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