High‐Resolution Observations of Interstellar Ca i Absorption—Implications for Depletions and Electron Densities in Diffuse Clouds

Abstract

We present high-resolution (FWHM ∼ 0.3-1.5 km s-1) spectra, obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudé spectrograph, and/or the KPNO coudé feed, of interstellar Ca I absorption toward 30 Galactic stars. Comparisons of the column densities of Ca I, Ca II, K I, and other species—for individual components identified in the line profiles and also when integrated over entire lines of sight—yield information on relative electron densities and depletions (dependent on assumptions regarding the ionization equilibrium). There is no obvious relationship between the ratio N(Ca I)/N(Ca II) [equal to ne/(Γ/αr) for photoionization equilibrium] and the fraction of hydrogen in molecular form f(H2) (often taken to be indicative of the local density nH). For a smaller sample of sight lines for which the thermal pressure (nHT) and local density can be estimated via analysis of the C I fine-structure excitation, the average electron density inferred from C, Na, and K (assuming photoionization equilibrium) seems to be independent of nH and nHT. While the electron density (ne) obtained from the ratio N(Ca I)/N(Ca II) is often significantly higher than the values derived from other elements, the patterns of relative ne derived from different elements show both similarities and differences for different lines of sight—suggesting that additional processes besides photoionization and radiative recombination commonly and significantly affect the ionization balance of heavy elements in diffuse interstellar clouds. Such additional processes may also contribute to the (apparently) larger than expected fractional ionizations (ne/nH) found for some lines of sight with independent determinations of nH. In general, inclusion of "grain-assisted" recombination does reduce the inferred ne, but it does not reconcile the ne estimated from different elements; it may, however, suggest some dependence of ne on nH. The depletion of calcium may have a much weaker dependence on density than was suggested by earlier comparisons with CH and CN. Two appendices present similar high-resolution spectra of Fe I for a few stars and give a compilation of column density data for Ca I, Ca II, Fe I, and S I.