Issue |
A&A
Volume 453, Number 2, July II 2006
|
|
---|---|---|
Page(s) | 555 - 565 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20054263 | |
Published online | 16 June 2006 |
Water destruction by X-rays in young stellar objects
1
Institute of Astronomy, ETH Zürich, 8092 Zürich, Switzerland e-mail: pascalst@astro.phys.ethz.ch
2
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3
Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
4
Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
Received:
28
September
2005
Accepted:
27
January
2006
Aims.We study the H2O chemistry in star-forming environments under the influence of a central X-ray source and a central far ultraviolet (FUV) radiation field. The X-ray models are applied to envelopes around low-mass Class 0 and I young stellar objects (YSOs).
Methods.The gas-phase water chemistry is modeled as a function of time, hydrogen
density and X-ray flux. To cover a wide range of physical environments,
densities between –109 cm-3 and temperatures
between
–1000 K are studied.
Results.Three different regimes are found: for K, the water abundance is
of order 10-7–10-6 and can be somewhat enhanced or reduced due to
X-rays, depending on time and density. For 100 K
K, H2O is reduced from initial
following ice evaporation to
for
erg s-1 cm-2 (
yr) and for
erg s-1 cm-2 (
yr). At higher
temperatures (
K) and hydrogen densities, water can persist with
even for high X-ray fluxes. Water is destroyed in both
Class 0 and I envelopes on relatively short timescales (
yr) for realistic X-ray fluxes, although the effect is less prominent
in Class 0 envelopes due to the higher X-ray absorbing densities there. FUV
photons from the central source are not effective in
destroying water.
Conclusions.X-rays reduce the water abundances especially in regions where the gas
temperature is –300 K for fluxes
–10-4 erg s-1 cm-2. The affected regions can be
envelopes, disks or outflow hot spots. The average water abundance in Class I
sources for
erg s-1 is predicted to be
. Central UV fields have a negligible
influence, unless the photons can escape through cavities.
Key words: stars: formation / ISM: molecules / X-rays: ISM / astrochemistry
© ESO, 2006
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