Copernicus observations of C I - Pressures and carbon abundances in diffuse interstellar clouds

Abstract

Using the Copernicus satellite, we 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), we used our equivalent widths to revise the f-values of some of the C I transitions measured by other investigators. We also observed H<SUB>2</SUB> from the J = 4 level so that we could compare the rotational excitation of H<SUB>2</SUB> with the fine-structure excitation of C I. <P />From the amount of fine-structure excitation of C I in each case, we have derived information on the thermal gas pressures within the diffuse clouds. Most clouds have p/k between 10<SUP>3</SUP> cm<SUP>-3</SUP> K and 10<SUP>4</SUP> cm<SUP>-3</SUP> K, but we found that at least 6% of the C I-bearing material is at p/k &gt; 10<SUP>4</SUP> cm<SUP>-3</SUP> K, and one-third of the gas has upper limits for pressure below 10<SUP>3</SUP> cm<SUP>-3</SUP> K, assuming temperatures are not appreciably below 20 K. An analysis of radial velocities for the absorption lines showed no distinctive trends for the kinematics of high- or low-pressure gas components. From the apparent lack of acceleration of high-pressure clouds, we conclude that it is unlikely that streaming intercloud material is causing significant ram pressurization. <P />We have compared our results with the predictions for pressure fluctuations caused by supernova explosions in the theory of McKee and Ostriker. The observed lower bound for the amount of C I at higher than normal pressures (p/k &gt; 10<SUP>4</SUP> cm<SUP>-3</SUP> K) is about an order of magnitude below the prediction, which is probably a consequence of the C I-bearing clouds not responding rapidly enough to the upward jumps in intercloud pressure before they decay. At the opposite pressure extreme (p/k &lt; 10<SUP>3</SUP> cm<SUP>-3</SUP> K), we find more C I than predicted. <P />Toward most of the stars, we calculated the total abundance of atomic carbon, both ionized and neutral, based on our evaluations of the ionization equilibrium at the relevant pressures. Our observations are consistent with there being little or no depletion of carbon, except for a few stars in the Scorpius region where gaseous carbon seems to have about one-seventh its solar abundance relative to hydrogen, presumably because of condensation by grains. <P />Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H<SUB>2</SUB> in the sense that at least half of the hydrogen atoms that strike grains emerge as part of H<SUB>2</SUB>.