The abundance of gaseous H

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