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J. Zsargo (JHU), K.R. Sembach (STScI), J.C. Howk (CASS, UCSD), B.D. Savage (U. Wisconsin)
Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactic halo stars are studied to determine the amount of \ion{O}{6} in the Galactic halo between ~0.3 and ~10 kpc from the Galactic mid-plane. Strong \ion{O}{6} \lambda1031.93 absorption was detected toward 21 stars, and a reliable 3 \sigma upper limit was obtained toward HD~97991. The weaker member of the \ion{O}{6} doublet at 1037.62~Å\ could be studied toward only six stars. The observed columns are reasonably consistent with a patchy exponential \ion{O}{6} distribution with a mid-plane density of 1.7\times10-8 cm-3 and scale height between 2.3 and 4 kpc. We do not see clear signs of strong high-velocity components in \ion{O}{6} absorption along the Galactic sight lines, which indicates the general absence of high velocity \ion{O}{6} within 2-5 kpc of the Galactic mid-plane. The correlation between the \ion{H}{1} and \ion{O}{6} intermediate velocity absorption is also poor. The \ion{O}{6} velocity dispersions are much larger than the value of ~18 km/s expected from thermal broadening for gas at T ~ 3\times105 K, the temperature at which \ion{O}{6} is expected to reach its peak abundance in collisional ionization equilibrium. Turbulence, inflow, and outflow must have an effect on the shape of the \ion{O}{6} profiles. Kinematical comparisons of \ion{O}{6} with \ion{Ar}{1} reveal that 9 of 21 sight lines are closely aligned in LSR velocity (|\Delta VLSR| \leq5 km/s ), while 8 of 21 exhibit significant velocity differences (|\Delta VLSR| \geq 15 km/s ). This dual behavior may indicate the presence of two different types of \ion{O}{6}-bearing environments toward the Galactic sight lines. Comparison of \ion{O}{6} with other highly-ionized species suggests that the high ions are produced primarily by cooling hot gas in the Galactic fountain flow, and that turbulent mixing also has a significant contribution. The role of turbulent mixing is most important toward sight lines that sample supernova remnants like Loop I and IV. We are also able to show that the \ion{O}{6} enhancement toward the Galactic center region that was observed in the FUSE survey of complete halo sight lines (Savage et al.) is likely associated with processes occurring near the Galactic center.
This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support to U. S. participants has been provided by NASA contract NAS5-32985.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.