INTERSTELLAR ENVIRONMENTS PROBED BY CA-I ABSORPTION AND THE EFFECTS OF DENSITY-DEPENDENT DEPLETIONS

The environments where absorption from interstellar Ca I arises have been studied. For lines of sight containing clouds of moderate density, such as those toward Cep OB3, the line profiles of Ca I and CH+ are very similar. However, they differ significantly from that for CH, which probes denser molecular gas. When normalized to the column density of CH, the column densities of CN, a molecule that samples relatively dense gas, and Ca I are inversely related. Thus, regions showing strong absorption of Ca I (and CH+) are different from those with strong CH and CN absorption. In the case for Ca I, the dependence of calcium depletion on density is the cause for the observed trends. In dense gas the depletion is so severe that absorption from neutral calcium may be below detection limits. These results are expected to apply equally well to other elements, particularly those displaying large gas-phase depletions.