1999A&A...352..266O

We have used the JCMT telescope to observe the J=5-4 and J=7-6 rotational transitions of CS and C³⁴S towards 19 ultracompact HII regions. The main goal was to extend the survey of Cesaroni et al. (1991A&A...252..278C) to a larger sample of sources and to higher energy transitions sensitive to the densest, hottest molecular gas. In all cases we find gas with n_H2≃10⁵-10⁶cm^–3 associated with the ultracompact HII regions, thus confirming that massive stars are born inside dense molecular clumps. In particular, the observed line intensities can be satisfactorily reproduced with a single density model, without the contribution of a radiative field. A comparison between the profiles of different transitions suggests that the regions traced by the CS molecule could be collapsing. Also, higher energy lines seem to trace more turbulent environments: it is shown that turbulence can be an efficient mechanism to confine the expansion of ultracompact HII regions, as proposed by Xie et al. (1996), although confinement by thermal pressure cannot be excluded on the basis of our data. For one of the sources, G10.47+0.03, we present a map in the CS(7-6) transition: this shows that the diameter of the emitting region is comparable to that measured in the lower excitation lines observed by Cesaroni et al. (1991A&A...252..278C).