ULTRAVIOLET INTERSTELLAR ABSORPTION TOWARD HD-156359, A HALO STAR AT 11 KILOPARSECS IN THE DIRECTION L = 329-DEGREES AND B = -15-DEGREES

UV interstellar absorption line measurements obtained by the IUE satellite toward HD 156359 in the direction l = 328.7-degrees and b = -14.5-degrees are analyzed. We assign HD 156359 an MK classification of O9.7 Ib-II on the basis of UV stellar photospheric and wind lines in its spectrum. The implied spectroscopic distance to HD 156359 is 11.1 kpc, placing it 2.8 kpc below the Galactic plane. By combining five SWP and two LWR spectra we produce a composite spectrum with a signal-to-noise ratio of 15 to 25. The UV interstellar absorption lines of neutral and weakly ionized species (Fe II, Mg II, Si II, S II) sample both dense and tenuous media along the sight line. The strong lines are saturated between -100 km s-1 and +20 km s-1 and reveal a high positive velocity cloud at +110 km s-1 having an equivalent neutral hydrogen column density of log N(H I) > 17.0. The highly ionized species (Si IV, C IV, and N V) illustrate the dependence of profile shapes on the effects of Galactic rotation. The Si IV, C IV, and N V profiles extend from -150 km s-1 to +40 km s-1 with half-absorption widths of nearly 100 km s-1. The C IV profiles also exhibit high positive velocity absorption extending from +40 km s-1 to +130 km s-1. Such large positive gas velocities are unexpected in the fourth quadrant of the Galaxy. The observed high ionization line profiles are converted into apparent optical depth, tau-a(upsilon), and apparent column density, N(a)(upsilon), profiles as functions of velocity. Comparison of the N(a)(upsilon) profiles between members within each doublet reveals that the observed Si IV and N V profiles are not significantly affected by unresolved saturated structure. The C IV N(a)(upsilon) profiles indicate that unresolved saturated structure may be present in the core of each component. The N(a)(upsilon) profiles are integrated over velocities between -150 km s-1 and +40 km s-1 to yield total column densities of log N = 14.10, 14.77, and 14.09 for Si IV, C IV, and NV, respectively. Kinematical modeling of the high ionization line profiles indicates that a simple model of halo gas corotating with the underlying disk can reproduce the observed line cores but not the high positive velocity C IV absorption. A class of models incorporating a breakdown of corotation at distances between 1.5 and 3.5 kpc away from the Galactic plane reproduces both the cores of the high ionization profiles and the high positive velocity C IV absorption. The detection of N V absorption strongly suggests the presence of gas with a temperature near 2 x 10(5) K toward HD 156359. The nonequilibrium cooling of gas in a Galactic fountain can explain the presence of both C IV and N V but falls short by a factor of 5 in producing the required amount of Si IV. Photoionized halo models are able to produce the observed amounts of Si IV and C IV but seem unable to produce sufficient N V. Although composite models are a possibility, the profile similarities for lines of N V, C IV, and Si IV suggests the creation of these ions by a common process.