O VI in the local interstellar medium

Abstract

We report the results of a search for O VI absorption in the spectra of 80 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In approximately 35% of cases (where photospheric material is detected), the velocity differences between the interstellar and photospheric components were beneath the resolution of the FUSE spectrographs. Therefore, in 65% of these stars the interstellar and photospheric contributions could be separated and the nature of the O VI component unambiguously determined. Furthermore, in other examples, where the spectra were of a high signal-to-noise, no photospheric material was found and any O VI detected was assumed to be interstellar. Building on the earlier work of Oegerle et al. (2005) and Savage & Lehner (2006), we have increased the number of detections of interstellar O VI and, for the first time, compared their locations with both the soft X-ray background emission and new detailed maps of the distribution of neutral gas within the local interstellar medium. We find no strong evidence to support a spatial correlation between O VI and SXRB emission. In all but a few cases, the interstellar O VI was located at or beyond the boundaries of the local cavity. Hence, any T ~ 300,000K gas responsible for the O VI absorption may reside at the interface between the cavity and surrounding medium or in that medium itself. Consequently, it appears that there is much less O VI-bearing gas than previously stated within the inner rarefied regions of the local interstellar cavity.

The trouble with the Local Bubble

“…In the new model this can occur at locations within the Local Chimney where infalling highly ionized million degree halo gas meets the cooler photoionized filler gas of the Local Cavity. The new O VI results support this scenario for sight-line distances < 100 pc (Barstow et al 2008;Barstow et al 2009). For sight-lines in the galactic disk that extend beyond the confines of the Local Cavity, O VI absorption can occur at interfaces between more distant hot bubble cavities and their surrounding neutral gas shells Bowen et al 2008).…”

“…The spatial distribution of O VI absorption shown in Figure 4 can be explained by our presently proposed model if local O VI absorption (d < 100 pc) only occurs in only two environments: (i) at the interfaces between the Local Cavity and the overlying galactic halo (at the 'top' and 'bottom' of the Local Chimney) and (ii) at the neutral walls between the Local Cavity and nearby hot bubbles. The former distribution of O VI absorption is confirmed by the new OVI results in which O VI is only detected at high latitudes for sources with distance > 60pc (Barstow et al 2008;Barstow et al 2009). The distribution of O VI at low latitudes is consistent with its formation at the interfaces between distant (d > 80pc) bubble cavities and their surrounding neutral gas shells (Bowen et al 2008;.…”

“…A new detailed analysis of the radial velocities of interstellar and (when present) stellar absorption lines in a sample of ∼ 100 absorption spectra recorded by FUSE for hot white dwarfs within 400pc of the Sun has recently been made available (Barstow et al 2008;Barstow et al 2009). For 2/3 of the sight-lines having significant OVI features the velocity of the OVI absorption is found to lie within the velocity uncertainty (i.e.…”

“…The idea for our new picture of the Local Cavity was largely driven by an interpretation of the results from a recent re-analysis of O VI absorption profiles detected by the FUSE satellite towards local hot white dwarfs (Barstow et al 2008;Barstow et al 2009).…”

OVI in the local interstellar medium:FUSEobservations towards B-stars within 200 pc

“…A field strength of 8 μG has recently been determined for the boundary wall to the LC, thus supporting the idea of an internally high magnetic pressure (Andersson & Potter 2006). We note that Barstow et al (2008) have recently carried out a reanalysis of the FUSE absorption data recorded towards ∼100 hot white dwarf stars within 200 pc, using a detailed comparison of the radial velocities of the interstellar and (when present) stellar absorption lines. In many cases, they find the velocity differences between the interstellar and photospheric components are below the resolution of the FUSE spectrographs.…”