The structure of molecular clumps around high-mass young stellar objects^*

¹ Dipartimento di Astronomia e Fisica dello spazio, Largo E. Fermi 2, 50125 Firenze, Italy
² INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ LMT/GTM Project, Dept. of Astronomy, 815J Lederle GRT Tower B, University of Massachusetts, 710 N. Pleasant st., Amherst, MA 01003, USA

Corresponding author: F. Fontani, fontani@arcetri.astro.it

Received: 14 December 2001
Accepted: 12 April 2002

Abstract

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 , 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 56 K. We estimate that the clumps have diameters of 0.2–1.6 pc, H₂ densities of 10⁵–, and masses of 10²–. 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 and , 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 (~10⁵ yr) entities, remnants of isothermal spheres currently undergoing gravitational collapse: the high mass accretion rates (~ yr^-1) lead to massive star formation at the centre of such clumps.