We present Far Ultraviolet Spectroscopic Explorer observations of the B0 III star HD 177989 ( l = 17.8 ^ { o } ,b = -11.9 ^ { 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 \sim 3.5 kpc , and spans \sim 5 ^ { 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 ^ { -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 \pm 0.09 ) \times 10 ^ { 13 } , N ( C iv ) = ( 1.78 \pm 0.04 ) \times 10 ^ { 14 } , N ( N v ) = ( 8.89 \pm 0.79 ) \times 10 ^ { 12 } , and N ( O vi ) = ( 7.76 \pm 0.75 ) \times 10 ^ { 13 } cm ^ { -2 } .
The corresponding column density ratios of N ( C iv ) /N ( Si iv ) = 4.96 \pm 0.17 , N ( C iv ) /N ( N v ) = 20.0 \pm 1.8 , and N ( C iv ) /N ( O vi ) = 2.29 \pm 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 \sim 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 .