Gas-grain chemical models of star-forming molecular clouds as constrained by ISO and SWAS observations

We have investigated the gaseous and solid state molecular composition of d ense interstellar material that periodically experiences processing in the shock waves associated with ongoing star formation. Our motivation is to co nfront these models with the stringent abundance constraints on CO2, H2O an d O-2, in both gas and solid phases, that have been set by ISO and SWAS. We also compare our results with the chemical composition of dark molecular c louds as determined by ground-based telescopes. Beginning with the simplest possible model needed to study molecular cloud gas-grain chemistry, we onl y include additional processes where they are clearly required to satisfy o ne or more of the ISO-SWAS constraints. When CO, N-2 and atoms of N, C and S are efficiently desorbed from grains, a chemical quasi-steady-state devel ops after about one million years. We find that accretion of CO2 and H2O ca nnot explain the [CO2/H2O](ice) ISO observations; as with previous models, accretion and reaction of oxygen atoms are necessary although a high O atom abundance can still be derived from the CO that remains in the gas. The ob servational constraints on solid and gaseous molecular oxygen are both met in this model. However, we find that we cannot explain the lowest H2O abund ances seen by SWAS or the highest atomic carbon abundances found in molecul ar clouds; additional chemical processes are required and possible candidat es are given. One prediction of models of this type is that there should be some regions of molecular clouds which contain high gas phase abundances o f H2O, O-2 and NO. A further consequence, we find, is that interstellar gra in mantles could be rich in NH2OH and NO2. The search for these regions, as well as NH2OH and NO2 in ices and in hot cores, is an important further te st of this scenario. The model can give good agreement with observations of simple molecules in dark molecular clouds such as TMC-1 and L134N. Despite the fact that S atoms are assumed to be continously desorbed from grain su rfaces, we find that the sulphur chemistry independently experiences an "ac cretion catastrophe". The S-bearing molecular abundances cease to lie withi n the observed range after about 3 x 10(6) years and this indicates that th ere may be at least two efficient surface desorption mechanisms operating i n dark clouds - one quasi-continous and the other operating more sporadical ly on this time-scale. We suggest that mantle removal on short time-scales is mediated by clump dynamics, and by the effects of star formation on long er time-scales. The applicability of this type of dynamical-chemical model for molecular cloud evolution is discussed and comparison is made with othe r models of dark cloud chemistry.