Element depletions in interstellar gas – II. The density-dependence of calcium and sodium depletions

Abstract

Complementary high-resolution optical (AAT) and ultraviolet (IUE) spectra of nine stars in the same region of sky have been obtained in order to study the behaviour of calcium and sodium depletions in interstellar gas. The optical absorption lines of Ca II and Na I have been analysed to determine column densities of these species in each line-of-sight. The ultraviolet data have been used to estimate hydrogen column densities and to perform ionization balance calculations in order to deduce the contributions to the abundances of Ca and Na from unobserved ion stages. The resultant depletions of these elements, relative to solar abundances, have been compared with estimates of the average density of interstellar gas in the different lines-of-sight. The depletion of calcium is found to be a strong function of present densities in the interstellar medium, while sodium appears to be depleted by a constant factor ∼4 over the range of densities sampled. These results have been compared with variations in the density-dependence of depletions for other elements found from a previous investigation based on ultraviolet data alone. It is concluded that shock sputtering of grains plays an important role in determining element depletions even in low-velocity interstellar gas. Our results suggest that calcium accretes predominantly on to silicate grains in the ambient interstellar medium, while sodium does not accrete efficiently on to grains, at least in regions of low to moderate density.