EDP Sciences
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Volume 395, Number 1, November III 2002
Page(s) 233 - 242
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20021280

A&A 395, 233-242 (2002)
DOI: 10.1051/0004-6361:20021280

The abundance of gaseous H $_{\mathsf 2}$O and O $_{\mathsf 2}$ in cores of dense interstellar clouds

H. Roberts1 and E. Herbst2

1  Department of Physics, The Ohio State University, Columbus, OH 43210, USA
2  Departments of Physics and Astronomy, The Ohio State University, Columbus, OH 43210, USA

(Received 21 May 2002 / Accepted 30 August 2002 )

We have used chemical models that include both gas-phase and grain-surface processes to try to understand the low water and molecular oxygen abundances inferred from SWAS observations towards dense molecular clouds. The models represent an improvement over pure gas-phase chemistries in that they are more realistic, and they are largely successful at reproducing the low O 2 abundances. For cold clouds, such as TMC-1 and L134N, the limits on the H 2O abundance are met by the models only after relatively long periods of accretion (10 6-10 7 yr), but we show that ground-based observations of these clouds do not necessarily contradict these ages, especially for L134N. If the upper limits on the H 2O abundance were to be revised downwards, however, or if water were to be observed in the cold clouds at the same level as in some star-forming regions, then even heavier depletions would be required. For this reason, the low H 2O abundance observed by SWAS in $\rho$ Oph cannot be reproduced by the models without calculating unphysically low abundances of CO.

Key words: ISM: abundances -- ISM: molecules -- molecular processes

Offprint request: E. Herbst, herbst@mps.ohio-state.edu

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