The abundance of gaseous HO and O in cores of dense interstellar clouds

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

Corresponding author: E. Herbst, herbst@mps.ohio-state.edu

Received: 21 May 2002
Accepted: 30 August 2002

Abstract

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 O₂ abundances. For cold clouds, such as TMC-1 and L134N, the limits on the H₂O abundance are met by the models only after relatively long periods of accretion (10⁶–10⁷ 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 H₂O 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 H₂O abundance observed by SWAS in ρ  Oph cannot be reproduced by the models without calculating unphysically low abundances of CO.