Origins of the highly ionized gas along the line of sight toward HD 116852

Andrew J. Fox*, Blair D. Savage, Kenneth R. Sembach, Dirk Fabian, Philipp Richter, David M. Meyer, James Lauroesch, J. Christopher Howk

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


We present Hubble Space Telescope Imaging Spectrograph (HST/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, b = -16°.1. The sight line passes underneath the Sagittarius-Carina and the Norma-Centaurus spiral arms. The STIS E140H grating observations provide high-resolution (FWHM ≈ 2.7 km-1) spectra of the resonance doublets of Si iv, C iv, and N v. These data are complemented by medium-resolution (FWHM ≈ 20 km s -1 ) FUSE spectra of O vi. The integrated logarithmic column densities are log N(Si iv) = 13.60 ± 0.02, log N(C iv) = 14.08 ± 0.03, log N(N v) = 13.34-0.06+0.05, and log N(O vi) = 14.28 ± 0.01. We find evidence for three distinct types of highly ionized gas present in the data. First, two narrow absorption components are resolved in the Si iv and C iv profiles, at approximate LSR velocities of -36 and -10 km 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 these narrow components, as implied by their b-values, suggests that the gas is photoionized. The ratio of C iv to Si iv in these narrow components is low compared to the Galactic average. Second, we detect an intermediate-width component in C iv and Si iv, at 17 km s-1, which we propose could arise at the conductive interface at the boundary between a low column density neutral or weakly ionized cloud and the surrounding hot medium. Finally, a broad collisionally ionized component of gas responsible for producing the smooth N v and O vi profiles is observed; such absorption is also present to a lesser degree in the profiles of Si iv and C iv. The broad O vi absorption is observed at a velocity displaced from the broad C iv component by almost 20 km s-1, an amount large enough to suggest that the two ions may not coexist in the same physical location. If these two ions do exist together, then the ratio JV(C iv)/ N(O vi) is too low to be consistent with turbulent mixing layer models, but could be explained by radiative cooling or conductive heating models. Combining our results with high resolution observations of four other sight lines from the literature, we find an average C iv component frequency of 1.0 ± 0.25 kpc-1.

Original languageEnglish (US)
Pages (from-to)793-809
Number of pages17
JournalAstrophysical Journal
Issue number2 I
StatePublished - Jan 10 2003


  • Galaxy: halo
  • ISM: structure
  • Stars: individual (HD 116852)
  • Ultraviolet: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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