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A. J. Fox, B. D. Savage, D. Fabian, P. Richter (UW - Madison), K. R. Sembach (STScI), D. M. Meyer, J. Lauroesch (Northwestern), J. C. Howk (Johns Hopkins)
We present Space Telescope Imaging Spectrograph (STIS) and Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of high ion interstellar ultraviolet absorption along the sight line to HD 116852. At a distance of 4.8 kpc, HD 116852 is an O9 III star lying in the low Galactic halo, -1.3 kpc from the plane of the Galaxy in the direction l = 304.9\circ, b = -6.1\circ. The sight line passes underneath the Sagittarius-Carina and the Norma-Centaurus spiral arms. The STIS E140H grating observations provide high-resolution (FWHM \approx 2.7km s-1) spectra of the resonance doublets of \ion{Si}{4}, \ion{C}{4} and \ion{N}{5}. These data are complemented by medium-resolution (FWHM \approx 20km s-1) FUSE spectra of \ion{O}{6}. We find evidence for three distinct types of absorbing gas present in the data. First, two narrow absorption components are resolved in the \ion{Si}{4} and \ion {C}{4} profiles, at approximate LSR velocities of -36 and -10km s-1. These narrow components appear to be produced in gas associated with the Norma and Sagittarius spiral arms, at approximate z-distances of -1.0 and -0.5 kpc respectively. The temperature of the gas in in these narrow components, as implied by their b-values, suggests that the gas is photoionized. The ratio of \ion{C}{4} to \ion{Si}{4} in these narrow components is low compared to the Galactic average. Second, we detect an intermediate-width component in \ion{C}{4} and \ion{Si}{4}, at +17km s-1, which we propose could arise at the conductive interface at the boundary between a dense cloud and the surrounding medium. Finally, a broad collisionally ionized component of gas responsible for producing the smooth \ion{N}{5} and \ion{O}{6} profiles is observed; such absorption is also present to a lesser degree in the profiles of \ion{Si}{4} and \ion {C}{4}. The broad \ion{O}{6} absorption is observed at a velocity displaced from the \ion{C}{4} profile by almost 20km s-1, an amount large enough to suggest that the two ions may not co-exist in the same physical location. If these two ions do exist together, then the ratio N(\ion{C}{4})/N(\ion{O}{6}) is too low to be consistent with turbulent mixing layer models, but could be explained by radiative cooling or conductive heating models.
We appreciate financial support from NASA contract NAS5-32985.
The author(s) of this abstract have provided an email address for comments about the abstract: fox@astro.wisc.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.