Properties of O VI Absorption in the Local Interstellar Medium
  We report on the properties of LISM O VI absorption observed with 20 km/s resolution 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 >2sigma 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)x10(-8) cm(-3) with an average value 3.6x10(-8) cm(-3) twice larger than found for more distant sight lines in the Galactic disk. The narrowest profiles are consistent with thermal Doppler broadening of O VI near its temperature of peak abundance, 2.8x10(5) K. 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 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 from the average velocity of C II. 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 the positive velocity O VI is instead tracing cooling hot Local Bubble (LB) 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. 
Blair D. Savage and Nicholas Lehner