A multi-wavelength study of star formation activity in the S235 complex

5.9 MB·

by  L. K. Dewangan

PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
A MULTI-WAVELENGTH STUDY OF STAR FORMATION ACTIVITY IN THE S235 COMPLEX
L. K. Dewangan1,3, D. K. Ojha2, A. Luna1, B. G. Anandarao3, J. P. Ninan2, K. K. Mallick2, and Y. D. Mayya1
ABSTRACT
We have carried out an extensive multi-wavelength study to investigate the star formation process
in the S235 complex. The S235 complex has a sphere-like shell appearance at wavelengths longer
than2µmandharborsanO9.5Vtypestarapproximatelyatitscenter. Near-infraredextinctionmap
traces eight subregions (having A > 8 mag), and five of them appear to be distributed in an almost
V
6 regularly spaced manner along the sphere-like shell surrounding the ionized emission. This picture is
1 also supported by the integrated 12CO and 13CO intensity maps and by Bolocam 1.1 mm continuum
0 emission. The position-velocity analysis of CO reveals an almost semi-ring like structure, suggesting
2 an expanding Hii region. We find that the Bolocam clump masses increase as we move away from
the location of the ionizing star. This correlation is seen only for those clumps which are distributed
n
near the edges of the shell. Photometric analysis reveals 435 young stellar objects (YSOs), 59% of
a
J which are found in clusters. Six subregions (including five located near the edges of the shell) are
very well correlated with the dust clumps, CO gas, and YSOs. The average values of Mach numbers
8
derived using NH data for three (East 1, East 2, and Central E) out of these six subregions are 2.9,
1 3
2.3, and 2.9, indicating these subregions are supersonic. The molecular outflows are detected in these

three subregions, further confirming the on-going star formation activity. Together, all these results
]
A are interpreted as observational evidence of positive feedback of a massive star.
G Subject headings: dust, extinction – Hii regions – ISM: clouds – ISM: individual objects (S235) –
stars: formation – stars: pre-main sequence
.
h
p
- 1. INTRODUCTION spanning near-infrared (NIR) to radio wavelengths. The
o S235 complex is known as an active site of star forma-
The energetics of massive stars can strongly influence
r tion, harboring young stellar clusters (e.g., Kirsanova et
t the surroundings (Zinnecker & Yorke 2007). Massive
s al. 2008; Dewangan & Anandarao 2011; Camargo et al.
starscandestroystar-formingclouds(i.e.,negativefeed-
a
2011; Chavarr´ıa et al. 2014) associated with known star-
[ back) and can also trigger star formation (i.e., positive
feedback), leading to the formation of a new generation forming subregions, namely East 1, East 2, and Central
1 of stars including young massive star(s) (Deharveng et (e.g. Kirsanova et al. 2008). In our previous work on the
v S235 complex using the Spitzer-IRAC data (Dewangan
al. 2010). However, the feedback processes of massive
8 & Anandarao 2011, hereafter Paper I), we detected sev-
stars are still poorly understood.
8 eral young stellar objects (YSOs) including a High Mass
The extended star-forming region S235 is known to
4 Protostellar Object (HMPO) candidate as well as signa-
be a part of the giant molecular cloud G174+2.5 in the
4 Perseus Spiral Arm (e.g. Heyer et al. 1996) and contains tures of outflow activities. Using 13CO (1−0) line data,
0
two known sites “S235 complex” and “S235AB region” Kirsanova et al. (2008) found three molecular gas com-
.
1 (seeFigure1inDewangan&Anandarao(2011)andalso ponents (i.e., −18 km s−1 < Vlsr < −15 km s−1 (red),
0 in Kirsanova et al. (2014)). The present work is focused −21 km s−1 < V < −18 km s−1 (central), and −25
lsr
6 on “S235 complex” and does not include “S235AB re- km s−1 <V <−21 km s−1 (blue)) in the direction of
lsr
1 gion”. Different values of the distance (1.36 kpc, 1.59 the S235 complex. However, the complex is well traced
:
v kpc, 1.8 kpc, 2.1 kpc, and 2.5 kpc) to the extended in mainly two molecular gas components (central and
i star-forming region S235 are reported in the literature blue). More recently, Kirsanova et al. (2014) derived
X
(e.g. Georgelin et al. 1973; Israel and Felli 1978; Evans physical parameters of dense gas (i.e., gas density and
r & Blair 1981; Brand & Blitz 1993; Burns et al. 2015; temperature) in subregions of the complex using ammo-
a
Foster&Brunt2015). Inthepresentwork,wehavecho- nia (NH ) line observations. However, the properties of
3
senadistanceof1.8kpcfollowingEvans&Blair(1981), dense gas are not explored with respect to the ionizing
which is an intermediate value of the published distance star location. Previous studies indicated that the S235
range. The Hii region associated with the S235 com- Hii region is interacting with its surrounding molecular
plex is predominantly ionized by a single massive star cloud and the S235 complex has been cited as a possible
BD+35o1201ofO9.5Vtype(Georgelinetal.1973). The site of triggered star formation (Kirsanova et al. 2008,
S235 complex has been studied using multiple datasets 2014; Camargo et al. 2011).
Spitzerimagesrevealedthatthecomplexhasasphere-
[email protected] like shell morphology and the ionizing star is approxi-
1InstitutoNacionaldeAstrof´ısica,O´pticayElectr´onica,Luis mately located at its center (see Figure 1 given in Pa-
EnriqueErro#1,Tonantzintla,Puebla,M´exicoC.P.72840
2DepartmentofAstronomyandAstrophysics,TataInstitute per I). In addition to the observed interesting morphol-
ofFundamentalResearch,HomiBhabhaRoad,Mumbai400005, ogy, the complex is a relatively nearby star-forming site,
India making it a promising site to study the feedback of a
3Physical Research Laboratory, Navrangpura, Ahmedabad -
massive star. In spite of the numerous existing observa-
380009,India
2 L. K. Dewangan et al.
tionsandinterpretations, thefeedbackofanO9.5Vtype We obtained deep NIR photometric JHK images and
star is not systematically explored in the S235 complex. the magnitudes of point sources in the region (∼17.(cid:48)6
Theaimofthepresentworkistostudythephysicalpro- × 15.(cid:48)5) from the UKIDSS 6th archival data release
cesses governing the interaction and feedback effect of a (UKIDSSDR6plus) of the Galactic Plane Survey (GPS;
massive star on its surroundings. Lawrenceetal.2007). ThesurveyusestheUKIRTWide
In order to address above aim, we revisited the Field Camera (WFCAM; Casali et al. 2007). Following
S235 complex using high sensitivity United Kingdom the selection conditions given in Lucas et al. (2008), we
Infra-Red Telescope (UKIRT) Infrared Deep Sky Survey retrieved only reliable NIR sources in the region. One
(UKIDSS)NIRdata,GiantMetre-waveRadioTelescope can also find more details about the selection procedure
(GMRT) 610 MHz radio continuum map, and dust con- of the GPS photometry in the work of Dewangan et al.
tinuum1.1mmdata, inconjunctionwithpublishednar- (2015). Two Micron All Sky Survey (2MASS; Skrut-
rowbandH2map,Spitzermid-infrared(MIR)data,NH3 skie et al. 2006) data were obtained for bright sources
linedata,NRAOVLASkySurvey(NVSS)1.4GHzcon- that were saturated in the GPS catalog. We found 2444
tinuummap,andCOlinedata. Weperformedadetailed sources detected in all the three NIR (JHK) bands. Ad-
studyofthedistributionandkinematicsofmoleculargas ditionally, 326 sources were selected having detections
in the complex. In order to systematically explore the only in the H and K bands.
feedback of a massive star, we estimated various pres-
sure components (such as pressure of an Hii region, ra- 2.3. Spitzer data
diation pressure, stellar wind pressure, pressure exerted
Thephotometricimages(3.6–24µm)oftheS235com-
bytheself-gravitatingmolecularcloud,andratioofther-
plex were obtained from the Spitzer Space Telescope
mal to non-thermal gas pressure). For a detailed study
Infrared Array Camera (IRAC; Fazio et al. 2004) and
of the embedded young population in the complex, we
the Multiband Imaging Photometer (MIPS; Rieke et al.
employed different color-color and color-magnitude dia-
2004). The final processed IRAC images (3.6–8.0 µm)
grams obtained using NIR and MIR data, as well as ex-
and photometry of point sources were taken from Pa-
tinction map generated from NIR data, and the surface
per I. MIPS 24 µm observations were retrieved from the
density analysis. Additionally, the physical properties Spitzer public archive1, which were carried out in MIPS
of gas derived using the line and continuum data were
scanmodeon15April2008(Programid40005; PI:Gio-
investigated.
vanniFazio). MIPSmosaicat24µmwasgeneratedusing
In Section 2, we provide the description of various
the Basic Calibrated Data (BCD) images.
datasets along with reduction procedures. In Section 3,
wesummarizetheresultsrelatedtothephysicalenviron- 2.4. Dust continuum 1.1 mm data
mentandpoint-likesources. Thepossiblestarformation
Bolocam1.1mmimage(Aguirreetal.2011)andBolo-
scenario is discussed in Section 4. Our main conclusions
cam source catalog at 1.1 mm (v2.1; Ginsburg et al.
are summarized in Section 5.
2013) were obtained from Bolocam Galactic Plane Sur-
2. DATAANDANALYSIS vey (BGPS). The effective full width at half maximum
(FWHM) of the 1.1 mm map is ∼33(cid:48)(cid:48).
Multi-wavelengthdataareemployedtostudythephys-
ical conditions in the S235 complex. The size of the
2.5. Radio continuum data
selected region is ∼17.(cid:48)6 × 15.(cid:48)5, centered at α =
2000
05h40m57.8s, δ = +35◦51(cid:48)13(cid:48)(cid:48), corresponding to a Weusedthearchivalradiocontinuumdataat610MHz
2000
physical scale of about 9.2 pc × 8.1 pc at a distance (50 cm) and 1.4 GHz (21 cm). The 1.4 GHz map was
of 1.8 kpc. retrieved from the NVSS archive. The beam size of the
NVSS image is ∼45(cid:48)(cid:48) (Condon et al. 1998). The 610
2.1. Narrow-band H image MHz continuum data were observed on 18–19 June 2005
2
(Project Code: 08SKG01) and were retrieved from the
Narrow-band H (ν = 1 − 0 S(1) at λ = 2.122 µm
2 GMRTarchive. GMRTradiodatareductionwascarried
(∆λ = 0.032µm)) imaging data of the S235 complex
outusingAIPSsoftware, inamannersimilartothatde-
were obtained from the survey of extended H emission
2 scribed in Mallick et al. (2013). Since the S235 complex
(Navareteetal.2015), conductedattheCanada-France-
was away from the center of the observed field, the pri-
Hawaii Telescope (CFHT), Mauna Kea, Hawaii, using
mary beam correction for 610 MHz was also carried out,
Wide-field InfraRed Camera (WIRCam). The observa-
using the AIPS PBCOR task and parameters from the
tions were performed with the average seeing of ∼0(cid:48).(cid:48)7. GMRT manual2. The synthesized beam size of the final
The survey also provided K-band continuum (λ = 2.218 610 MHz map is ∼48(cid:48)(cid:48) × 44(cid:48).(cid:48)2.
µm; ∆λ = 0.033µm) images, which were used to get
the final continuum-subtracted H2 map. Note that the 2.6. Molecular CO line data
H image published by Navarete et al. (2015) contains
2 The J=1−0 lines of 12CO and 13CO data were ob-
onlytheEast2andCentralstar-formingsubregions(see
servedfromtheFiveCollegeRadioAstronomyObserva-
source IDs 164 and 165 in Table B2 given in Navarete et
tory (FCRAO) 14 meter telescope in New Salem, Mas-
al. 2015), while the H image in this work is presented
for a larger area ∼10.(cid:48)72× 9.(cid:48)9 toward the complex. One sachusetts. The FCRAO beam sizes were 45(cid:48)(cid:48) and 46(cid:48)(cid:48)
for12COand13CO,respectively. TheS235complexwas
can find more details about the survey in the work of
observed as part of the Extended Outer Galaxy Survey
Navarete et al. (2015).
1 seehttp://sha.ipac.caltech.edu/applications/Spitzer/SHA/
2.2. Near-infrared data
2http://gmrt.ncra.tifr.res.in/gmrt hpage/Users/doc/obs manual.pdf
Study of star formation activity in S235 3
(E-OGS, Brunt 2004), that extends the coverage of the The Central E subregion contains two bright sources,
FCRAO Outer Galaxy Survey (OGS, Heyer et al. 1998) namelyEBIRS1/G173.6328+02.8064(hereafter,IRS1)
to Galactic longitude l = 193◦, over a latitude range of and EB IRS 2/G173.6339+02.8218 (hereafter, IRS 2)
−3◦.5 ≤ b ≤ +5◦.5. However, the data cubes of S235 (Evans & Blair 1981) with previously estimated lumi-
complex were further re-processed and a document de- nosities of ∼2.5 × 103 L and ∼1.5 × 103 L (Evans
(cid:12) (cid:12)
scribingthere-processingdatamethodsisgiveninBrunt et al. 1981), respectively. Furthermore, radio continuum
(2004) (also Brunt C.M. et al.; in preparation). These emissions are absent toward these two sources (IRS 1
CO data cubes were obtained from M. Heyer and C. andIRS2),aspointedoutbyNordhetal.(1984). Com-
Brunt (through private communication). bining these information with the radio continuum data
presentedinthiswork,onecannotethatnoradiocontin-
2.7. Other Data uumemissionpeaksareseentowardthesubregions. Fig-
We utilized the publicly available archival WISE3 ure1bshowstheenhanced4.5µmandK-bandextended
(Wright et al. 2010) image at 12 µm (spatial resolution emissionfeaturesintheEast1andEast2subregions. In
∼6(cid:48)(cid:48)). We also used previously published NH line data Paper I, the signatures of outflow activities were found
3
from Kirsanova et al. (2014). towardEast1andEast2subregionsduetothepresence
of shock-excited H emissions as revealed by the IRAC
2
3. RESULTS ratio maps. Note that the embedded sources in East 1
3.1. Mid-infrared and radio continuum images subregionappeartobeseparatedbytheeasternemission
wall from the sphere-like shell (see Figure 1a).
The most prominent feature of the S235 complex ob-
servableintheinfraredregime,thesphere-likeshellmor-
phology,isdetectedatwavelengthslongerthan2µm(see 3.2. Near-infrared extinction map
Figure 1). Figure 1a is a three-color composite image Thevisualextinction(A )mapoftheS235complexis
V
madeusingMIPS24µminred,WISE12.0µmingreen, generated using the publicly available GPS NIR (JHK)
andIRAC8.0µminblue. Figure1bshowsathree-color photometric data. The extinction value of individual
compositeimageusingMIPS24µm(red), IRAC4.5µm stars was computed using a color-excess along the line
(green), and UKIDSS 2.2 µm (blue). Radio continuum of sight (E(H −K) = (H −K) − < (H −K) >; see
obs 0
contours at 1.4 GHz and 610 MHz are also overlaid on equation given in Lada et al. 1994) using the reddening
thecolor-compositeimagesinFigures1aand1b,respec- lawsofIndebetouwetal.(2005). The<(H-K) >isthe
0
tively. Theseradiocontinuumcontourstracetheionized meanintrinsiccolorofthestars,whichwasdeterminedto
emission in the S235 complex. The distribution of ion- be∼0.25fromthenearbycontrolfield(similarsizeastar-
izedemissionisalmostsphericalinboththeradiomaps. get region; central coordinates: α = 05h43m35s.8,
J2000
The location of a previously characterized O9.5V star δ =+35◦20(cid:48)20(cid:48)(cid:48).4). Theextinctionmapwascreated
J2000
(BD +35o1201) appears at the peak of radio continuum by mean value of 40 nearest neighbours. The resultant
emission. The bulk of the ionized emission, as traced at A map of S235 complex is shown in Figure 2a, which
V
the sensitivity levels shown in the Figures 1a and 1b, is allows to trace several embedded subregions in the com-
well located inside the sphere-like shell, as highlighted plex. All these subregions are indicated in Figure 2a,
by a circle in Figure 1a. The radio emission at 610 MHz which are already highlighted in Figure 1b based on the
appears elongated in north direction, as highlighted by visual appearance of stellar contents. The A values
V
an arrow (see Figure 1b). This particular feature does vary between 2 and 12.5 mag with an average of about
not appear to be seen in the NVSS 1.4 GHz map. The 5.8 mag. The A map also resembles the sphere-like
V
WISE 12 µm band is dominated by the 11.3 µm poly- morphology as highlighted in Figure 1a, except in the
cyclic aromatic hydrocarbon (PAH) emission as well as north direction, where it seems to be broken (shown by
the warm dust continuum emission. The 24 µm image an arrow in Figure 2a).
shows the warm dust emission, whereas the 8 µm band Fifteen Bolocam dust continuum clumps at 1.1 mm
contains PAH emission features at 7.7 and 8.6 µm (in- were retrieved from Bolocam source catalog (v2.1) and
cluding the continuum). In Figure 1, we notice that the areshowninFigure2a. Figure2aillustratesacorrelation
warmdustemissionissurroundedbythe8µmemission. betweenextinctionanddustclumps. Mostoftheclumps
Furthermore, the correlation of the warm dust and ion- are located in the region having A greater than 8 mag.
V
ized emission is evident, which has generally been found
inHiiregions(e.g.Deharvengetal.2010;Paladinietal. 3.3. Clump properties
2012). In addition to the extended emission, the embed-
Dust continuum emission is known to trace the dense
ded stellar contents are visually seen around the sphere-
and cold regions. In Figure 2b, we show Bolocam dust
like shell. The different subregions (viz. East 1, East 2,
continuum emission at 1.1 mm, which is seen toward
North, North-West, Central W, Central E, South-West,
all the marked subregions in Figure 1b. It is impor-
and South) in the S235 complex are shown in Figure 1b.
tant to note that previously published SCUBA/JCMT
Note that three subregions (East 1, South-West, and
850 µm data (beam size ∼14(cid:48)(cid:48)) are not available toward
South)arelocatedawayfromtheedgesofthesphere-like
the “North” and “North-West” subregions of the S235
shell(seeFigure1). Thereisnoticeablenebularemission
complex (e.g. Klein et al. 2005; Francesco et al. 2008).
seen between East 1 subregion and the sphere-like shell,
Therefore, the 1.1 mm data (beam size ∼33(cid:48)(cid:48)) provide a
whichisdesignatedaseasternemissionwallinFigure1a.
morecompleteinformationofthedenseandcoldregions
3 WideField InfraredSurvey Explorer, whichis ajoint project oftheS235complex. Theidentifiedfifteenclumpsinthe
oftheUniversityofCaliforniaandtheJPL,Caltech,fundedbythe 1.1mmmaparealsopresentedinFigure2b. Theassoci-
NASA ationofclump(s)witheachsubregionislistedinTable1.
4 L. K. Dewangan et al.
√
Fivesubregions(i.e.,North-West,CentralW,CentralE, solidangleΩ (=π(FWHM/(206265× 4ln2))2 =
beam
East 1, and East 2) contain at least two clumps, while 2.9×10−8 forBolocambeamFWHM=33(cid:48)(cid:48))isinsteradi-
other two subregions (i.e., South-West and North) are ans, and N(H ) (= 2.0×1022 S (Jy) at T = 20 K; see
2 ν D
associated with a single clump. In order to derive gas Bally et al. 2010) is per square centimeter. The column
mass Mg for each clump at 1.1 mm emission, the follow- densities thus computed are listed in Table 1. Bolocam
ing formula has been utilized (Hildebrand 1983): clumps have column densities between 0.33 × 1022 and
13.5 × 1022 cm−2.
D2S R
M = ν t (1)
g B (T )κ
ν d ν
3.4. IRAC ratio and H maps
2
whereS istheintegrated1.1mmflux(Jy),Disthedis-
ν In order to trace molecular H emission in the com-
tance (kpc), R is the gas-to-dust mass ratio (assumed 2
t plex, we examined the Spitzer-IRAC ratio (4.5 µm/3.6
to be 100), B is the Planck function for a dust temper-
ν µm) map and the narrow H (2.12 µm) map. The
ature T , and κ is the dust absorption coefficient. In 2
d ν Spitzer-IRACratiomapshavebeenutilizedbymanyau-
the calculation, we use κ = 1.14cm2g−1 (e.g. Enoch et
ν thors to study the interaction between a massive star
al. 2008; Bally et al. 2010), D = 1.8 kpc, and T = 20 K
d and its surrounding interstellar medium (ISM) (Povich
(an average temperature value estimated from NH line
3 et al. 2007; Watson et al. 2008; Kumar Dewangan &
data; see Kirsanova et al. 2014, for more details). The
Anandarao 2010; Kumar & Anandarao 2010; Dewangan
massescomputedfromtheobserveddustcontinuumdata
& Anandarao 2011; Dewangan et al. 2012). IRAC 3.6
arelistedinTable1. Thetotalgasmassofthe15clumps
µm and 4.5 µm bands have almost identical point re-
comes out to be ∼1179 M . The clump masses vary be-
(cid:12) sponse function (PRF), therefore, one can directly take
tween 7 M and 285 M . All the clumps have masses
(cid:12) (cid:12) the ratio of 4.5 µm to 3.6 µm bands. In Figure 4, we
below 100 M except three clumps. These three clumps
(cid:12) display a ratio map of 4.5 µm/3.6 µm emission. The
are associated with Central E (clump IDs 6 and 9) and
spherical shell-like morphology is evident and the map
East 1 (clump ID 15) subregions and have masses more
depicts the edges of the shell. Additionally, several fea-
than 200 M (see Table 1). In an earlier work of the
(cid:12) tures are seen that are present toward the edges of the
S235 complex (excluding North and North-West subre-
shell and in the vicinity of a massive star. In general,
gions),Kleinetal.(2005)selectedthirteendustemission
the bright emission region in ratio 4.5 µm/3.6 µm map
clumps in the JCMT SCUBA 850 µm survey (beam size
indicates the excess 4.5 µm emission, while the remain-
∼14(cid:48)(cid:48); see Fig. 1 of IRAS 05377+3548 in Klein et al.
ing black or dark gray regions suggest the domination of
2005). They estimated a mass range of clumps between
3.6 µm emission. IRAC 4.5 µm band contains a promi-
10and120M ,usinganaveragedusttemperatureof20
(cid:12) nent molecular hydrogen line emission (ν = 0–0 S(9);
K, a gas-to-dust mass ratio of 150, and a distance of 1.8
4.693 µm), which can be excited by outflow shocks, and
kpc. Kirsanovaetal.(2014)alsocomputedamassrange
a hydrogen recombination line Brα (4.05 µm). IRAC
of dense gas in the clumps between 12 and 250 M (see
(cid:12) 3.6 µm band harbors PAH emission at 3.3 µm as well
Table6inKirsanovaetal.2014). Consideringtheclump
as a prominent molecular hydrogen feature at 3.234 µm
masses estimatedin thepresent work, onecan finda no-
(ν = 1–0 O(5)). In ratio map, we also notice a bright
ticeable difference in the clump mass at the high end of
region in the vicinity of the massive star that coincides
theestimatedrangecomparedtothepreviousworks. At
withwarmdustemissiontracedat24µmaswellaswith
the low end of the range, the gas mass is almost same in
the peak of the radio emission (see arrow in Figure 4).
the present calculations compared to the earlier works.
This bright emission region probably traces the Brα fea-
More extensive high sensitivity and high resolution ob-
ture originated by photoionized gas. In East 1, East 2,
servations at mm/sub-mm wavelengths would allow to
Central E, and North-West subregions, we found bright
further explore the clump mass at the high end of the
emissions that are marked in the ratio map. Note that
estimated mass range.
there are no radio continuum emission detected toward
In Figure 3, we show the distribution of clump masses
these subregions. Therefore, these emissions are proba-
as a function of distance from the location of the O9.5V
blyH featurestracingoutflowactivities(seeSection3.5
star. Interestingly, we notice that the clump masses in- 2
for molecular outflows). Figure 5 shows a continuum-
creaseaswemoveawayfromthelocationoftheionizing
subtracted 2.12 µm H (ν = 1−0 S(1)) image, revealing
star. Thiscorrelationisseenonlyforthoseclumpswhich 2
thepresenceofH emissioninthecomplex. Thecomplex
aredistributedneartheedgesofthesphere-likeshell(e.g. 2
containsdiffuseorfilamentary-likeH featuresaswellas
Central E, East 2, North, and North-West subregions). 2
extendedH emission. TheextendedH emissionisseen
Theimplicationofthiscorrelationforthestarformation 2 2
as polar structures (i.e., monopolar, bipolar, and multi-
process is discussed in Section 4.
polar; see East 1, East 2, and Central E subregions in
The column densities of clumps can also be computed
Figures 5 and 6). The bipolar emission structure is of-
using the Bolocam data. The column density per beam
ten interpreted as a bipolar outflow excited by a young
is given by (see Deharveng et al. 2010):
star. The multipolar structures could be originated due
S R to presence of bipolar outflows associated with multiple
N(H2)= κ B (T ) µν tm Ω , (2) young stars. The monopolar H2 structure could be ex-
ν ν d H2 H beam plainedbythe presenceof abipolar outflowexcited by a
In the equation above, S , R , κ , and B (T ) are de- young star, however its red-shifted component is hidden
ν t ν ν d
fined as in Equation 1, µ = 2.8 is the mean molecular fromviewduetoinclinationangle,extinction,andopac-
H2
weight per hydrogen molecule (Kauffmann et al. 2008), ity of the ISM around the young star. In Figure 6, we
the hydrogen atom mass m is in grams, and the beam presentH ,4.5µm,and24µmimagesofthreesubregions
H 2
Study of star formation activity in S235 5
(i.e., East 1, East 2, and Central-E), where polar struc- locity better than 12CO. Considering this fact, we pre-
turesaretracedintheH map. InEast1subregion,one ferred to show only 13CO channel maps of the complex.
2
multipolarstructureisclearlytraced. Additionally,some In the velocity range from −22.75 to −21.25 km s−1,
H knotsareseen. TheH mapconfirmsintenseoutflow Central W, East 2, and North-West subregions are well
2 2
or jet activities in this subregion. The extended 4.5 µm detected. In the velocity range from −21 to −19.75 km
emission is also detected similar to those seen in the H s−1, Central E, Central W, East 1, East 2, North, and
2
map. The prominent 4.5 µm emission was reported in North-West subregions are traced. In the velocity range
Paper I using the IRAC ratio map 4.5 µm/8.0 µm (see from−19.50to−18.0kms−1,CentralEandEast1sub-
Figure 11 in Paper I). This subregion contains a clus- regions are seen. The 13CO gas kinematics presented in
ter of embedded YSOs which makes difficult to pinpoint this work is consistent with the previous work of Kir-
the exact exciting source(s) of outflows. However, some sanova et al. (2008). They suggested the presence of
probable driving sources (i.e. YSOs) of outflows, that three molecular gas components in the complex, which
are taken from Paper I, are marked in the 4.5 µm and correspond to quiescent undisturbed primordial gas (i.e.
24µmimages. ThesehighlightedYSOsappearbrightin −18kms−1 <V <−15kms−1 (red)),gascompressed
lsr
the 24 µm image. In East 2 subregion, a bipolar struc- by the shock from expanding S235 Hii region (i.e. −21
ture is detected and is also seen in the 4.5 µm image km s−1 < V < −18 km s−1 (central)), and gas ex-
lsr
(also see Figure 12 in Paper I). In Paper I, the driving pulsion from the embedded young star clusters driven
sourceofthisoutflowwascharacterizedasaHMPOcan- by the combined effect of the cluster stars (i.e. −25 km
didate. The position of this YSO is also marked in the s−1 < V < −21 km s−1 (blue)). Furthermore, Kir-
lsr
4.5 µm and 24 µm images. The YSO is very bright in
sanovaetal.(2014)utilizedNH linedataandfoundthe
3
the 24 µm image. In Central-E subregion (see Figure 6),
densestmolecularclumpsassociatedwithEast1,East2,
wefindabipolarandtwomonopolarstructures(alsosee
and Central subregions, mostly belong to the “central”
source IDs 164 and 165 in Table B2 given in Navarete
molecular component.
et al. 2015). The probable exciting sources of one bipo- Figure 8 shows the 12CO and 13CO integrated gray
larstructure(Outflow-ce1)andonemonopolarstructure
scale maps, obtained by integrating over the velocity
(Jet-ce) are marked in the 4.5 µm and 24 µm images rangebetween−23and−18kms−1. Bolocamdustcon-
(see Figure 6). The 24 µm counterparts of these driv- tinuum emission contours are also overlaid on the 12CO
ing sources are also seen. However, the driving source
integrated map to illustrate the physical association of
of one monopolar structure (Outflow-ce2; Figure 6) is
dust emission with the complex. The integrated CO
not detected. In general, one can refer H nebulosity
2 mapstracearegionemptyofmoleculargasbetweentwo
as an outflow when it traces back to a source. In the
subregions, namely North and North-West. It is to be
present case, considering the morphology of H nebulos-
2 noted that the region empty of CO gas is exactly coinci-
ity (e.g. Takami et al. 2010), we refer this monopolar
dent with the broken part in the extinction map, where
structure as the outflow signature. It is generally ac-
the radio continuum emission at 610 MHz is elongated
cepted that the molecular outflows directly indicate the
(see Figures 1b and 2a). This region also appears to
presence of star formation processes. A comparison of
be associated with Brα emission which is surrounded by
the 2.12 µm H emission with the features detected in
2 H and/or PAH features (see Figure 4). It suggests a
2
the ratio map suggest that the black region in the ra-
close interaction between ionized and molecular gas in
tio map probably traces the H features (see Figures 4
2 the complex. It seems that the ionizing photons have
and5). However,thepresenceof3.3µmPAHfeaturecan
managed to escape in this direction. We propose that
notbeignored. Theprominentdiffuseorfilamentary-like
these features are best explained by the existence of a
H features traced in the H map could be explained by
2 2 cavity between North and North-West subregions (also
ultra-violet (UV) fluorescence. In summary, the IRAC
see Section 4.1).
ratio and H maps trace photodissociation regions (or
2 In Figure 9, we show position-velocity (p-v) analysis
photon-dominated regions, or PDRs) around the Hii re-
of 12CO and 13CO gas. The p-v plots (right ascension-
gion and star formation activities in the complex.
velocity and declination-velocity) of 12CO and 13CO gas
have revealed noticeable velocity gradients and show es-
3.5. CO molecular gas associated with the S235 complex sentially the same structure. However, 13CO data pro-
In the following, we present molecular line data anal- vide more information related to the velocity structure
ysis, which enables us to examine the ongoing physical of the denser regions compared to 12CO emission (see
processesandvelocitystructurespresentinthecomplex. declination-velocity plots in Figure 9). The p-v plots of
12COand13COgasrevealanalmostsemi-ring-likeorC-
3.5.1. 12CO and 13CO Distributions like structure at larger scale (about 8 parsec extended).
In Figure 7, we present the velocity channel maps of The detection of such morphology in the p-v plot is of-
the J = 1−0 line of 13CO (at intervals of 0.25 km s−1). ten interpreted as signature of an expanding shell (e.g.
The maps reveal the distribution of molecular gas com- Wilson et al. 2005; Arce et al. 2011; Dewangan et al.
ponents along the line of sight. The maps are shown for 2015). We find an expansion velocity of the gas to be
thevelocityrangeof−22.75kms−1 to−15kms−1. One ∼3 km s−1. Detailed discussion about this morphology
maynoticefromFigure7thatthedistributionofmolecu- can be found in Section 4.1. Note that the S235 com-
largasisclumpy,asalsopreviouslypointedoutbyHeyer plexhoststheO9.5Vtypestar, thereforethepresenceof
et al. (1996). In general, the 13CO line is more optically theC-likestructureinthecomplexcanbeinterpretedas
thin compared to 12CO line. Therefore, the 13CO line caused by the expanding Hii region. Similar result was
data can trace dense condensation and its associated ve- also observed in W42 star-forming region (see Figure 8
6 L. K. Dewangan et al.
in Dewangan et al. 2015). more details about P in the work of Lada et al.
TNT
(2003). The sound speed a (= (kT /µm )1/2) can be
s kin H
3.5.2. Molecular outflows estimatedwiththeknowledgeofgaskinetictemperature
In addition to the C-like structure in the p-v plots, we (Tkin) and µ=2.37 (approximately 70% H and 28% He
have also found noticeable velocity gradients toward the by mass). The non-thermal velocity dispersion is given
East 1, East 2, Central E, and North-West subregions, by:
suggesting the presence of outflow activities in each of (cid:115) (cid:114)
subregions. Following Paper I, we know that the East 1, σ = ∆V2 − kTkin = ∆V2 −σ2, (3)
East 2, and Central E subregions contain a cluster of NT 8ln2 17m 8ln2 T
H
YSOs. We have searched for outflows toward these sub-
regions using the doppler shifted velocity components where ∆V is the measured FWHM linewidth of the ob-
(red, green, and blue). Due to the coarse beam of CO servedNH3spectra,σT(=(kTkin/17mH)1/2)isthether-
data (beam size ∼46(cid:48)(cid:48)), we cannot pinpoint the exact mal broadening for NH3 at Tkin (e.g. Dunham et al.
exciting source of the outflows in each subregion. Con- 2011), and mH is the mass of hydrogen atom. The com-
sidering this limitation, we have not shown the doppler puted values of as, σNT, and PTNT are given in Table 2.
shifted components (red, green, and blue) toward the The value of PTNT is computed for each of the observed
East 2, Central E, and North-West subregions. In Fig- positions. In each subregion, the variation of PTNT as a
ure 10, we present the noticeable receding, approaching, functionofdistancefromthelocationofamassivestaris
and rest gas components only in East 1 subregion. We shown in Figure 11b. We find an average PTNT value of
have also marked the position of at least one probable 0.13,0.21,and0.22forEast1,East2,andCentralEsub-
driving source of the CO outflow (also see Section 3.4). regions, respectively. It suggests that non-thermal pres-
Highresolutionmolecularlineobservationsarenecessary sure is dominant in these densest subregions. The lower
to examine better insight into the molecular outflows in values of PTNT might be due to contributions from the
the complex. bipolaroutflows,asinvestigatedinprevioussection. The
average values of Mach numbers (σ /a ) estimated for
NT s
3.6. NH radial velocity and non-thermal velocity East1,East2,andCentralEsubregionsare2.9,2.3,and
3
dispersion 2.9,respectivelywhichindicatethatthesesubregionsare
supersonic.
As mentioned in the introduction, in general, it is
knownthatNH3 datatracethedensestregionsofmolec- 3.7. Feedback of a massive star
ularcloudinagivenstar-formingregion. Theproperties
In this section, we derive the various pressure com-
of densest gas traced by NH line observations have not
3 ponents (pressure of an Hii region (P ), radiation
yet been studied with respect to the ionizing star in the HII
pressure (P ), and stellar wind ram pressure (P ))
S235 complex. The locations of NH line observations rad wind
3 driven by a massive star to study its feedback in the
are marked in Figure 2b, which were mainly observed in
vicinity.
East 1, East 2, and Central E subregions (see Kirsanova
It is found that the massive star is approximately lo-
etal.2014,formoredetails). InFigure11a,weshowthe
cated at the center of the sphere-like shell. The radio
variationofNH (1,1)radialvelocityineachsubregionas
3 continuum images presented in this work can be used
afunctionofdistancefromthelocationoftheO9.5Vstar.
to estimate the number of Lyman continuum photons
The NH (1,1) radial velocities vary between −22 and
3 (N ). The expression of N is given by (Matsakis et
−18.5kms−1 inthesethreesubregions. Ingeneral,there uv uv
al. 1976):
are noticeable velocity variations within East 1 (−21.5
to −19.0 km s−1), East 2 (−21.5 to −20.5 km s−1), and (cid:18)S (cid:19)(cid:18) D (cid:19)2(cid:18) T (cid:19)−0.45 (cid:16) ν (cid:17)0.1
Central E (−22 to −18.5 km s−1) subregions. These ob- Nuv(s−1)=7.5×1046 Jνy kpc 104eK × GHz
servational characteristics are also evident in CO data
(4)
(see Figure 9). Balser et al. (2011) reported the veloc-
where S is the measured total flux density in Jy, D is
ity of ionized gas to be about −25.61(±0.12) km s−1 ν
the distance in kpc, T is the electron temperature, and
in the S235 Hii region using a hydrogen radio recombi- e
ν is the frequency in GHz. The calculation of N is
uv
nation line (H87-93α). Therefore, the molecular gas is
performed for both the radio frequencies (0.61 GHz and
red-shifted with respect to the ionized gas in the S235
1.4 GHz) separately. Substituting D = 1.8 kpc, T =
Hii region. Interestingly, we investigate that the radial e
10000 K, and S = 1.92 Jy, we compute N = 4.9 ×
1.4 uv
velocity observed in Central E subregion shows a linear 1047 s−1. Similarly, we find N = 8.2 × 1047 s−1 for D,
uv
trend as we move away from the location of a massive
T ,andS =3.49Jy. Theestimatesofionizingphoton
e 0.61
star, which is not seen in other two subregions. In each
fluxvaluesatdifferentfrequenciescorrespondtoasingle
subregion, wehavelineofsightvelocitycomponentsand
ionizingstarofO9.5Vspectraltype(Martinsetal.2005),
itseemsthattheradialvelocityofgasinCentralEsubre-
which is also in agreement with the previously reported
gion might be almost equal to the real velocity, however
spectral type of the ionizing source of the complex (e.g.
it is not the case for other two subregions. The observed
Georgelin et al. 1973).
trend could be interpreted as the direct influence of the
The different pressure components (P , P , and
expandingHiiregionexcitedbytheO9.5Vstar. Further HII rad
P ; as mentioned above) are defined as below (e.g.
wind
discussion on this result is presented in Section 4.
Bressert et al. 2012):
We computed thermal sound speed (a ), non-thermal
s (cid:32)(cid:115) (cid:33)
vneolno-ctihtyermdiaspleprrseisosnur(eσN(PTT)N, Tan=d ath2se/σrN2aTti)o. oOfntehecramnafilntdo PHII =µmHc2s 4π3αNuvD3 ; (5)
B s
Study of star formation activity in S235 7
P =L /4πcD2; (6) atedwithatypicalcoolmolecularcloud,weinferthatan
rad bol s
additional physical process has been acted to compress
Pwind =M˙wVw/4πDs2; (7) thesurroundingmoleculargastoenhancethepressurein
the complex. Consequently, this process may stimulate
In the equations above, N , µ, and m are defined
uv H the initial collapse and fragmentation of the extended
earlier, the radiative recombination coefficient is “α ”
B molecular cloud. The pressure calculations indicate that
(= 2.6 × 10−13 × (104 K/T )0.7 cm3 s−1; see Kwan
e the photoionized gas can be considered as the important
1997), c is the sound speed in the photo-ionized region
s contributorforthefeedbackmechanismintheS235com-
(=10kms−1), M˙ isthemass-lossrate, V isthewind
w w plex.
velocity of the ionizing source, L is the bolometric
bol
luminosity of the complex, and D is the projected
s 3.8. Young stellar objects in S235
distance from the location of the O9.5V type star to
the subregions where the pressure components are 3.8.1. Identification of young stellar objects
estimated. Onecaninferfromequations5, 6, and7that
A population of YSOs can be identified using NIR
the pressures, P and P , scale as D−2 while P
rad wind s HII and MIR color schemes as utilized by many authors
scales as D−3/2. (e.g. Allen et al. 2004; Lada et al. 2006; Gutermuth et
s
al. 2009). In Paper I, we identified YSOs using only
Note that the highlighted subregions are not located IRAC data. In the present work, we also employed
at the same projected distance from the location of a the UKIDSS-GPS NIR data, in combination with
massive star (see Figures 2 and 3). Therefore, we com- IRAC data for finding more deeply embedded and faint
pute the pressure components driven by a massive star sources. NotethattheUKIDSS-GPSNIRdataarethree
(i.e., P , P , and P ) at D = 3.4 pc (East 1), magnitudes deeper than 2MASS. Here, we describe the
HII rad wind s
3.0 pc (East 2), 1.5 pc (Central E), 2.6 pc (North), 2.7 procedure of YSOs identification and classification using
pc(North-West),1.9pc(CentralW),and3.0pc(South- photometric IRAC and GPS data.
West) (see Figures 2 and 3).
Evans&Blair(1981)estimatedatotalgasmassofthe 1. We selected sources having detections in all four
S235 cloud to be about 3000 M in a radius of 3 pc. Spitzer-IRACbands,whichweretakenfromPaperI.We
(cid:12)
Nordh et al. (1984) computed a total luminosity of S235 found131YSOs(52ClassIand79ClassII),4ClassIII,
of ∼8 × 104 L . We use M ≈ 3000 M , R ≈ 3.0 133 photospheres, and 38 contaminants in our selected
(cid:12) cloud (cid:12) c
pc, L ≈ 8 × 104 L , M˙ ≈ 1.58 × 10−9 M yr−1 region shown in Figure 1a. The details of YSOs clas-
bol (cid:12) w (cid:12)
(for O9.5V star; Marcolino et al. 2009), V ≈ 1500 km sification can be found in Paper I. Figure 12a displays
w
s−1 (for O9.5V star; Marcolino et al. 2009) in the above the different selected sources. In Figure 12a, we show
Class I YSOs (red circles), Class II YSOs (open blue
equations to compute the pressure contributions driven
triangles), Class III YSOs (black squares), photospheric
by a massive star on different subregions (see the list in
emissions (gray dots), and PAH-emission-contaminated
Table 3). The error associated with each pressure com-
apertures (magenta multiplication symbols).
ponentrelatedtoeachsubregionisalsogiveninTable3.
In the S235 complex, we find that the pressure of the
Hii region exceeds the radiation pressure and the stellar 2. Then, we considered sources lacking detections
in two longer wavelengths of IRAC bands (5.8 and
windpressure(seeTable3). Thetotalpressure(P =
total
8.0 µm). For such sources, GPS-IRAC (H, K, 3.6,
P + P + P ) driven by a massive star on each
HII rad wind
and 4.5 µm) classification scheme was utilized, as
subregion is also tabulated in Table 3, which is found to
be ∼10−10 dynes cm−2 for each subregion. described in details by Gutermuth et al. (2009). In this
method, the dereddened color-color space ([K−[3.6]]
Additionally,pressureexertedbytheself-gravityofthe 0
and [[3.6]−[4.5]] ) was used to identify infrared-excess
surrounding molecular gas is also estimated using the 0
sources. These dereddened colors were computed
relation (e.g. Harper-Clark & Murray 2009):
using the color excess ratios given in Flaherty et al.
P ≈πGΣ2 (8) (2007). This scheme also offered to identify possible dim
cloud
extragalactic contaminants from YSOs with additional
where Σ (=Mcloud/πRc2) is the mean mass surface den- conditions (i.e., [3.6]0 < 15 mag for Class I and [3.6]0
sityofthecloud,Mcloud isthemassofthemoleculargas, < 14.5 mag for Class II). We used the observed color
andRcistheradiusofthemolecularregion. Thevalueof and the reddening law (from Flaherty et al. 2007) to
Pcloud for the entire cloud is estimated to be ∼(1.0±0.5) compute the dereddened 3.6 µm magnitudes. In the
× 10−10 dynes cm−2. The values of Pcloud associated end, 16 Class I and 133 Class II YSOs are obtained
with different subregions are also found to be ∼0.1–3.0 using GPS-IRAC data (see Figure 12b).
×10−10 dynes cm−2 (see Table 1 and Figure 2a). In
subregions, we find Ptotal (cid:38) Pcloud (see Table 3). This 3. NIR color-color diagram (H−K vs J−H) is a very
argument is still valid if we put all the subregions at useful tool to select infrared-excess sources. We applied
the same projected distance (i.e. Ds = 3.0 pc; see pres- this scheme for sources, having detections in all the
sure calculations related to East 2 in Table 3). Addi- three JHK bands. Figure 12c shows a NIR color-color
tionally, the Pcloud is relatively higher than the pressure diagram of such sources. The reddening lines are drawn
associated with a typical cool molecular cloud (∼10−11 using the extinction law of Indebetouw et al. (2005).
– 10−12 dynes cm−2 for a temperature ∼20 K and the The color-color diagram is divided into three different
particledensity∼103–104 cm−3)(seeTable7.3ofDyson regions, namely “I”, “II”, and “III”. One can find more
&Williams1980). Consideringthepressurevalueassoci- details about NIR YSOs classification in Sugitani et
8 L. K. Dewangan et al.
al. (2002) and Dewangan et al. (2012). The sources ysis, such as empirical cumulative distribution (ECD) of
distributed in the “I” region represent the likely Class I YSOs as a function of NN distance, is often studied to
objects (protostellar objects). T Tauri-like sources infer the clustered YSO populations in star-forming re-
(Class II objects) are identified within the “II” region gions (see Chavarr´ıa et al. 2008; Gutermuth et al. 2009;
along the T Tauri locus with large NIR excess. The Dewangan & Anandarao 2011, for more details). In the
sources that fall between the reddening bands of the ECD analysis, a cutoff length (also referred as the dis-
main-sequence and giant stars are located in the “III” tance of inflection d ) is chosen, to delineate the low-
c
region. With this method, we obtain 5 Class I and 96 density/distributed populations. For the present case,
Class II sources (see Figure 12c). we selected a cutoff distance of d ∼52(cid:48)(cid:48) (0.45 pc at a
c
distance of 1.8 kpc), which separates the cluster mem-
4. Finally, we considered sources (significant in num- bers within the contour level of 10 YSOs/pc2. Such
ber) that have detections only in the H and K bands. cutoff distance has been determined for different star-
In order to further identify infrared excess sources from formingregions, suchastheDiamondRingregioninthe
such sample, we utilized a color-magnitude (H−K/K) Cygnus-X star-forming complex (d ∼61(cid:48)(cid:48) or 0.43 pc at
c
diagram (see Figure 12d). The diagram allows to select 1450 pc; Beerer et al. 2010), the Cygnus-OB2 region in
red sources having H−K > 1.04. The color-magnitude theCygnus-Xcomplex(d ∼72(cid:48)(cid:48) or0.51pc;Guarcelloet
c
analysis of the nearby control field allowed us to infer al. 2013), and the W5 star-forming region (d ∼33(cid:48)(cid:48) or
c
this color criterion. This condition provides us 54 0.32pcat2kpc;Chavarr´ıaetal.2014). Thecomparison
additional deeply embedded infrared excess sources. of d values suggests that W5 is more densely populated
c
with YSOs compared to Cygnus-X. In general, the value
Recently, Chavarr´ıa et al. (2014) also presented pho- of d offers to infer the most densely YSOs populated
c
tometry of the entire extended star-forming S235 region star-forming regions. The ECD analysis yielded a clus-
(including S235AB region) using NIR and Spitzer data. teredfractionofabout59%YSOs(i.e. 258fromatotalof
These authors obtained K-band observations using the 435 YSOs). The YSO clusters are mainly distributed in
2.1-m telescope located at Kitt Peak National Observa- East1,East2,CentralE,CentralW,North-West,South,
tory. Additionally, they observed J and H bands using and South-West subregions. In Central E and East 1
the 6.5-m Multiple Mirror Telescope (MMT) telescope subregions, we found the highest level of surface density
located at Fred Lawrence Whipple Observatory. They contours. North subregion is not associated with any
identified 690 YSOs in the entire S235 complex. How- YSO cluster; however, it contains a few Class II YSOs.
ever, they did not separate the contaminations in their TheclusteredandscatteredYSOlocationsareidentified
analysis. Additionally,theydidnotprovidethepositions in Figure 13 by filled and open symbols, respectively.
of their identified YSOs. Therefore, a comparison of our Wealsoconstructedseparatelythesurfacedensitycon-
selected YSOs to the sources identified by Chavarr´ıa et tour maps of Class I and Class II YSOs, using the same
al. (2014) cannot be thoroughly performed in this work algorithm as explained above. The surface density con-
(see Figure 13 in this work and Figure B2 in Chavarr´ıa tour maps of Class I and Class II YSOs are shown in
et al. (2014)). In our YSOs analysis, we have carefully Figure 14a. The surface density of Class I YSOs lies be-
separatedthecontaminationsfromYSOpopulations. As tween 0.04 and 42.75 YSOs/pc2 with a dispersion (σ)
a result of the four schemes explained above, a total of of 2.64 and is shown by contour levels of 5, 8, 10, and
435 YSOs are yielded in the complex (S235AB region is 20 YSOs/pc2. Similarly, the surface density of Class II
not included). The positions of all YSOs are shown in YSOs is found to lie between 0.17 and 174 YSOs/pc2,
Figure 13. with a dispersion of 6.62 and is drawn at contour levels
of 10, 20, and 40 YSOs/pc2. All the Class I and Class II
3.8.2. Spatial distribution of YSOs
YSO clusters are exclusively found in the regions with
In recent years, the surface density of young stellar A > 8 mag. The Class I YSO clusters are traced in
V
populations in star-forming regions has been adopted East 1, East 2, Central E, South, and South-West sub-
to identify and to study the young stellar clusters (e.g. regions (see Figure 14a).
Gutermuth et al. 2009; Bressert et al. 2010). In Paper I,
thesurfacedensitymapofYSOswascomputedusingthe
4. DISCUSSION
nearest-neighbour (NN) technique (also see Gutermuth
4.1. Expanding Hii region
et al. 2009; Bressert et al. 2010, for more details). Here,
we also followed the same procedure and generated the Oneoftheimportantresultsofthepresentworkisthe
surface density map of all the selected 435 YSOs. The identification of an almost semi-ring-like or C-like struc-
map was created using a 5(cid:48)(cid:48) grid and 6 NN at a distance tureinthep-vplots,whichissimilartotheoneobserved
of1.8kpc. Figure13showstheresultantsurfacedensity inOrionnebula(seeFigure10binWilsonetal.2005)and
contours of YSOs overlaid on the extinction map. The to that in W42 (see Figure 8 in Dewangan et al. 2015).
contourlevelsareshownat10,20,40,and70YSOs/pc2, Arce et al. (2011) also studied such observed structures
increasingfromtheoutertotheinnerregions. Thefigure in Perseus molecular cloud along with the modeling of
clearly illustrates the spatial correlation between YSOs expanding bubbles in a turbulent medium. These au-
surface density and extinction. thorssuggestedthatthesemi-ring-likeorC-likestructure
The surface density analysis allows to trace the indi- in the p-v plots is the signature of an expanding shell.
vidualgroupsorclustersofYSOs. Now,wewanttoiden- They also mentioned that the ring-like morphology can
tify the clustered populations from distributed sources. beseeninthep-vplotwhenthepowerfulionizingsource
However, there is no unique method to find a cutoff dis- is situated at the center of the region. Additionally, the
tancefortracingthesetwopopulations. Statisticalanal- NH radial velocity observed in Central E subregion ex-
3
Study of star formation activity in S235 9
hibitsalineartrendasonemovesawayfromthelocation traced toward these subregions in a velocity range −17
of a massive star. The analysis of pressure contributions km s−1 to −15 km s−1 (see Figure 7). In Figures 15a
driven by a massive star (i.e., P , P , and P ) and 15b, we present the distribution of molecular gas
HII rad wind
indicates that the P value is higher than the other toward the “S235 complex” and the “S235AB region”.
HII
pressurecomponents. Therefore,itseemsthattheC-like The CO map is integrated in the [-25,-15] km s−1 veloc-
structure in the p-v plots corresponds to the expanding ity range (see Figure 15a). In Figure 15b, the p-v plot
HiiregionexcitedbytheO9.5Vstaratthecenterofthe of13COgasrevealsanalmostbroadbridgefeature. The
sphere-like shell. Negative feedback of the O9.5V star p-v plot shows that two peaks (a red shifted and a blue
is also evident by the presence of a cavity (i.e. empty shifted; as mentioned above) are separated by lower in-
of molecular CO gas region) between North and North- tensity intermediated velocity emission. The presence of
West subregions. Previously, Evans & Blair (1981) also such broad bridge feature in the p-v plot might indicate
suggestedthattheS235molecularcloudisheatedbythe the signature of a collision between two clouds (e.g. Ha-
excitingstar. Alltheseobservedfeaturessuggesttheim- worth et al. 2015a,b). Therefore, there is a possibility
pact of ionizing photons on the low density regions in of the formation of YSO clusters associated with South
the complex, through which the ionized gas might have and South-West subregions by the interaction between
escaped. these two clouds. In the present work, our analysis is
mainlyfocusedontheS235complex,thereforetheresults
4.2. Morphology and signposts of star formation related to the S235AB region are not presented in this
work. Hence, a detailed investigation of the interaction
TheNIRextinctionmapoftheS235complexprovides
between these two clouds is beyond the scope of present
a more complete picture of the spatial distribution of
work. It is important to note that the broad bridge fea-
embedded subregions. There are eight subregions which
ture is not seen in the p-v maps of “S235 complex” (see
arewellcorrelatedwiththelocationsofthedustclumps,
Figure9),suggestingtheapplicabilityofcloud-cloudcol-
CO gas, and YSOs. Five of these subregions (i.e. Cen-
lision scenario is unlikely in the S235 complex. Hence,
tral E, East 2, North, North-West, and Central W) ap-
we rule out the cloud-cloud collision process within the
pear to be nearly regularly spaced along the sphere-like
S235 complex.
shell surrounding the ionized emission. Deharveng et al.
(2003) also reported similar configuration in Sh 104 re- 4.3. Triggered star formation
gion that was classified as a site of triggered star for-
InSection3.7,weshowedthatthetotalpressuredriven
mation. Furthermore, three subregions (i.e. East 1,
by a massive star is in equilibrium with the pressure ex-
South, and South-West) in the S235 complex are spa-
erted by the self-gravitating molecular cloud. In Sec-
tially away from the sphere-like shell. The presence of
tion 4.1, we discussed that the signature of the expand-
H2 and PAH emissions at the boundaries of the sphere- ing Hii region is evident. Additionally, we inferred that
like shell indicates the presence of PDR surrounding the
the densest subregions (i.e. East 1, East 2, and Cen-
ionized emission (see Figure 4). In the present work, we
tral E) traced by NH data are supersonic (see Sec-
3
investigated more number of YSOs using GPS, in addi-
tion 3.6). We also pointed out that the observed higher
tiontoIRACdata, comparedtoPaperI.TheYSOclus-
velocity dispersions could be due to the presence of out-
ters are found in all the subregions except North, where
flows in the subregions. Morgan et al. (2010) studied 44
only Class II YSOs are seen without any clustering. Ad-
bright-rimmedclouds(BRCs)usingNH dataandfound
3
ditionally, the clusters of Class I YSOs are associated
that the potentially triggered samples of BRCs are asso-
with Central E, East 1, East 2, South, and South-West
ciatedwithhighervelocitydispersionscomparedtonon-
subregions. Themoleculardataalsorevealedoutflowsig-
triggeredsources. Intheirsamples,non-triggeredsources
naturesintheCentralE,East1,East2,andNorth-West
were largely subsonic, whereas the triggered samples of
subregions. We find the probable outflow driving candi-
BRCs were supersonic. These results indicate that the
date (i.e. Class I YSO) in each subregion. All together,
S235complexcouldbeasiteoftriggeredstarformation.
the early stages of star formation activity are evident in
This interpretation is further supported by the observed
all the eight subregions.
configuration as traced in the extinction map, dust con-
Concerning the molecular gas distribution toward the
tinuum emission, molecular gas, ionized emission, and
SouthandSouth-Westsubregions(seeFigure7),wesug-
distribution of YSOs. Therefore, the on-going star for-
gest that these two subregions seem to be located at the
mation in the S235 complex may have been influenced
interfacebetweentheS235molecularcloud(−23kms−1 by the expanding Hii region.
<Vlsr <−18 km s−1) and the S235AB molecular cloud Theoretically, there are two main scenarios discussed
(−17 km s−1 <Vlsr <−15 km s−1). As previously also in the literature which explain the triggered star for-
reported by Evans & Blair (1981), the extended star- mation by the expansion of the Hii region (Elmegreen
forming region S235 is comprised of two velocity compo- 1998; Deharveng et al. 2005): “collect and collapse” (see
nents at −20 and −17 km s−1. The velocity information Elmegreen & Lada 1977; Whitworth et al. 1994; Dale
suggests that the molecular gas associated with S235AB et al. 2007) and radiation-driven implosion (RDI; see
region is red-shifted with respect to the S235 molecu- Bertoldi 1989; Lefloch & Lazareff 1994). In the “col-
lar cloud. In the integrated 12CO map, some molecu- lect and collapse” scenario, massive and dense shell of
lar gas is seen toward the South and South-West subre- cool neutral material can be accumulated around an ex-
gions(seeFigure8a),howeverthemoleculargasisabsent pandingHiiregion,andstarformationoccurswhenthis
there in the integrated 13CO map (see Figure 8b). Ad- material becomes gravitationally unstable. In the RDI
ditionally, A value is found significant there (see Fig- model, the expanding Hii region causes the instability
V
ure 2a). Furthermore, the molecular gas is noticeably and aids in the collapse of a pre-existing dense clump in
10 L. K. Dewangan et al.
themolecularcloud. Kirsanovaetal.(2014)utilizedNH etal.2009),respectively. Consideringtheseages,onecan
3
line data and suggested that the subregions (i.e. S235 notice that the dynamical age of the Hii region is com-
East 2 and S235 Central) were formed via triggering by parable to the ages of YSOs. These calculations suggest
a “collect-and-collapse” process. Furthermore, they sug- that the molecular materials have been fragmented into
gested that S235 East 1 region was formed as a result of several condensations around the sphere-like shell (also
aninteractionoftheshockfrontfromS235complexwith seeFigure13). Therefore,the“collectandcollapse”sce-
a pre-existing dense clump. nario seems to be applicable in the S235 complex, which
In the present work, Bolocam data analysis revealed could be responsible for the origin of Central E, East 2,
that the clump masses grow as one goes away from the North, North-West, and Central W subregions.
location of the ionizing star. This pattern is observed NotethatEast1subregionisfarawayfromthesphere-
only for those clumps which are located near the edges like shell and is the most massive clump in the com-
of the sphere-like shell (e.g. Central E, East 2, North, plex. Additionally, this subregion is separated by the
and North-West subregions). It seems that the material eastern emission wall from the sphere-like shell and is in
has been collected on the edges of the sphere-like shell pressure equilibrium and is associated with Class I and
by the expanding Hii region (also see Figure 2a), sug- Class II YSO clusters. This subregion is considered as
gesting the “collect and collapse” scenario applicable in the youngest star-forming site in the complex (see Kir-
thecomplex. Inordertocheckthe“collectandcollapse” sanovaetal.2014)andissupersonic. Therefore,itseems
process as triggering mechanism, we calculated the dy- that the YSO clusters in East 1 subregion could be orig-
namical age (t ) of the Hii region and the fragmenta- inated by compression of the pre-existing dense material
dyn
tion time scale (t ) following the equations given in by the expanding Hii region through the RDI process.
frag
Dyson & Williams (1980) and Whitworth et al. (1994),
respectively. Onecanfindmoredetailsofsimilaranalysis 5. SUMMARYANDCONCLUSIONS
in Dewangan et al. (2012). The condition for the “col- Inthispaper,wehavemadeanextensiveinvestigation
lect and collapse” process is t ≥ t . To find the of the S235 complex using the multi-wavelength data
dyn frag
values of ambient density (n =) for t ≥ t con- covering radio through NIR wavelengths. The results
0 dyn frag
dition, the diagram of t and t as a function of obtained in this work are based on the study of ion-
frag dyn
“n ” is shown in Figure 15c. The t is estimated for ized emission, molecular emission, cold dust emission,
0 frag
different“a ”valuesof0.2, 0.3, and0.4kms−1 (seeSec- embedded young populations, and various physical
s
tion 3.6 for “a ” estimation). Here, we use N = 4.9 × calculations. The important findings of this work are as
s uv
1047 s−1 (see Section 3.7) for the entire radio emission follows:
of the region (spatial extent ∼6.(cid:48)40 or 3.35 pc) and α • The most prominent structure in the S235 complex
B
= 2.6 × 10−13 cm3 s−1 at T = 10000 K. If the condi- is the sphere-like shell morphology, as traced at wave-
e
lengths longer than 2 µm.
tion “t ≥ t ” is satisfied then the values of “n ”
dyn frag 0
• The distribution of ionized emission observed in the
should be greater than 5750, 7700, and 9240 cm−3 for
GMRT 610 MHz and NVSS 1.4 GHz continuum maps
different “a ” values of 0.2, 0.3, and 0.4 km s−1, respec-
s is almost spherical. The ionizing photon flux values
tively (see Figure 15c). On the other hand, if n < 5750
0 estimated at both the frequencies correspond to a single
cm−3, then the condition “t ≥ t ” is not fulfilled.
dyn frag ionizing star of O9.5V spectral type. The location of
Consequently, the fragmentation of the molecular mate-
the counterpart of this ionizing star (i.e. BD +35o1201)
rials into clumps will not occur due to the “collect and
appears at the peak of radio continuum emissions as
collapse” process in the complex.
well as approximately at the center of the sphere-like
Using 13CO, NH , H CO, HCO+, and HCN line data,
3 2 shell.
a significant density variation (i.e. 1 × 103 – 2 × 104
• The NIR extinction map depicts eight subregions
cm−3) has been reported in the S235 complex (Evans &
(East 1, East 2, North, North-West, Central W, Cen-
Blair 1981; Kirsanova et al. 2014). Nordh et al. (1984)
tral E, South-West, and South; having A > 8 mag),
V
noted that the density structure of the S235 cloud is
and five of them seem to be located in an almost
non-homogeneous with high density clumps engulfed in
regularly spaced manner along the sphere-like shell
a medium with lower density. Kirsanova et al. (2014)
surrounding the ionized emission.
adopted the value of volume density of 7000 cm−3 to • Bolocam dust continuum emission at 1.1 mm is
compute the age of the S235 Hii region. In order to found toward all the eight subregions and provides
compare the previous results, following Kirsanova et al. more insight of the dense and cold regions of the S235
(2014), we also adopt the same value of volume density complex,whichislackinginthepublishedsub-mmmaps
(i.e. n0 = 7000 cm−3) in the present work. Substituting in the literature.
Nuv,n0,andanaverageradiusoftheHiiregionR≈1.67 • Bolocam clump masses increase as one moves away
pc, we find tdyn ∼1 Myr. Our estimated tdyn value is in fromthelocationoftheionizingstar. Thischaracteristic
agreementwiththeworkofKirsanovaetal.(2014). The is found only for those clumps which are located near
tfrag is computed to be about 0.85 Myr, 1.1 Myr, and the edges of the shell.
1.3 Myr, respectively, for a = 0.2, 0.3, and 0.4 km s−1. • The position-velocity analysis of 12CO and 13CO
s
WefindthatthedynamicalageoftheHiiregion(t ≈ emissions depicts an almost semi-ring like structure,
dyn
1 Myr) is comparable with the fragmentation time scale indicating the signature of an expanding Hii region.
(tfrag ≈ 0.85–1.3 Myr) for n0 = 7000 cm−3 and as = • The pressure calculations (PHII, Prad, and Pwind)
0.2–0.4 km s−1. In general, the average ages of Class I indicate that the photoionized gas associated with
andClassIIYSOsare∼0.44Myrand∼1–3Myr(Evans the S235 Hii region can be considered as the major
contributor for the feedback mechanism in the S235