Simulations of High Mass Star Formation in the Milky Way

Abstract

Massive star formation takes place in the dense cores of molecular clouds where the stars may be obscured at optical wavelengths. An excellent signpost of a massive young stellar object is the presence of an ultra-compact HII region (UCHii), which is a dense photo-ionised cocoon of gas surrounding the newly formed star. The aim of this project is to develop an assembly of numerical tools, caravela, that can simulate realistic data streams representing high-mass star forming regions in our Galaxy. The synthetic output consists in images and photometric point source catalogues, in the IRAS and Herschel wavebands. In an era when large observational surveys are increasingly important, this tool can produce simulated infrared point-source catalogues of high-mass star forming regions on a Galactic scale. The approach used is to construct a synthetic Galaxy of star-forming regions represented by SED templates. The star-forming regions are distributed randomly along a four spiral arm morphology, although a wide range of geometries can be used including rings and different numbers of spiral arms. The caravela code then observes the synthetic Galaxy to produce simulated images and point source catalogues with appropriate sensitivity and angular resolution. caravela was first used to model the simulated Galaxy by constraining the synthetic output to observations made by IRAS. This numerical tool will allow the user to infer physical properties of the Galactic population of high-mass star forming regions from such observations. Second, the selected model was again observed with caravela in Herschel mode. These are therefore predictive results for the future Herschel observations. A model with 4.0×104 compact proto-stars embedded in larger grey-body envelopes (with T = 40 K and linear size scale lIII = 5.0 × 106 AU) is the best-fit model to the IRAS observational data set studied. We found a level of contamination from low- and intermediate-mass objects of " 90%. The modelled data set resulting from the Herschel simulation resulted in the detection of approximately twice as many Herschel objects than IRAS, which is consistent, in a limited way, with the real observed companion clump fraction (CCF) of 0.90 ± 0.07 (Thompson et al., 2006) means that on average there were observed 2 sources per one IRAS source. Our caravela and the real observed CCF are therefore consistent. caravela was coupled with an independent diffuse emission model (Paladini et al., 2007) and the resulting analysis is presented as an interesting seed for the future.