MOLECULE FORMATION IN INTERSTELLAR CLOUDS

Abstract. — Considerable progress has been made during the past five years toward a quan-titative understanding of the formation and destruction processes for interstellar molecules. Two areas have been most successful-investigations of the formation process for molecular hydro-gen on the surfaces of dust grains and studies of reactions involving positive ions in the gas. Exten-sive studies at ultraviolet wavelengths from the Copernicus satellite make possible detailed compa-risons between predictions and observations for H2 and other species in the diffuse interstellar gas. These confirm that a hydrogen atom is converted into an H2 molecule at approximately every collision with an interstellar dust grain. In the more dense interstellar gas, observations with radio telescopes provide vast data on complex molecules and have recently identified a num-ber of reactive intermediate species — HCO+, N2H+, CCH — whose presence strongly supports the proposed reaction processes. Observed fractionation of (deuterium/hydrogen) and possibly (carbon-13/carbon-12) in certain interstellar molecules provide additional challenges and infor-mation for studies of reactions. Although there is semiquantitative agreement between predic-tions and observed abundances for a wide range of small molecular species, the tests are suffi-ciently precise in only a few cases to reach reasonably definitive conclusions. To a large degree this is due to poor knowledge of the physical conditions in the gas where the molecules are locat