Interstellar molecule reactions

Abstract

Considerable progress has been made during the past five years toward a quantitative understanding of the formation and destruction processes for interstellar molecules. Two areas have been most successful—investigations of the formation process for molecular hydrogen on the surfaces of dust grains and studies of reactions involving positive ions in the gas. Laboratory measurements for ion-molecule reaction rates have provided strong support for the latter. Extensive studies at ultraviolet wavelengths from the Copernicus satellite make possible detailed comparisons between predictions and observations for ${\mathrm{H}}_2$ and other species in the diffuse interstellar gas. These confirm that a hydrogen atom is converted into an ${\mathrm{H}}_2$ 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 number of reactive intermediate species—HC${\mathrm{O}}^{+}$, ${\mathrm{N}}_2$ ${\mathrm{H}}^{+}$, HNC, CCH—whose presence strongly supports the proposed reaction processes. The quite recent development of laboratory methods to measure the microwave frequencies of reactive molecules and molecular ions has been an essential contribution to these identifications. Observed fractionation of (deuterium/hydrogen) and possibly (carbon-13/carbon-12) in certain interstellar molecules provide additional challenges and information for studies of reactions. Although there is semiquantitative agreement between predictions and observed abundances for a wide range of small molecular species, the tests are sufficiently 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 located. Certain laboratory data are needed—especially, photodissociation cross sections and radiative lifetimes in the ultraviolet, as well as some charge-transfer and reaction rate coefficients under low temperature/density conditions. Information on surface reactions applicable to the astrophysical situation is also desirable. At present the chief problems for interstellar molecule reactions are understanding the formation of larger molecules (larger than triatomic), and the role of surface reactions on dust grains for molecules other than ${\mathrm{H}}_2$. A lengthy introduction to the interstellar medium is provided for the nonastronomer. The status of information on the basic surface and gas phase processes is reviewed. Finally, the reaction schemes which seem to be of most importance for the major species of small interstellar molecules are discussed and quantitatively compared with observations when possible. Reactions that produce isotope fractionation are treated in some detail.