Abstract
Abstract Massive star formation occurs in the interior of giant molecular clouds and proceeds through many stages. In this work, we focus on massive young stellar objects (MYSOs) and ultracompact H ii regions (UCH ii), where the former are enshrouded in dense envelopes of dust and gas, the latter of which has begun dispersing. By selecting a complete sample of MYSOs and UCH ii from the Red MSX Source (RMS) survey database, we combine Planck and IRAS data and build their spectral energy distributions. With these, we estimate the physical properties (dust temperatures, mass, luminosity) of the sample. Because the RMS database provides unique solar distances, it also allows the instantaneous star formation efficiency (SFE) to be investigated as a function of Galactocentric radius. We find that the SFE increases between 2 and 4.5 kpc, where it reaches a peak, likely in correspondence with the accumulation of molecular material at the end of the Galactic bar. It then stays approximately constant up to 9 kpc, after which it linearly declines, in agreement with predictions from extragalactic studies. This behavior suggests the presence of a significant amount of undetected molecular gas at R G > 8 kpc. Finally, we present diagnostic colors that can be used to identify sites of massive star formation.
Highlights
The initial stages of a massive star can be traced back to a giant molecular cloud (GMC), i.e., a cloud with mass ranging from 105–106 Me (e.g., Dame et al 2001; Miville-Deschenes et al 2017) and a linear size up to hundreds of parsecs
Planck whole-sky submillimeter maps provide an unprecedented opportunity to carry out an unbiased study of the environmental conditions in which massive star formation takes place
Using Planck High Frequency Instrument (HFI) upper-frequency bands (350, 500, and 850 μm) complemented by IRAS/IRIS 100, 60 and 25 μm bands, we have estimated dust temperatures, luminosities, masses, and surface densities of clumps associated with a complete sample of candidate massive young stellar objects (MYSOs) and ultracompact H II regions (UCH II) regions selected from the Red MSX Source (RMS) survey database
Summary
The initial stages of a massive star can be traced back to a giant molecular cloud (GMC), i.e., a cloud with mass ranging from 105–106 Me (e.g., Dame et al 2001; Miville-Deschenes et al 2017) and a linear size up to hundreds of parsecs. Within GMCs, very dense molecular cores (n > 105 cm−3; e.g., Giannetti et al 2013) collapse and evolve first into massive young stellar objects (MYSOs), and later, when the OB star begins to ionize the surrounding material, into ultracompact H II regions (UCH II) In this framework, characterizing clumps hosting massive star formation, an intermediate stage between GMCs and cores, is of primary importance: massive star formation is known to take place in cold (Td < 25 K), massive (M > 100 Me), luminous (L > 103 Le) environments, so constraining dust temperatures, luminosities, and masses of the clumps allows one to assess whether the clump under investigation is able to effectively form massive stars. The Appendix contains an estimate of the colors that can be used as diagnostics to identify regions of massive star formation
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