A Survey of OviAbsorption in the Local Interstellar Medium

We report the results of a survey of OVI 1032 absorption along the lines ofsight to 25 white dwarfs in the local interstellar medium (LISM) obtained withthe Far Ultraviolet Spectroscopic Explorer (FUSE). We find that interstellarOVI absorption along all sightlines is generally weak, and in a number ofcases, completely absent. No OVI absorption was detected with significancegreater than 2 sigma for 12 of the 25 stars, where the 1 sigma uncertainty is 4mA, equivalent to an OVI column density of ~3 x 10^12 cm^-2. Of the remainingstars, most have column densities N(OVI) < 10^13 cm^-2 and no column densitiesexceed 1.7 x 10^13 cm^-2. For lines of sight to hot (T_eff > 40,000 K) whitedwarfs, there is some evidence that the OVI absorption may be at leastpartially photospheric or circumstellar in origin. We interpret the "patchy"distribution of OVI absorption in terms of a model where OVI is formed inevaporative interfaces between cool clouds and the hot, diffuse gas in theLocal Bubble (LB). If the clouds contain tangled or tangential magnetic fields,then thermal conduction will be quenched over most of the cloud surface, andOVI will be formed only in local "patches" where conduction is allowed tooperate. We find an average OVI space density in the LISM of 2.4 x 10^-8 cm^-3,which is similar to, or slightly larger than, the value in the Galactic diskover kpc scales. This local density implies an average OVI column density of ~7x 10^12 cm^-2 over a path length of 100 pc within the LB. The OVI datapresented here appears to be inconsistent with the model proposed byBreitschwerdt & Schmutzler (1994), in which highly ionized gas at low kinetictemperature (~50,000 K) permeates the LB. Our survey results are consistentwith the supernova-driven cavity picture of Cox & Smith (1974).Comment: 40 pages, 9 figures, AASTeX, submitted to Ap