Interstellar Na I and Ca II absorption in intercloud regions

Abstract

The weak interstellar absorption lines of Na I, Ca II, and H I formed in the intercloud medium toward eight nearly unreddened stars are analyzed. Compared with the corresponding column densities within a standard cloud, the Na I/H column-density ratio is smaller by a factor probably in excess of 17, while the Ca II/H column-density ratio is about normal. These results imply that the local intercloud medium extensively contains low-density H I regions in which the temperature is at least about 2500 K and the depletion of calcium is less severe than in the clouds, by a factor of 20.