HIGHLY IONIZED ATOMS TOWARD HD-93521

Absorption features of interstellar C IV and Si IV have been observed in the spectrum of the O9.5 halo star HD 93521, located some 1500 pc from the Galactic plane. Photon counts with a signal-to-noise ratio of about 20/1 were obtained with the Hubble Space Telescope, using the Goddard High Resolution Spectrograph in the echelle mode (lambda/DELTA-lambda congruent-to 85,000). The full width at half-maximum depth (FWHM) is about 50 km s-1 for both features, in contrast to the 7 km s-1 found for one of the several S II features; the line centers for C IV and Si IV are at upsilon = -67 km s-1 and -60 km s-1, respectively. The widths must be attributed chiefly to macroscopic motion, since at the 10(6) K required for the Si IV width the fraction of Si atoms in the Si+3 state would likely be very small. The velocity cannot be due to Galactic rotation but could result from infall of gas, as from a galactic fountain. The Si IV/C IV ratio of N(upsilon), the column density per unit velocity interval, increases from about 0.2 to 0.4 as -upsilon decreases from 90 to 50 km s-1. This result, which is uncertain because of possible stellar wind absorption features, is qualitatively consistent with a fountain model if the slower gas has cooled and recombined more than the faster gas. Detailed models of such gas flow should take into account heating by this decrease of kinetic energy, presumably in shocks.