Interstellar Matter, Nebulae

Abstract

Using the Copernicus satellite, the authors observed ultraviolet absorption lines of interstellar neutral carbon atoms toward 27 stars. In addition to deriving column densities of C I (both in its ground state and the two excited fine-structure levels), the equivalent widths were used to revise the f-values of some of the C I transitions measured by other investigators. The authors also observed H2 from the J = 4 level. From the amount of fine-structure excitation of C I in each case, information on the thermal gas pressures within the diffuse clouds was derived. An analysis of radial velocities for the absorption lines showed no distinctive trends for the kinematics of high- or low-pressure gas components. Toward most of the stars, the total abundance of atomic carbon, both ionized and neutral, was calculated. The observations show, that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains emerge as part of H2.