Ionization equilibria of calcium and sodium in interstellar clouds

Variations in the ionization equilibria for calcium and sodium as a function of position in interstellar clouds are calculated, including effects due to attenuation of the ultraviolet ionizing radiation by dust grains. It is found that while the ionization rates for Ca, Na, and Ca+ may vary by more than an order of magnitude within particular clouds, the ratio of number densities (Cah/n(Na0) does not exhibit such large variations Possible influences on this ratio due to cosmic-ray ionization and the charge-transfer reaction NaQC+ Na+tC* are considered. Some improvement in the agreement between the theoretical ratio and the observed average n(Ca+)/n(Na0) 0 87 is evident over calculations in which ultraviolet extinction is neglected, but a significant discrepancy remains Several effects which would lower the theoretical ratio still further are briefly discussed.