EDP Sciences
Free access
Volume 389, Number 2, July II 2002
Page(s) 446 - 463
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020597

A&A 389, 446-463 (2002)
DOI: 10.1051/0004-6361:20020597

Chemistry as a probe of the structures and evolution of massive star-forming regions

S. D. Doty1, E. F. van Dishoeck2, F. F. S. van der Tak2, 3 and A. M. S. Boonman2

1  Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
2  Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
3  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 31 October 2001 / Accepted 15 April 2002)

We present detailed thermal and gas-phase chemical models for the envelope of the massive star-forming region AFGL 2591. By considering both time- and space-dependent chemistry, these models are used to study both the physical structure proposed by van der Tak et al. (1999, 2000), as well as the chemical evolution of this region. The model predictions are compared with observed abundances and column densities for 29 species. The observational data cover a wide range of physical conditions within the source, but significantly probe the inner regions where interesting high-temperature chemistry may be occurring. Taking appropriate care when comparing models with both emission and absorption measurements, we find that the majority of the chemical structure can be well-explained. In particular, we find that the nitrogen and hydrocarbon chemistry can be significantly affected by temperature, with the possibility of high-temperature pathways to HCN. While we cannot determine the sulphur reservoir, the observations can be explained by models with the majority of the sulphur in CS in the cold gas, SO 2 in the warm gas, and atomic sulphur in the warmest gas. Because the model overpredicts CO 2 by a factor of 40, various high-temperature destruction mechanisms are explored, including impulsive heating events. The observed abundances of ions such as HCO + and N 2H + and the cold gas-phase production of HCN constrain the cosmic-ray ionization rate to ~ $5.6 \times 10^{-17}$ s -1, to within a factor of three. Finally, we find that the model and observations can simultaneously agree at a reasonable level and often to within a factor of three for $7 \times 10^{3} \leq t({\rm yrs}) \leq 5 \times 10^{4}$, with a strong preference for $t \sim 3 \times 10^{4}$ yrs since the collapse and formation of the central luminosity source.

Key words: stars: formation -- stars: individual: AFGL 2591 -- ISM: molecules

Offprint request: S. Doty, doty@cc.dension.edu

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