ATLASGAL - properties of compact H II regions and their natal clumps

We present a complete sample of molecular clumps containing compact and ultracompact H?ii (UC H?ii) regions between ? = 10° and 60° and |b| < 1°, identified by combining the APEX Telescope Large Area Survey of the Galaxy submm and CORNISH radio continuum surveys with visual examination of archival infrared data. Our sample is complete to optically thin, compact and UC H?ii regions driven by a zero-age main-sequence star of spectral type B0 or earlier embedded within a 1000?M? clump. In total we identify 213 compact and UC H?ii 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 H?ii 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 UC H?ii 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 UC H?ii-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.