Molecular Counterparts of Ultracompact HiiRegions with Extended Envelopes

We carried out 13CO J=1-0, CS, and C34S J=2-1 and J=3-2 line observations ofmolecular clouds associated with 16 ultracompact (UC) HII regions with extendedenvelopes. The molecular clouds are the ones that give birth to rich stellarclusters and/or very massive (O7-O4) stars. Our data show that the clouds arevery clumpy and of irregular morphology. They usually have much larger masses,velocity dispersions, and fractions of dense gas than molecular clouds thatform early B or late O stars. This is compatible with earlier findings thatmore massive stars form in more massive cores. 13CO cores are in generalassociated with compact HII regions regardless of the presence of UC HIIregions therein. In contrast, CS cores are preferentially associated withcompact HII regions that contain UC HII regions. As with the fact that thecompact HII regions containing UC HII regions are more compact than those notassociated with UC HII regions, these indicate that the former may be in anearlier evolutionary phase than the latter. The diffuse extended envelopes ofHII regions often develop in the direction of decreasing molecular gas density.Based on detailed comparison of molecular line data with radio continuum andrecombination line data, the extended ionized envelopes are likely the resultsof champagne flows in at least 10 sources in our sample. Together these resultsappear to support a published suggestion that the extended emission around UCHII regions can be naturally understood by combining the champagne flow modelwith the hierarchical structure of molecular clouds. We discuss the implicationof our results for the blister model of HII regions.Comment: 36 pages, including 30 figures, accepted for publication in Ap