A HIGH-RESOLUTION SURVEY OF INTERSTELLAR CA-II ABSORPTION

We present high-resolution (0.3-1.2 km s(-1)) spectra, obtained with t he AAT UHRF and/or the KPNO coude feed, of interstellar Ca II absorpti on toward 44 Galactic stars. For most stars in the sample, these new s pectra are characterized by higher resolution, higher S/N, and/or wide r velocity coverage than previously available spectra. As was found in a similar high-resolution survey of Na I absorption, the Ca II profil es show complex structure in many lines of sight. Multicomponent fits to the line profiles yield estimates for the column densities, line wi dths, and velocities for 417 individual interstellar cloud components; an analysis of the component separations suggests that we may have di scerned only about 40% of the components actually present, however. Th e mean LSR velocities of the Ca II and Na I absorption are similar, bu t the Ca II velocity distribution is broader, due to outlying componen ts seen only in Ca II. The Ca II line widths imply that at most 40% of the Ca II components in the present sample could arise in warm gas at T greater than or similar to 6000 K. If most gas traced by Ca II is w armer than 500 K, then the turbulent velocities in that gas must gener ally be subsonic. In general, the Ca II component line widths are larg er than those of corresponding Na I components seen at the same veloci ty, suggesting that the Ca II occupies a somewhat larger volume, chara cterized by a larger temperature and/or turbulent velocity, than that occupied by the Na I. Ca II absorption can arise both in cold, relativ ely dense gas, where Ca is typically heavily depleted onto grains and Ca II can be its dominant ionization state, and also in warmer, lower density gas, where Ca is less depleted but Ca II is a trace ionization state. The interplay between the strong effects of Ca ionization bala nce and Ca depletion, for different types of neutral gas, can explain the similarities between the line profiles seen for Ca II and for vari ous dominant first ions of less depleted elements. High-resolution spe ctra of Ca II can thus be useful for interpreting lower resolution UV spectra of many other species found in neutral clouds. The ratio of th e column densities of Na I and Ca II is not an unambiguous indicator o f Ca depletion and cloud physical conditions, and it can vary by facto rs of more than 30 for adjacent components separated by only several k m s(-1); ''individual'' components derived from lower resolution spect ra thus may often be blends of several distinct and physically differe nt parcels of gas.