  We carried out 13CO J=1-0, CS, and C34S J=2-1 and J=3-2 line observations of
molecular clouds associated with 16 ultracompact (UC) HII regions with extended
envelopes. The molecular clouds are the ones that give birth to rich stellar
clusters and/or very massive (O7-O4) stars. Our data show that the clouds are
very clumpy and of irregular morphology. They usually have much larger masses,
velocity dispersions, and fractions of dense gas than molecular clouds that
form early B or late O stars. This is compatible with earlier findings that
more massive stars form in more massive cores. 13CO cores are in general
associated with compact HII regions regardless of the presence of UC HII
regions therein. In contrast, CS cores are preferentially associated with
compact HII regions that contain UC HII regions. As with the fact that the
compact HII regions containing UC HII regions are more compact than those not
associated with UC HII regions, these indicate that the former may be in an
earlier evolutionary phase than the latter. The diffuse extended envelopes of
HII regions often develop in the direction of decreasing molecular gas density.
Based on detailed comparison of molecular line data with radio continuum and
recombination line data, the extended ionized envelopes are likely the results
of champagne flows in at least 10 sources in our sample. Together these results
appear to support a published suggestion that the extended emission around UC
HII regions can be naturally understood by combining the champagne flow model
with the hierarchical structure of molecular clouds. We discuss the implication
of our results for the blister model of HII regions.
