2002ApJ...567..354S

We present Far Ultraviolet Spectroscopic Explorer observations of the B0 III star HD 177989 (l=17°.8, b=-11°.9, 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° in diameter. The O VI λ1031.93 line is compared with Space Telescope Imaging Spectrograph (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(SiIV)=(3.59±0.09)x10¹³, N(CIV)=(1.78±0.04)x10¹⁴, N(NV)=(8.89±0.79)x10¹², and N(OVI)= (7.76±0.75)x10¹³ cm^–2. The corresponding column density ratios of N(CIV)/N(SiIV)=4.96±0.17, N(CIV)/N(NV)=20.0±1.8, and N(CIV)/N(OVI)=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.