Properties of O vi Absorption in the Local Interstellar Medium

Abstract

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 ≥2 σ significance along 24 of 39 lines of sight. The column densities range from log N(O ) = 12.38 to 13.60 with a median value of 13.10. The line-of-sight volume density, n(O ) = N(O )/d, exhibits a large dispersion ranging from 0.68 × 10-8 to 13.0 × 10-8 cm-3 with an average value n(O ) = 3.6 × 10-8 cm-3 twice larger than that found for more distant sight lines in the Galactic disk. The Doppler parameter, b, for the ≥4 σ O VI detections ranges 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 × 105 K. log N(O ) is observed to correlate with b. The broader profiles are tracing a combination of O VI with T > 2.8 × 105 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 7-29 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 that the positive-velocity O VI is instead tracing cooling hot Local Bubble gas. The properties of the O VI absorption in the LISM are broadly consistent with the expectations of the theory of conductive interfaces caught in the old condensing phase and possibly in the young evaporative phase of their evolution.