Properties of O VI Absorption in the Local Interstellar Medium

The measurement of absorption over short distances in the local interstellar medium (LISM) provides an excellent opportunity to study the properties of O VI absorption and its kinematic relationship to absorption by tracers of cool and warm gas. We report on the properties of LISM O VI absorption observed with 20 km s −1 resolution Far Ultraviolet Spectroscopic Explorer (FUSE) observations of 39 white dwarfs (WDs) ranging in distance from 37 to 230 pc with a median distance of 109 pc. LISM O VI is detected with ≥ 2σ significance along 24 of 39 lines of sight. The column densities range from log N(O VI) = 12.38 to 13.60 with a median value of 13.10. The line of sight volume density, n(O VI) = N(O VI)/d, exhibits a large dispersion ranging from (0.68 to 13.0)×10 −8 cm −3 with an average value n(O VI) = 3.6 × 10 −8 cm −3 twice larger than found for more distant sight lines in the Galactic disk. The Doppler parameter, b, for the ≥ 4σ O VI detections range from 15.0±3.5 to 36.2±7.3 km s −1 , with average and dispersion of 23.0±5.6 km s −1. The narrowest profiles are consistent with thermal Doppler broadening of O VI near its temperature of peak abundance, 2.8 × 10 5 K. log N(O VI) is observed to correlate with b. The broader profiles are tracing a combination of O VI with T > 2.8 × 10 5 K, multiple component O VI absorption either in interfaces or hot bubbles, and/or the evaporative outflow of conductive interface gas. Comparison of the average velocities of O VI and C II absorption reveals 10 cases where the O VI absorption is closely aligned with the C II absorption as expected if the O VI is formed in a condensing interface between the cool and warm absorption traced by C II (and also O I and C III) and a hot exterior gas. The comparison also reveals 13 cases where O VI absorption is displaced to positive velocity by 7 to 29 km s −1 from the average velocity of C II. The positive velocity O VI is mostly found in the north Galactic hemisphere. The positive velocity O VI appears to be tracing the evaporative flow of O VI from a young interface between warm gas and a hot exterior medium. However, it is possible – 2 – …