FUSE Observations of O VI Overlying the Scutum Supershell

We present Far Ultraviolet Spectroscopic Explorer observations of the B0 III star HD 177989 ($l = 17.8^{rm \o}, b = -11.9^{\rm o}, d$ = 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 ~ 5$^{\rm o}$ in diameter. The O VI $\lambda$1031.93 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. 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 density ratios 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 the 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.