SIMBAD references

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 (10⁶-10⁷yr), 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 ρOph cannot be reproduced by the models without calculating unphysically low abundances of CO.