ULTRAVIOLET INTERSTELLAR ABSORPTION TOWARD HD-163522, A HALO STAR AT 9 KILOPARSECS IN THE DIRECTION L = 350-DEGREES AND B = -9-DEGREES

UV interstellar absorption line measurements obtained by the IUE satellite toward HD 163522 in the direction l = 349°.6 and b = -9°.1 are analyzed. The UV stellar spectrum of HD 163522 is consistent with the B1 Ia MK classification which implies a distance of 9.4-1.9+2.4 kpc, placing the star beyond the Galactic center at a distance away from the Galactic plane of z = -1.5 kpc. By combining four short-wavelength spectra and two long-wavelength spectra, we have produced an absorption-line spectrum with S/N ≈ 15-25. The UV interstellar absorption lines for neutral, weakly ionized, and highly ionized gas are strong and broad and show asymmetrical extensions to νLSR = -100 km s-1. The observed profiles are strongly influenced by the effects of Galactic rotation over the large distance to HD 163522. The profiles for interstellar species known to have large scale heights (e.g., Si IV, C IV, and N V) are significantly more affected by Galactic rotation than the profiles for those species more confined to the plane (e.g., H I, Fe II, Mg II, S II, etc.). Simple model calculations are performed to quantitatively understand the characteristics of the observed profiles. Although the sight line passes over the expanding 3 kpc arm at a z distance estimated to be about -0.9 kpc, we find no evidence in the data for absorption at velocities associated with the 3 kpc arm in the underlying disk. The observed absorption line profiles for A1 III, Si IV, C IV, and N V are converted into plots of optical depth versus velocity. A comparison of these plots for the weak and strong member of each doublet reveals that the profiles are not strongly influenced by the presence of unresolved saturated absorption and the optical depth profiles can therefore be directly converted into measures of N(ν), the column density per km s-1. Integration of the N(ν) over the velocity range from -100 to +60 km s-1 yields log N = 13.53, 13.96, 14.55, and 14.09 for A1 III, Si IV, C IV, and N V, respectively. The definite detection of N V with a profile shape that is similar to that of C IV and Si IV favors the origin of these ions in collisionally ionized intermediate temperature gas that may be associated with the nonequilibrium cooling of gas in a Galactic fountain. The observed N V and C IV column densities are compatible with the time dependent ionization calculations published by Edgar and Chevalier in 1986 for a fountain flow rate of 4 M⊙ yr-1 to each side of the Galactic plane. However, that calculation predicts four times less Si IV than is observed.