A Survey of O VI Absorption in the Local Interstellar Medium

We report the results of a survey of O VI λ1032 absorption along the lines of sight to 25 white dwarfs in the local interstellar medium (LISM) obtained with FUSE. We find that interstellar O VI absorption along all sight lines is generally weak, and in a number of cases, completely absent. No O VI absorption was detected with significance greater than 2 σ for 12 of the 25 stars, where the 1 σ uncertainty is ~4 mÅ, equivalent to an O VI column density of ~3 × 1012 cm-2. Of the remaining stars, most have column densities N(O ) < 1013 cm-2, and no column densities exceed 1.7 × 1013 cm-2. For lines of sight to hot (Teff > 40,000 K) white dwarfs, there is some evidence that the O VI absorption may be at least partially photospheric or circumstellar in origin. We interpret the "patchy" distribution of O VI absorption in terms of a model in which O VI is formed in evaporative interfaces between cool clouds and the hot, diffuse gas in the Local Bubble (LB). If the clouds contain tangled or tangential magnetic fields, then thermal conduction will be quenched over most of the cloud surface, and O VI will be formed only in local "patches" where conduction is allowed to operate. We find an average O VI space density in the LISM of 2.4 × 10-8 cm-3, which is similar to, or slightly larger than, the value in the Galactic disk over kiloparsec scales. This local density implies an average O VI column density of ~7 × 1012 cm-2 over a path length of 100 pc within the LB. The O VI data presented here appear to be inconsistent with the model proposed by Breitschwerdt & Schmutzler, in which highly ionized gas at low kinetic temperature (~50,000 K) permeates the LB. Our survey results are consistent with the supernova-driven cavity picture of Cox & Smith for the LB, and in particular, the recent model by Cox & Helenius for the creation of cool clouds in the LB by magnetic flux tubes and their subsequent magnetic shielding from conduction.