Dense Interstellar Cloud Chemistry

Abstract

Significant advances in our understanding of the gas phase chemistry of dense interstellar clouds have occurred in the last few years. These advances include the delineation of the reaction pathways by which complex molecules are produced, a preliminary understanding of the varied chemistries existing in and near regions of star formation, an appreciation that photodissociation caused by internally generated photons is important, and a realization that PAH’s affect the chemistry of less complex species. However, all of these advances and indeed all of our understanding of the chemistry of dense interstellar clouds is dependent on knowledge of rate coefficients for a variety of processes that occur under the unusual conditions present in these sources. Our knowledge of some types of reactive processes is sufficiently poor that model results dependent on these proceses are often of questionable validity. Highest on the list of poorly understood processes are dissociative recombination and neutral-neutral reactions. In the former type of reaction, the neutral product branching ratios are not known definitively, while in the latter it is unclear whether or not reactions involving radicals are inhibited at low temperature by small activation energy barriers. Further progress in understanding the chemistry of dense clouds depends on significant laboratory and/or theoretical advances in the treatment of these and other processes.