Optical interstellar lines in dark clouds

Abstract

Observations of the equivalent widths and radial velocities of the interstellar lines of Ca ii, Na i, CH, and CH^+ are presented for 30 stars. A special effort was made to observe stars behind or within dense clouds with up to 3 mag of absorption. The interstellar lines in the cloud stars are compared with those in a group of highly reddened supergiants whose reddening arises from a large distance, i.e., a long path length through relatively thin material. The atomic interstellar lines of the cloud stars are weak compared to those of supergiants with the same color excess; the molecular lines are of comparable strength in the two groups, except that the ratio n_(CH)/n_(CH^+) is larger in the cloud stars. The ionization equilibrium is discussed, and it is concluded that the deficiency of atomic Ca and Na relative to hydrogen in the clouds is about a factor of 100 larger than in the supergiants.