2002A&A...389..603F

We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular clumps associated with 12 ultracompact (UC) HII regions in the J=6-5, 8-7 and 13-12 rotational transitions of methyl-acetylene (CH₃C₂H). Under the assumption of LTE and optically thin emission, we have derived temperature estimates ranging from 30 to 56K. We estimate that the clumps have diameters of 0.2-1.6 pc, H₂ densities of 10⁵-10⁶cm^–3, and masses of 10²-2x10⁴M_☉. We compare these values with those obtained by other authors from different molecular tracers and find that the H₂ density and the temperature inside the clumps vary respectively like n_H2∝R^–2.6 and T∝R^–0.5, with R distance from the centre. We also find that the virial masses of the clumps are ∼3 times less than those derived from the CH₃C₂H column densities: we show that a plausible explanation is that magnetic fields play an important role to stabilise the clumps, which are on the verge of gravitational collapse. Finally, we show that the CH₃C₂H line width increases for decreasing distance from the clump centre: this effect is consistent with infall in the inner regions of the clumps. We conclude that the clumps around UC HII regions are likely to be transient (∼10⁵yr) entities, remnants of isothermal spheres currently undergoing gravitational collapse: the high mass accretion rates (∼10^–2M_☉/yr) lead to massive star formation at the centre of such clumps.