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The G305 star-forming complex: Embedded Massive Star Formation Discovered by Herschel Hi-GAL

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 Added by Alessandro Faimali
 Publication date 2012
  fields Physics
and research's language is English




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We present a Herschel far-infrared study towards the rich massive star- forming complex G305, utilising PACS 70, 160 {mu}m and SPIRE 250, 350, and 500 {mu}m observations from the Hi-GAL survey of the Galactic plane. The focus of this study is to identify the embedded massive star-forming population within G305, by combining far-infrared data with radio continuum, H2O maser, methanol maser, MIPS, and Red MSX Source survey data available from previous studies. By applying a frequentist technique we are able to identify a sample of the most likely associations within our multi-wavelength dataset, that can then be identified from the derived properties obtained from fitted spectral energy distributions (SEDs). By SED modelling using both a simple modified blackbody and fitting to a comprehensive grid of model SEDs, some 16 candidate associations are identified as embedded massive star-forming regions. We derive a two-selection colour criterion from this sample of log(F70/F500)geq 1 and log(F160/F350)geq 1.6 to identify an additional 31 embedded massive star candidates with no associated star-formation tracers. Using this result we can build a picture of the present day star-formation of the complex, and by extrapolating an initial mass function, suggest a current population of approx 2 times 10^4 young stellar objects (YSOs) present, corresponding to a star formation rate (SFR) of 0.01-0.02 Modot yr^-1. Comparing this resolved star formation rate, to extragalactic star formation rate tracers (based on the Kennicutt-Schmidt relation), we find the star formation activity is underestimated by a factor of geq 2 in comparison to the SFR derived from the YSO population.



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We present 109-115 GHz (3 mm) wide-field spectral line observations of 12^CO, 13^CO and C^18O J=1-0 molecular emission and 5.5 and 8.8 GHz (6 and 3 cm) radio continuum emission towards the high-mass star forming complex known as G305. The morphology of G305 is dominated by a large evacuated cavity at the centre of the complex driven by clusters of O stars surrounded by molecular gas. Our goals are to determine the physical properties of the molecular environment and reveal the relationship between the molecular and ionised gas and star formation in G305. This is in an effort to characterise the star-forming environment and constrain the star formation history in an attempt to evaluate the impact of high-mass stars on the evolution of the G305 complex. Analysis of CO emission in G305 reveals 156 molecular clumps with the following physical characteristics. The 5.5 and 8.8GHz radio continuum emission reveals an extended low surface brightness ionised environment within which we identify 15 large-scale features with a further eight smaller sources projected within these features. By comparing to mid infrared emission and archival data, we identify nine HII regions, seven compact HII regions, one UC HII region, four extended regions. The total integrated flux of the radio continuum emission at 5.5 GHz is ~180 Jy corresponding to a Lyman continuum output of 2.4x10^50 photons s^-1. We compare the ionised and molecular environment with optically identified high-mass stars and ongoing star formation, identified from the literature. Analysis of this dataset reveals a star formation rate of 0.008--0.016 and efficiency of 7--12%, allows us to probe the star formation history of the region and discuss the impact of high-mass stars on the evolution of G305.
The Herschel survey of the Galactic Plane (Hi-GAL) provides a unique opportunity to study star formation over large areas of the sky and different environments in the Milky Way. We use the best studied Hi-GAL fields to date, two 2x2 tiles centered on (l, b) = (30, 0) deg and (l, b) = (59, 0) deg, to study the star formation activity using a large sample of well selected young stellar objects (YSOs). We estimate the star formation rate (SFR) for these fields using the number of candidate YSOs and their average time scale to reach the Zero Age Main Sequence, and compare it with the rate estimated using their integrated luminosity at 70 micron combined with an extragalactic star formation indicator. We measure a SFR of (9.5 +- 4.3)*10^{-4} Msol/yr and (1.6 +- 0.7)*10^{-4} Msol/yr with the source counting method, in l=30 deg and l=59 deg, respectively. Results with the 70 micron estimator are (2.4 +- 0.4)*10^{-4} Msol/yr and (2.6 +- 1.1)*10^{-6} Msol/yr. Since the 70 micron indicator is derived from averaging extragalactic star forming complexes, we perform an extrapolation of these values to the whole Milky Way and obtain SFR_{MW} = (0.71 +- 0.13) Msol/yr from l = 30 deg and SFR_{MW} = (0.10 +- 0.04) Msol/yr from l=59 deg. The estimates in l=30 deg are in agreement with the most recent results on the Galactic star formation activity, indicating that the characteristics of this field are likely close to those of the star-formation dominated galaxies used for its derivation. Since the sky coverage is limited, this analysis will improve when the full Hi-GAL survey will be available.
171 - M.T. Beltran 2013
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We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key-project that will map the inner Galactic Plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2{deg} x 2{deg} tiles approximately centered at l=30{deg} and l=59{deg}. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around A_V of about 1 is exceeded for the regions in the l=59{deg} field; a A_V value between 5 and 10 is found for the l=30{deg} field, likely due to the relatively larger distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm-2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.
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