HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF INTERSTELLAR LINES IN 3 HIGH-LATITUDE STARS

To study changes in the gas-phase abundances of elements in the Galact ic halo, we have used the Goddard High Resolution Spectrograph instrum ent on the Hubble Space Telescope to observe interstellar absorption l ines of O I, Mg II, Al II, Al III, Si II, S II, Fe II, and Ni II in th e spectra of three stars at high Galactic latitudes: HD 22586, HD 4979 8, and HD 120086. Recognizing that we would probably observe a decreas e in the amount of depletions of certain heavy elements onto dust grai ns, we also made comparison observations of HD 72089 behind the Vela s upernova remnant. This star has a number of high-velocity gas componen ts that have been subjected to strong shocks, allowing us to determine whether the patterns of abundances, in a medium where the grains have been partially destroyed, had any resemblance to abundances in the ha lo. When we compare various element abundances relative to that of sul fur, a normally undepleted element, we find that as a rule the element depletions of gas toward the halo stars are intermediate between high - and low-velocity gas in the spectrum of HD 72089. Exceptions to this are the extraordinarily light depletions of Si toward HD 22586 and HD 120086 (-0.1 and 0.0 dex, respectively), and a very mild depletion of Mg in HD 120086 (-0.2 dex). We see no evidence that iron-peak element s are enhanced relative to the others (beyond the effects of changes i n depletion), as one might expect if the ejecta from Type Ia supernova e were enriching the gas in the halo. It therefore seems apparent that either there is not enough change in the Type II supernovae to Type I a supernovae ratio from the plane to the halo to produce a noticeable enrichment, or else one or more gas transport processes between the di sk and halo, such as mass exchange in a Galactic fountain or the gener al turbulence of the interstellar medium, are sufficiently strong to c ycle material between the disk and halo at a rate that can stifle any abundance contrasts.