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Session 17 - The Galactic Interstellar Gas.
Display session, Monday, January 13
I will describe a multiyear observing program using the GHRS\/ in its highest resolution modes to study the properties of individual interstellar clouds in the Galactic disk and in the halo. This program emphasizes three particular areas of investigation: the elemental abundances and depletions in neutral H I gas; (2) the physical conditions in the clouds, particularly T, n_e, and n(H^0); and (3) the relationship between highly-ionized gas and the low-ionization clouds. In this poster, I concentrate on results obtained so far on the determination of electron densities in interstellar clouds.
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^+\textstyle *)/N(C^+) and N(Si^+\textstyle *)/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 (perhaps) 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.
Program listing for Monday