Issue |
A&A
Volume 425, Number 2, October II 2004
|
|
---|---|---|
Page(s) | 577 - 589 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20048091 | |
Published online | 21 September 2004 |
Influence of UV radiation from a massive YSO on the chemistry of its envelope
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 ( eV) from a massive young stellar
object (YSO) on the chemistry of its own envelope by extending the
models of Doty et al. ([CITE]) 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
–100 at 200 AU from the star with an optical depth
–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 H2O abundance caused by
evaporation of ices in the inner region. In particular, the
/
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 H2 in the inner dense and
warm region and produces more HCN. It is found that the
abundance is strongly enhanced and grows steadily with increasing UV
field. In addition, the ratio
/
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
© ESO, 2004
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