carried out an extensive study of the
clusters such as RCW38. They further suggested that a MCG35.2. In the following, the important outcomes of
single O star can be formed at molecular column den- this work are presented.
sity of 1022 cm−2. There are some star-forming regions • The molecular gas emission in the direction of the
reported in the literature where the cloud-cloud colli- MCG35.2 is traced in a velocity range of 30–40 km s−1.
sion process is likely and Torii et al. (2016) provided a There are two noticeable molecular components (having
Table 2 containing the physical parameters of the cloud- velocity peaks at 33 and 37 km s−1) detected in the
cloud collisions estimated in the earlier studies. MCG35.2.
In Section 3.5, the photometric analysis of point-like • The most prominent structure in the MCG35.2 is the
sources has revealed ongoing star formation activities horseshoe-like morphology traced in the infrared and
throughout the MC35.20. However, the majority of the millimeter images.
YSOs clusters are distributed toward the horseshoe-like • The distribution of ionized emission toward Ap2-1
structurewheretenmassiveclumpsaredetected. Thein- observed in the MAGPIS 20 cm continuum map and
tensestarformationactivitiesarealsoinvestigatedatthe Hα image is almost spherical. The ionizing photon flux
base of this structure where the previously known sites, valueestimatedat20cmcorrespondstoasingleionizing
G35.20N and G35.20S are located. Recently, Beltr´an star of B0.5V–B0V spectral type.
et al. (2016) reported the presence of a binary system •Theposition-velocityanalysisof13COemissiondepicts
of ultra-compact/hyper-compact Hii regions at the geo- a broad bridge feature which is defined as a structure
metricalcenteroftheradiojetinG35.20N.Theyfurther having two velocity peaks (a red-shifted (at 37 km s−1)
foundthatthebinarysystem, whichisassociatedwitha and a blue-shifted (at 33 km s−1)) interconnected by
Keplerian rotating disk, contains two B-type stars of 11 lower intensity intermediated velocity emission. Such
and6M(cid:12). Thesites,G35.20NandAp2-1arerevealedas feature can indicate the signature of a collision between
theactivesitesofMSFandareextendedbyabout4.5pc. molecular components in the MCG35.2. These two
Furthermore, thesites, G35.20NandG35.20S(including velocity peaks are separated by ∼4 km s−1. The study
Ap2-1 and Mercer 14) are found at the intersection of of molecular line data also shows a possible complemen-
two molecular components (having peaks at 33 and 37 tary pair at 32-33 km s−1 and 37-38 km s−1 within the
km s−1) in the MC35.20 (see Section 3.3). The velocity extended MCG35.2 complex.
separationofthetwocloudcomponentsisdepictedtobe • Fourteen Herschel clumps are identified in the
Star Formation Activity in the molecular cloud G35.20−0.74 7
MCG35.2 and ten out of these fourteen clumps are seen Bressert,E.,Bastian,N.,Gutermuth,R.,etal.2010,MNRAS,
toward the horseshoe-like structure. 409,54
• The analysis of photometric data at 1–24 µm reveals Carey,S.J.,Noriega-Crespo,A.,Price,S.D.,etal.2005,BAAS,
37,1252
a total of 499 YSOs and these YSOs are found in
Casali,M.,Adamson,A.,AlvesdeOliveira,C.,etal.2007,A&A,
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Table 1
Listofsurveys(coveringfromradio,NIR,andoptical-Hαwavelengths)utilizedinthiswork
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17.6pcatadistanceof2.0kpc);centeredatl=35◦.144;b=−0◦.845). TheCOcontoursare61.78Kkms−1 ×(0.2,0.25,0.3,0.35,0.4,0.5,0.6,
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Star Formation Activity in the molecular cloud G35.20−0.74 9
Figure 2. Three-colorimagesofthestar-formingcomplex,G35.20−0.74usingtheSpitzer,WISE,andHerschelimages. a)Theimageisaresult
ofthecombinationofthreebands: 24µm(red),12µm(green),and8.0µm(blue). TheCOcontour(dottedwhite)isshownwithalevelof12.356
K km s−1. The locations of G35.2N, G35.2S, and Ap2-1 are highlighted. Figure reveals the dark clouds located within the MCG35.2 and the
distribution of dark clouds appears as a horseshoe-like morphology. A dotted-dashed box shows the field of Figure 3a. b) Color-composite map
usingtheHerschel350µm(red),250µm(green),and160µm(blue)images. TheCOemissioncontour(dotted-dashedcyan)isalsoshownfrom
35to36kms−1 withalevelof3Kkms−1, tracinganalmosthorseshoe-likemorphologycontainingthesourcesG35.2NandG35.2Satitsbase
(alsoseeFigure5). Thescalebarcorrespondingto3pc(atadistanceof2.0kpc)isshowninboththepanels.
10 L. K. Dewangan
Figure 3. Zoomed-in view of G35.2N, G35.2S, and Ap2-1. a) The image is a result of the combination of three bands: 70 µm (red), 8.0 µm
(green), and5.8µm(blue). TheMAGPIS20cmemissionisoverplottedbycyansolidcontourswithlevelsof5, 30, 60, 80, and95%ofthepeak
value (i.e., 0.0257 Jy/beam). A dotted-dashed box shows the field of Figure 3b. b) The UWISH2 continuum-subtracted H2 map (gray-scale) at
2.12µm(seedotted-dashedboxinFigure3a). Asolidbox(inred)showsthefieldofFigures3cand3d. c)Zoomed-inviewofAp2-1nebulausing
theSpitzer8.0µmimage(invertedgray-scale). d)Zoomed-inviewofAp2-1nebulausingtheIPHASHαimage(invertedgray-scale). Inlasttwo
panels(candd),theimagesareoverlaidwiththeMAGPIS20cmemission. The20cmcontoursareshownwithlevelsof5,10,20,30,and98%of
thepeakvalue(i.e.,0.0257Jy/beam).