Interstellar electron density measurements with the GHRS

The measurement of n(e) in interstellar clouds is crucial for understanding the ionization state of the gas and for determining the nature of the ionization mechanisms. The GHRS allows n(e) to be estimated through a number of different techniques, including collisional excitation of fine-structure levels, using the column-density ratios N(C+*)/N(C+) and N(Si+*)/N(Si+), and ionization equilibrium, using the column-density ratios N(C-0)/N(C+), N(Mg-0)/N(Mg+), and N(S-0)/N(S+). In addition, the UV data provide indirect estimates of the column densities of Ca++ and Na+, allowing n(e) to be determined using ground-based measurements of N(Ca+) and N(Na-0). This wealth of diagnostic information allows systematic differences between the techniques to be evaluated. Current results suggest that n(e) values derived using ionization equilibrium of the neutral species C-0, Mg-0, S-0, and Na-0 are systematically and substantially higher than those derived using C+ excitation equilibrium or Ca+ ionization equilibrium. Possible explanations for this effect are discussed.