Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aas...189.1714f&link_type=abstract
American Astronomical Society, 189th AAS Meeting, #17.14; Bulletin of the American Astronomical Society, Vol. 28, p.1297
Astronomy and Astrophysics
Astronomy
2
Scientific paper
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: (1) 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.
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