The Abundance of Interstellar CO

Abstract

The current understanding of the abundance of CO in interstellar clouds is reviewed from both observational and theoretical points of view. In special circumstances, the CO and H2 column densities can be measured directly by ultraviolet and infrared absorption line techniques. Indirect methods invoke mean relations between CO line intensities and extinction, diffuse far-infrared flux, and diffuse gamma ray flux, or assume that the clouds are in virial equilibrium. The problems associated with these indirect methods are considered, and the range of applicability of the empirical relations is investigated. A better understanding of the abundance of CO is necessary if it is to be used effectively as a quantitative tracer of molecular material in the Milky Way and other galaxies. Theoretical models of the structure and chemistry of interstellar clouds can be used to investigate the depth dependence of the abundance of CO, as well as the isotope fractionation. The models treat the photochemical zone in detail and incorporate the most recent laboratory data on poorly understood photodissociation processes. Such models can be tested by comparison with observations of CO in diffuse and translucent molecular clouds. Models of dense clouds must account for the observed fractions of CO in solid form in such clouds.