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Issue A&A
Volume 389, Number 2, July II 2002
Page(s) 603 - 617
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20020579



A&A 389, 603-617 (2002)
DOI: 10.1051/0004-6361:20020579

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

F. Fontani1, R. Cesaroni2, P. Caselli2 and L. Olmi3

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

(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 J=6-5, 8-7 and 13-12 rotational transitions of methyl-acetylene (CH 3C 2H). 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, H 2 densities of 105- $10^6~\rm {cm^{-3}}$, and masses of 102- $2\times 10^4~M_\odot$. We compare these values with those obtained by other authors from different molecular tracers and find that the H 2 density and the temperature inside the clumps vary respectively like $n_{\rm H_2}\propto R^{-2.6}$ and $T\propto R^{-0.5}$, 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 CH 3C 2H 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 CH 3C 2H 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 (~ $10^{-2}~M_\odot$ yr -1) lead to massive star formation at the centre of such clumps.


Key words: stars: formation -- radio lines: ISM -- ISM: molecules

Offprint request: F. Fontani, fontani@arcetri.astro.it

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© ESO 2002


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