Observational signatures of cloud-cloud collision in the extended star-forming region S235

We present a multi-wavelength data analysis of the extended star-forming region S235 (hereafter E-S235), where two molecular clouds are present. In E-S235, using the 12CO (1-0) and 13CO (1-0) line data, a molecular cloud linked with the site "S235main" is traced in a velocity range [-24, -18] km s^{-1}, while the other one containing the sites S235A, S235B, and S235C (hereafter "S235ABC") is depicted in a velocity range [-18, -13] km s^{-1}. In the velocity space, these two clouds are separated by ~4 km s^{-1}, and are interconnected by a lower intensity intermediate velocity emission, tracing a broad bridge feature. In the velocity channel maps, a possible complementary molecular pair at [-21, -20] km s^{-1} and [-16, -15] km s^{-1} is also evident. The sites, "S235ABC", East 1, and South-West are spatially seen in the interface of two clouds. Together, these observed features are consistent with the predictions of numerical models of the cloud-cloud collision (CCC) process, favoring the onset of the CCC in E-S235 about 0.5 Myr ago. Deep UKIDSS near-infrared photometric analysis of point-like sources reveals significant clustering of young stellar populations toward the sites located at the junction, and the "S235main". The sites, "S235ABC" harbor young compact H II regions having dynamical ages of ~0.06-0.22 Myr, and these sites (including South-West and East 1) also contain dust clumps (having M_clump ~40 to 635 M_sun). Our observational findings suggest that the star formation activities (including massive stars) appear to be influenced by the CCC mechanism at the junction.Comment: 16 pages, 12 figures, Accepted for publication in The Astrophysical Journa

Spitzer IRAC imaging photometric study of the massive star forming region AFGL 437

We present {\it Spitzer} IRAC mid-infrared photometry on the massive star forming region AFGL 437 (IRAS 03035+5819). From the IRAC colour-colour diagram, we identify several new embedded YSOs within 64 arcsec of the central compact cluster. Using the IRAC ratio images, we investigate the molecular outflows associated with the highly embedded young stellar object WK34 in the central cluster. We attribute the lobes seen (extended to $\sim$ 0.16 pc in the north) in the ratio map to shocked molecular hydrogen emission. IRAC images reveal a large diffuse nebulosity associated with the central cluster and extending up to $\sim$ 8.0 pc from south-west to north-east direction with its brightness gradually increasing from 3.6 to 8.0 $\mu$m. A dense box-car-shaped nebula (more than 2.0 pc in size) situated to the south-west of the cluster shows molecular hydrogen emission that may have been caused by shock waves from the compact cluster sources. It seems that these sources are also responsible for the infrared-bright nebulosity. Using a 2D radiative transfer model, we derive from the spectral energy distributions, the mass, age and luminosity of all the YSOs identified within the central cluster. The SED modelling shows that the driving engine of the outflows, WK34, appears to be massive but very young and deeply embedded. The weighted mean values of the masses and ages of the 21 YSOs derived from the model are in the range 1-10 \msun and 10$^{4.1 - 6.4}$ yr respectively; while their luminosities are in the range of 10$^{0.47-3.48}$ L$_{\odot}$.Comment: 11 pages, 6 figures, 2 tables; Accepted in MNRAS Main Journal, 2009; The article is proof-corrected version; also some numerical (sizes in arcsec/pc) corrections have been incorporated. The essential content and conlusions remain unchange

A study of the massive star forming region M8 using Spitzer IRAC images

Spitzer IRAC images (3.6, 4.5, 5.8 and 8.0$\mu$m) and photometry of the star forming region M8 are presented. IRAC photometry reveals ongoing star formation in the M8 complex, with 64 Class 0/I and 168 Class II sources identified in several locations in the vicinity of sub-mm gas cores/clumps. Nearly 60% of these YSOs occur in about 7 small clusters. The spatial surface density of the clustered YSOs is determined to be about 10-20 YSOs/pc$^{2}$. Fresh star formation by the process of "collect and collapse" might have been triggered by the expanding HII regions and winds from massive stars. IRAC ratio images are generated and studied in order to identify possible diagnostic emission regions in M8. The image of 4.5$\mu$m/8.0$\mu$m reveals Br$\alpha$ counterpart of the optical Hourglass HII region, while the ratio 8.0$\mu$m/4.5$\mu$m indicates PAH emission in a cavity-like structure to the east of the Hourglass. The ratio maps of 3.6$\mu$m/4.5$\mu$m, 5.8$\mu$m/4.5$\mu$m and 8.0$\mu$m/4.5$\mu$m seem to identify PAH emission regions in the sharp ridges and filamentary structures seen East to West and NE to SW in M8 complex.Comment: 16 pages, 2 tables, 6 figures; Accepted for publication in MNRAS, May 201

New insights in the mid-infrared bubble N49 site: a clue of collision of filamentary molecular clouds

We investigate the star formation processes operating in a mid-infrared bubble N49 site, which harbors an O-type star in its interior, an ultracompact HII region, and a 6.7 GHz methanol maser at its edges. The 13CO line data reveal two velocity components (at velocity peaks ~88 and ~95 km/sec) in the direction of the bubble. An elongated filamentary feature (length >15 pc) is investigated in each molecular cloud component, and the bubble is found at the interface of these two filamentary molecular clouds. The Herschel temperature map traces all these structures in a temperature range of ~16-24 K. In the velocity space of 13CO, the two molecular clouds are separated by ~7 km/sec, and are interconnected by a lower intensity intermediate velocity emission (i.e. a broad bridge feature). A possible complementary molecular pair at [87, 88] km/sec and [95, 96] km/sec is also observed in the velocity channel maps. These observational signatures are in agreement with the outcomes of simulations of the cloud-cloud collision process. There are also noticeable embedded protostars and Herschel clumps distributed toward the filamentary features including the intersection zone of the two molecular clouds. In the bubble site, different early evolutionary stages of massive star formation are also present. Together, these observational results suggest that in the bubble N49 site, the collision of the filamentary molecular clouds appears to be operated about 0.7 Myr ago, and may have triggered the formation of embedded protostars and massive stars.Comment: 19 pages, 11 figures, 2 tables. Accepted for publication in The Astrophysical Journa

The embedded ring-like feature and star formation activities in G35.673-00.847

We present a multi-wavelength study to probe the star formation (SF) process in the molecular cloud linked with the G35.673-00.847 site (hereafter MCG35.6), which is traced in a velocity range of 53-62 km/s. Multi-wavelength images reveal a semi-ring-like feature (associated with ionized gas emission) and an embedded face-on ring-like feature (without the NVSS 1.4 GHz radio emission; where 1-sigma ~ 0.45 mJy/beam) in the MCG35.6. The semi-ring-like feature is originated by the ionizing feedback from a star with spectral type B0.5V-B0V. The central region of the ring-like feature does not contain detectable ionized gas emission, indicating that the ring-like feature is unlikely to be produced by the ionizing feedback from a massive star. Several embedded Herschel clumps and young stellar objects (YSOs) are identified in the MCG35.6, tracing the ongoing SF activities within the cloud. The polarization information from the Planck and GPIPS data trace the plane-of-sky magnetic field, which is oriented parallel to the major axis of the ring-like feature. At least five clumps (having M_clump ~ 740 - 1420 M_sun) seem to be distributed in an almost regularly spaced manner along the ring-like feature and contain noticeable YSOs. Based on the analysis of the polarization and molecular line data, three subregions containing the clumps are found to be magnetically supercritical in the ring-like feature. Altogether, the existence of the ring-like feature and the SF activities on its edges can be explained by the magnetic field mediated process as simulated by Li & Nakamura (2002).Comment: 26 pages, 12 figures, 5 tables. Accepted for publication in The Astrophysical Journa

Uncovering distinct environments in an extended physical system around the W33 complex

We present a multi-wavelength investigation of a large-scale physical system containing the W33 complex. The extended system (~50 pc x 37 pc) is selected based on the distribution of molecular gas at [29.6, 60.2] km/s and of 88 ATLASGAL 870 micron dust clumps at d ~2.6 kpc. The extended system/molecular cloud traced in the maps of 13CO and C18O emission contains several HII regions excited by OB stars (age ~0.3-1.0 Myr) and a thermally supercritical filament ("fs1", length ~17 pc). The filament, devoid of the ionized gas, shows dust temperature (T_d) of ~19 K, while the HII regions are depicted with T_d of ~21-29 K. It suggests the existence of two distinct environments in the cloud. The distribution of Class I young stellar objects (mean age ~0.44 Myr) traces the early stage of star formation (SF) toward the cloud. At least three velocity components (around 35, 45, and 53 km/s) are investigated toward the system. The analysis of 13CO and C18O reveals the spatial and velocity connections of cloud components around 35 and 53 km/s. The observed positions of previously known sources, W33 Main, W33 A and O4-7I stars, are found toward a complementary distribution of these two cloud components. The filament "fs1" and a previously known object W33 B are seen toward the overlapping areas of the clouds, where ongoing SF activity is evident. A scenario concerning the converging/colliding flows from two different velocity components appears to explain well the observed signposts of SF activities in the system.Comment: 18 Pages, 10 Figures, 2 Tables, Accepted for publication in MNRA

New evidences in IRDC G333.73+0.37: colliding filamentary clouds, hub-filament system, and embedded cores

To unravel the star formation process, we present a multi-scale and multi-wavelength study of the filamentary infrared dark cloud (IRDC) G333.73+0.37, which hosts previously known two HII regions located at its center. Each HII region is associated with a mid-infrared source, and is excited by a massive OB star. Two filamentary structures and a hub-filament system (HFS) associated with one HII region are investigated in absorption using the Spitzer 8.0 $\mu$m image. The $^{13}$CO(J = 2-1) and C$^{18}$O(J = 2-1) line data reveal two velocity components (around $-$35.5 and $-$33.5 km s$^{-1}$) toward the IRDC, favouring the presence of two filamentary clouds at different velocities. Nonthermal (or turbulent) motions are depicted in the IRDC using the C$^{18}$O line data. The spatial distribution of young stellar objects (YSOs) identified using the VVV near-infrared data traces star formation activities in the IRDC. Low-mass cores are identified toward both the HII regions using the ALMA 1.38 mm continuum map. The VLT/NACO adaptive-optics L$^{\prime}$-band images show the presence of at least three point-like sources and the absence of small-scale features in the inner 4000 AU around YSOs NIR31 and MIR 16 located toward the HII regions. The HII regions and groups of YSO are observed toward the central part of the IRDC, where the two filamentary clouds intersect. A scenario of cloud-cloud collision or converging flows in the IRDC seems to be applicable, which may explain star formation activities including HFS and massive stars.Comment: 19 pages, 12 figures; Accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS) Journa

Cluster-forming site AFGL 5157: colliding filamentary clouds and star formation

We observationally investigate star formation (SF) process occurring in AFGL 5157 (area ~13.5 pc X 13.5 pc) using a multi-wavelength approach. Embedded filaments are seen in the {\it Herschel} column density map, and one of them is identified as an elongated filamentary feature (FF) (length ~8.3 pc; mass ~1170 Msun). Five Herschel clumps (Mclump ~45-300 Msun) are traced in the central part of FF, where an extended temperature structure (Td ~13.5-26.5 K) is observed. In the direction of the central part of FF, the warmer region at Td ~20-26.5 K spatially coincides with a mid-infrared (MIR) shell surrounding a previously known evolved infrared cluster. Diffuse H-alpha emission is traced inside the infrared shell, suggesting the presence of massive stars in the evolved cluster. Based on the surface density analysis of young stellar objects (YSOs), embedded clusters of YSOs are traced toward the central part of FF, and are distributed around the infrared shell. Previously detected H2O masers, H2 knots, massive protostar candidates, and HII region are also seen toward the embedded clusters. Using the 12CO and 13CO line data, the central part of FF is observed at the overlapping zones of two filamentary molecular clouds (length ~12.5 pc) around -20 and -17 km/s, which are also connected in velocity. Our observational results suggest that the formation of massive stars appears to be triggered by a collision of two filamentary molecular clouds, which might have also influenced the birth of YSOs in AFGL 5157.Comment: 21 pages, 14 figures, Accepted for publication in The Astrophysical Journa

Cloud-Cloud Collision Induced Star Formation in IRAS 18223-1243

In the direction of l = 17.6 - 19 deg, the star-forming sites Sh 2-53 and IRAS 18223-1243 are prominently observed, and seem to be physically detached from each other. Sh 2-53 has been investigated at the junction of the molecular filaments, while a larger-scale environment of IRAS 18223-1243 remains unexplored. The goal of this paper is to investigate the star formation processes in the IRAS site (area ~0.4 deg x 0.4 deg). Based on the GRS 13CO line data, two molecular clouds, peaking at velocities of 45 and 51 km/s, are found. In the position-velocity plots, a relatively weak 13CO emission is detected at intermediate velocities (i.e. 47.5 - 49.5 km/s) between these two clouds, illustrating a link between two parallel elongated velocity structures. These clouds are physically connected in both space and velocity. The MAGPIS data at 20 cm trace free-free continuum emission toward the IRAS 18223-1243 source. Using the Spitzer and UKIDSS photometric data, we have identified infrared-excess young stellar objects (YSOs), and have observed their groups toward the intersection zones of the clouds. IRAS 18223-1243 is also spatially seen at an interface of the clouds. Considering these observational findings, we propose the onset of the collision of two clouds in the IRAS site about 1 Myr ago, which triggered the birth of massive star(s) and the YSO groups. A non-uniform distribution of the GPIPS H-band starlight mean polarization angles is also observed toward the colliding interfaces, indicating the impact of the collision on the magnetic field morphology.Comment: 18 pages, 11 figures, 2 tables, Accepted for publication in The Astrophysical Journa

Embedded filaments in IRAS 05463+2652: early stage of fragmentation and star formation activities

We present a multi-wavelength data analysis of IRAS 05463+2652 (hereafter I05463+2652) to study star formation mechanisms. A shell-like structure around I05463+2652 is evident in the Herschel column density map, which is not associated with any ionized emission. Based on the Herschel sub-millimeter images, several parsec-scale filaments (including two elongated filaments, "s-fl" and "nw-fl" having lengths of ~6.4 pc and ~8.8 pc, respectively) are investigated in I05463+2652 site. Herschel temperature map depicts all these features in a temperature range of ~11-13 K. 39 clumps are identified and have masses between ~70-945 M$_\odot$. A majority of clumps (having M_clump >= 300 M$_\odot$) are distributed toward the shell-like structure. 175 young stellar objects (YSOs) are selected using the photometric 1-5 microns data and a majority of these YSOs are distributed toward the four areas of high column density >= 5 x 10^{21} cm^{-2}; A_V ~5.3 mag) in the shell-like structure, where massive clumps and a spatial association with filament(s) are also observed. The knowledge of observed masses per unit length of elongated filaments and critical mass length reveals that they are supercritical. The filament "nw-fl" is fragmented into five clumps (having M_clump ~100-545 M$_\odot$) and contains noticeable YSOs, while the other filament "s-fl" is fragmented into two clumps (having M_clump ~170-215 M$_\odot$) without YSOs. Together, these observational results favor the role of filaments in star formation process in I05480+2545. This study also reveals the filament "s-fl", containing two starless clumps, at an early stage of fragmentation.Comment: 13 pages, 6 figures, 1 table, Accepted for publication in The Astrophysical Journa