Optical interstellar lines in southern supergiants

Photoelectric photometry of 35 southern supergiants reveals a good correlation of the central residual intensity of the diffuse interstellar line at 4430 A with color excess. From spectra of the brighter southern supergiants, I have derived column densities of Na i and Ca ii, and, in a few cases, of CH and CH+. These results are averaged and compared with previous studies of the interstellar medium at high galactic latitudes and in the denser regions of the plane. The basic peculiarities of the interstellar lines in these two regions, as discussed in Papers I and II, are more sharply delineated by this comparison. Models for the gas in the halo are derived, and I obtained the mass of gas in the halo (from 1times107 to 6times107 M/sub sun/) and a maximum value of the metal depletion of the halo gas relative to the disk gas (from 300 to 90, depending on whether grains exist in the halo). This maximum value implies that the gas in the halo has been significantly contaminated by material which has undergone nuclear processing