Calcium is always found to be highly depleted in the interstellar medium. New observations of the interstellar Ca II H and K lines toward 10 stars along varied lines of sight are combined with existing data in order to study the nature of its depletion. The depletion of Ca along with that of Ti is found to be a stronger function of mean line-of-sight density than P, Mg, Mn, and Fe. This result indicates that both Ca and Ti are so highly depleted in cold diffuse clouds that Ca II and Ti II essentially trace only the warm intercloud medium. This conclusion is based on the fact that the Ca II/Ti II abundance ratio remains constant rather than exhibiting the systematic differences which would be expected from relative changes in the Ca III/Ca II ionization balance. Furthermore, the present results place strong constraints on the efficiency of grain destruction by shocks; the data indicate efficiences of less than or equal to 5%.
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Crinklaw et al. (1994) studied this question.