ATLASGAL --- properties of compact HII regions and their natal clumps

TL;DR

This study compiles a comprehensive catalog of molecular clumps with compact and ultra-compact HII regions, analyzing their properties, distribution, and relation to massive star formation, revealing key thresholds and correlations in star-forming environments.

Contribution

It provides the first complete sample of such clumps in a specific Galactic region, deriving their physical properties and comparing them with other star formation tracers, highlighting the clump characteristics linked to massive star formation.

Findings

Surface density threshold for massive star formation is 0.05 g/cm^2.

Mass of embedded stars correlates with natal clump mass.

Clump properties are similar across different star formation tracers.

Abstract

We present a complete sample of molecular clumps containing compact and ultra-compact (UC) HII regions between \ell=10\degr and 60\degr\ and $|b|<1\degr, identified by combining the the ATLASGAL submm and CORNISH radio continuum surveys with visual examination of archival infrared data. Our sample is complete to optically thin, compact and UCHII regions driven by a zero age main sequence star of spectral type B0 or earlier embedded within a 1,000 Msun clump. In total we identify 213 compact and UCHII regions, associated with 170 clumps. Unambiguous kinematic distances are derived for these clumps and used to estimate their masses and physical sizes, as well as the Lyman continuum fluxes and sizes of their embedded HII regions. We find a clear lower envelope for the surface density of molecular clumps hosting massive star formation of 0.05 g cm^{-2}, which is consistent with a similar sample of clumps associated with 6.7 GHz masers. The mass of the most massive embedded stars is closely correlated with the mass of their natal clump. Young B stars appear to be significantly more luminous in the ultraviolet than predicted by current stellar atmosphere models. The properties of clumps associated with compact and UCHII regions are very similar to those associated with 6.7 GHz methanol masers and we speculate that there is little evolution in the structure of the molecular clumps between these two phases. Finally, we identify a significant peak in the surface density of compact and UCHII regions associated with the W49A star-forming complex, noting that this complex is truly one of the most massive and intense regions of star formation in the Galaxy.