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N. C. Sterling (University of Texas - Austin), B. D. Savage, P. Richter, D. Fabian (University of Wisconsin - Madison), K. R. Sembach (Johns Hopkins University)
We present \emph{Far Ultraviolet Spectroscopic Explorer} observations of the B0 III star HD177989 (l = 17.8o, b = -11.9o, d = 4.9 kpc). This line of sight passes through the high latitude outflow from the Scutum Supershell (GS 018-04+44), a structure that lies at a kinematic distance of ~ 3.5 kpc, and spans ~ 5o in diameter. The O VI \lambda1031.93 line is compared with STIS observations of Si IV, C IV, and N V to examine the ionizing mechanisms responsible for producing the high ion absorption, as well as to study the processes by which gas is expelled into the halo. The O VI profile spans a similar velocity range as the other highly ionized atoms, from -70 to +80 km s-1. Component fits reveal very broad absorption at the kinematic velocity of the Scutum shell, which differs from the narrow Si IV, C IV, and N V components, suggesting that these other species occupy a more confined region. It is possible that the O VI is present in hot gas from the shell interior that is diffusing into the high latitude region above it. The column densities in the Scutum Supershell component are N(Si IV) = (3.59 ±0.09) \times 1013, N(C IV) = (1.78 ± 0.04) \times 1014, N(N V) = (8.89 ±0.79) \times 1012, and N(O VI) = (7.76 ±0.75) \times 1013 cm-2. The corresponding column density ratios of N(C IV)/N(Si IV) = 4.96 ±0.17, N(C IV)/N(N V) = 20.0 ± 1.8, and N(C IV)/N(O VI) =2.29 ±0.23 show that while the C IV and Si IV columns are amplified in this region, the enhancement is not reflected in N V or O VI. We suggest that such ionic ratios and column densities could be produced by ~ 150-200 turbulent mixing layers lying in a fragmented medium above the polar cap of the superbubble. We note through a sight line comparison that although these absorption characteristics are similar to those near the center of Radio Loops I and IV, they differ considerably from those near the edges of the loops. It is apparent that the traits of high ion absorption in a supershell, as well as the ionizing mechanisms responsible, can vary among sight lines through the shell.
Financial support for this work was provided through NASA contract NAS5-32985.