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W.R. Oegerle (JHU), E.B. Jenkins (Princeton), R.L. Shelton (JHU), D.V. Bowen (Princeton), FUSE Team
We report the results of an initial FUSE survey of \ion{O}{6} \lambda1032 absorption along the lines of sight to eleven nearby white dwarfs, ten of which are within the Local Bubble (LB; d \la 100 pc). A goal of this survey is to investigate the possible formation of \ion{O}{6} in the conductive interfaces between cool (~104 K) clouds immersed in the presumably hot (106 K) gas within the LB. This mechanism is often invoked to explain the widespread presence of \ion{O}{6} throughout the Galactic disk.
We find no \ion{O}{6} absorption toward two stars, and the column densities along three additional sight lines are quite low; N(\ion{O}{6}) ~5 \times 1012 cm-2. In several directions, we observe rather broad, shallow absorption with N(\ion{O}{6}) ~1-2 \times 1013 cm-2. Models of conductive interfaces predict narrow profiles with N(\ion{O}{6}) \ga 1013 cm-2 per interface (Slavin 1989, Borkowski et al 1990), in the absence of a significant transverse magnetic field. Hence, our observations of weak \ion{O}{6} absorption indicate that conduction is being quenched, possibly by non-radial magnetic fields.
Alternatively, the gas within the LB may not be hot. Breitschwerdt & Schmutzler (1994) have proposed a model for the LB in which an explosive event within a dense cloud created rapid expansion and adiabatic cooling, resulting in a cavity containing gas with a kinetic temperature of T~ 50,000 K, but with an ionization state characteristic of much hotter gas. This model has a number of attractive features, but appears to predict significantly more \ion{O}{6} than we observe.
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.