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

¹ Space Science Division, NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
² Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands

Corresponding author: P. Ehrenfreund, pascale@strwchem.strw.leidenuniv.nf

Received: 20 June 2001
Accepted: 27 August 2001

Abstract

We have investigated the gaseous and solid state molecular composition of dense interstellar material that periodically experiences processing in the shock waves associated with ongoing star formation. Our motivation is to confront these models with the stringent abundance constraints on CO₂, H₂O and O₂, 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 clouds as determined by ground-based telescopes. Beginning with the simplest possible model needed to study molecular cloud gas-grain chemistry, we only include additional processes where they are clearly required to satisfy one or more of the ISO-SWAS constraints. When CO, N₂ and atoms of N, C and S are efficiently desorbed from grains, a chemical quasi-steady-state develops after about one million years. We find that accretion of CO₂ and H₂O cannot explain the 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 observational constraints on solid and gaseous molecular oxygen are both met in this model. However, we find that we cannot explain the lowest abundances seen by SWAS or the highest atomic carbon abundances found in molecular clouds; additional chemical processes are required and possible candidates 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 of H₂O, O₂ and NO. A further consequence, we find, is that interstellar grain 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 test 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 surfaces, we find that the sulphur chemistry independently experiences an "accretion catastrophe" . The S-bearing molecular abundances cease to lie within the observed range after about years and this indicates that there may be at least two efficient surface desorption mechanisms operating in dark clouds -one quasi-continous and the other operating more sporadically 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 longer time-scales. The applicability of this type of dynamical-chemical model for molecular cloud evolution is discussed and comparison is made with other models of dark cloud chemistry.