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Register a new userProbing the physical conditions and star formation processes in the Galactic HII region S305
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We present multi-scale and multi-wavelength observations of the Galactic HII region S305, which is excited by massive O8.5V and O9.5V stars. Infrared images reveal an extended sphere-like shell (extension ~7.5 pc; at T_d = 17.5-27 K) enclosing the S305 HII region (size ~5.5 pc; age ~1.7 Myr). The extended structure observed in the Herschel temperature map indicates that the molecular environment of S305 is heated by the massive O-type stars. Regularly spaced molecular condensations and dust clumps are investigated toward the edges of the infrared shell, where the PAH and H_2 emission is also observed. The molecular line data show a signature of an expanding shell of molecular gas in S305. GMRT 610 and 1280 MHz continuum maps reveal overdensities of the ionized emission distributed around two O-type stars, which are surrounded by the horseshoe envelope (extension ~2.3 pc). A molecular gas deficient region/cavity is identified toward the center of the horseshoe envelope, which is well traced with PAH, H_2, molecular, and dust emission. The edges of the infrared shell are found to be located in the front of the horseshoe envelope. All these outcomes provide the observational evidence of the feedback of O-type stars in S305. Moreover, non-thermal radio emission is detected in S305 with an average spectral index alpha ~-0.45. The variations in alpha, ranging from -1.1 to 1.3, are explained due to soft synchrotron emission and either optically-thicker thermal emission at high frequencies or a suppression of the low-frequency emission by the Razin-Tsytovich effect.
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The expansion of HII regions can trigger the formation of stars. An overdensity of young stellar objects (YSOs) is observed at the edges of HII regions but the mechanisms that give rise to this phenomenon are not clearly identified. Moreover, it is difficult to establish a causal link between HII-region expansion and the star formation observed at the edges of these regions. A clear age gradient observed in the spatial distribution of young sources in the surrounding might be a strong argument in favor of triggering. We have observed the Galactic HII region RCW120 with herschel PACS and SPIRE photometers at 70, 100, 160, 250, 350 and 500$mu$m. We produced temperature and H$_2$ column density maps and use the getsources algorithm to detect compact sources and measure their fluxes at herschel wavelengths. We have complemented these fluxes with existing infrared data. Fitting their spectral energy distributions (SEDs) with a modified blackbody model, we derived their envelope dust temperature and envelope mass. We computed their bolometric luminosities and discuss their evolutionary stages. The herschel data, with their unique sampling of the far infrared domain, have allowed us to characterize the properties of compact sources observed towards RCW120 for the first time. We have also been able to determine the envelope temperature, envelope mass and evolutionary stage of these sources. Using these properties we have shown that the density of the condensations that host star formation is a key parameter of the star-formation history, irrespective of their projected distance to the ionizing stars.
The southern Galactic high mass star-forming region, G351.6-1.3, is a HII region-molecular cloud complex with a luminosity of 2.0 x 10^5 L_sun, located at a distance of 2.4 kpc. In this paper, we focus on the investigation of the associated HII region, embedded cluster and the interstellar medium in the vicinity of G351.6-1.3. We address the identification of exciting source(s) as well as the census of stellar populations. The ionised gas distribution has been mapped using the Giant Metrewave Radio Telescope (GMRT), India at three continuum frequencies: 1280, 610 and 325 MHz. The HII region shows an elongated morphology and the 1280 MHz map comprises six resolved high density regions encompassed by diffuse emission spanning 1.4 pc x 1.0 pc. The zero age main-sequence (ZAMS) spectral type of the brightest radio core is O7.5. We have carried out near-infrared observations in the JHKs bands using the SIRIUS instrument on the 1.4 m Infrared Survey Facility (IRSF) telescope. The near-infrared images reveal the presence of a cluster embedded in nebulous fan-shaped emission. The log-normal slope of the K-band luminosity function of the embedded cluster is found to be 0.27 +- 0.03 and the fraction of the near-infrared excess stars is estimated to be 43%. These indicate that the age of the cluster is consistent with 1 Myr. The champagne flow model from a flat, thin molecular cloud is used to explain the morphology of radio emission with respect to the millimetre cloud and infrared brightness.
Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms.
By means of different physical mechanisms, the expansion of HII regions can promote the formation of new stars of all masses. RCW 120 is a nearby Galactic HII region where triggered star formation occurs. This region is well-studied - there being a wealth of existing data - and is nearby. However, it is surrounded by dense regions for which far infrared data is essential to obtain an unbiased view of the star formation process and in particular to establish whether very young protostars are present. We attempt to identify all Young Stellar Objects (YSOs), especially those previously undetected at shorter wavelengths, to derive their physical properties and obtain insight into the star formation history in this region. We use Herschel-PACS and -SPIRE images to determine the distribution of YSOs observed in the field. We use a spectral energy distribution fitting tool to derive the YSOs physical properties. Herschel-PACS and -SPIRE images confirm the existence of a young source and allow us to determine its nature as a high-mass (8-10 MSun) Class 0 object (whose emission is dominated by a massive envelope) towards the massive condensation 1 observed at (sub)-millimeter wavelengths. This source was not detected at 24 micron and only barely seen in the MISPGAL 70 micron data. Several other red sources are detected at Herschel wavelengths and coincide with the peaks of the millimeter condensations. SED fitting results for the brightest Herschel sources indicate that, apart from the massive Class 0 that forms in condensation 1, young low mass stars are forming around RCW 120. The YSOs observed on the borders of RCW 120 are younger than its ionizing star, which has an age of about 2.5 Myr.
Massive clumps tend to fragment into clusters of cores and condensations, some of which form high-mass stars. In this work, we study the structure of massive clumps at different scales, analyze the fragmentation process, and investigate the possibility that star formation is triggered by nearby HII regions. We present a high angular resolution study of a sample of 8 massive proto-cluster clumps. Combining infrared data, we use few-arcsecond resolution radio- and millimeter interferometric data to study their fragmentation and evolution. Our sample is unique in the sense that all the clumps have neighboring HII regions. Taking advantage of that, we test triggered star formation using a novel method where we study the alignment of the centres of mass traced by dust emission at multiple scales. The eight massive clumps have masses ranging from 228 to 2279 $M_odot$. The brightest compact structures within infrared bright clumps are typically associated with embedded compact radio continuum sources. The smaller scale structures of $R_{rm eff}$ $sim$ 0.02 pc observed within each clump are mostly gravitationally bound and massive enough to form at least a B3-B0 type star. Many condensations have masses larger than 8 $M_odot$ at small scale of $R_{rm eff}$ $sim$ 0.02 pc. Although the clumps are mostly infrared quiet, the dynamical movements are active at clump scale ($sim$ 1 pc). We studied the spatial distribution of the gas conditions detected at different scales. For some sources we find hints of external triggering, whereas for others we find no significant pattern that indicates triggering is dynamically unimportant. This probably indicates that the different clumps go through different evolutionary paths. In this respect, studies with larger samples are highly desired.
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