2001A&A...370..557V

We present results of chemical models for a variety of types of regions shown by SWAS observations to contain less O₂ and H₂O than previously expected. We identify time-dependent models for which O₂ and H₂O abundances meet the SWAS constraints and for which calculated abundances of other species are in harmony with measurements made primarily at millimeter wavelengths. The phases of acceptable composition are transient in these time-dependent models but are of very substantial length in many models for which CO and N₂ are assumed to be returned promptly and unaltered to the gas phase though other species, except for H₂ and He, freeze-out onto the dust. We also consider whether the presence of bistability in some steady-state models can account for the SWAS and other observations.