EDP Sciences
Free access
Volume 425, Number 2, October II 2004
Page(s) 577 - 589
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20048091

A&A 425, 577-589 (2004)
DOI: 10.1051/0004-6361:20048091

Influence of UV radiation from a massive YSO on the chemistry of its envelope

P. Stäuber1, S. D. Doty2, E. F. van Dishoeck3, J. K. Jørgensen3 and A. O. Benz1

1  Institute of Astronomy, ETH-Zentrum, 8092 Zurich, Switzerland
    e-mail: pascalst@astro.phys.ethz.ch
2  Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
3  Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands

(Received 15 April 2004 / Accepted 23 June 2004)

We have studied the influence of far ultraviolet (UV) radiation ( $6 < h\nu < 13.6$ eV) from a massive young stellar object (YSO) on the chemistry of its own envelope by extending the models of Doty et al. (2002) to include a central source of UV radiation. The models are applied to the massive star-forming region AFGL 2591 for different inner UV field strengths. Depth-dependent abundance profiles for several molecules are presented and discussed. We predict enhanced column densities for more than 30 species, especially radicals and ions. Comparison between observations and models is improved with a moderate UV field incident on the inner envelope, corresponding to an enhancement factor $G_{0}
\approx 10$ - 100 at 200 AU from the star with an optical depth $\tau \approx 15$- 17. The chemical networks of various species are explored. Subtle differences are found compared with traditional models of Photon Dominated Regions (PDRs) because of the higher temperatures and higher gas-phase H 2O abundance caused by evaporation of ices in the inner region. In particular, the $\element{CN}$/ $\element{HCN}$ ratio is not a sensitive tracer of the inner UV field, in contrast with the situation for normal PDRs: for low UV fields, the extra CN reacts with H 2 in the inner dense and warm region and produces more HCN. It is found that the $\element{CH^+}$ abundance is strongly enhanced and grows steadily with increasing UV field. In addition, the ratio $\element{CH^+}$/ $\element{CH}$ is increased by a factor of 103- 105 depending on the inner UV flux. High- J lines of molecules like CN and HCN are most sensitive to the inner dense region where UV radiation plays a role. Thus, even though the total column density affected by UV photons is small, comparison of high- J and low- J lines can selectively trace and distinguish the inner UV field from the outer one. In addition, future Herschel-HIFI observations of hydrides can sensitively probe the inner UV field.

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

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