O vi IN THE LOCAL INTERSTELLAR MEDIUM

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. and Savage & Lehner, 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 ~ 300,000 K 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.

[1]  Martin Barstow,et al.  A SEARCH FOR HIDDEN WHITE DWARFS IN THE ROSAT EXTREME ULTRAVIOLET SURVEY , 1997 .

[2]  Dan McCammon,et al.  ROSAT Survey Diffuse X-Ray Background Maps. II. , 1997 .

[3]  M. Freyberg,et al.  A Catalog of Soft X-Ray Shadows, and More Contemplation of the ¼ keV Background , 2000 .

[4]  M. Barstow,et al.  Hubble Space Telescope Ultraviolet Spectroscopy of Two Hot White Dwarfs , 1997 .

[5]  R. Willingale,et al.  Evidence for a far-ultraviolet spacecraft glow in the ROSAT wide-field camera , 1994 .

[6]  T. P. Sasseen,et al.  Observations of Diffuse Extreme-Ultraviolet Emission with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) , 2004, astro-ph/0411121.

[7]  J. Vallerga.  The Stellar Extreme-Ultraviolet Radiation Field , 1998 .

[8]  D. York.  Highly ionized atoms observed with Copernicus , 1974 .

[9]  D. Morton,et al.  Atomic Data for Resonance Absorption Lines. III. Wavelengths Longward of the Lyman Limit for the Elements Hydrogen to Gallium , 2003 .

[10]  S. Vennes.  Properties of Hot White Dwarfs in Extreme-Ultraviolet/Soft X-Ray Surveys , 1999 .

[11]  T. Cravens,et al.  Heliospheric X-ray Emission Associated with Charge Transfer of the Solar Wind with Interstellar Neutrals , 2000, The Astrophysical journal.

[12]  C. Heiles.  Whence the Local Bubble, Gum, Orion? GSH 238+00+09, A Nearby Major Superbubble toward Galactic Longitude 238° , 1998 .

[13]  Donald P. Cox,et al.  Modeling the local bubble , 1997 .

[14]  John R. Thorstensen,et al.  Hot White Dwarfs in the Extreme-Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions , 1998 .

[15]  R. Lallement,et al.  OVI in the Local Interstellar Medium: FUSE observations towards B-stars within 200 pc , 2008 .

[16]  S. Snowden,et al.  Deconstructing the Spectrum of the Soft X-Ray Background , 2000 .

[17]  G. Field,et al.  Detection of an Anisotropic Soft X-ray Background Flux , 1968, Nature.

[18]  F. Crifo,et al.  3D mapping of the dense interstellar gas around the Local Bubble , 2003 .

[19]  Dan McCammon,et al.  The soft X-ray diffuse background , 1983 .

[20]  M. Wolleben,et al.  A New Model for the Loop I (North Polar Spur) Region , 2007, 0704.0276.

[21]  R. Shelton.  Simulations of Supernova Remnants in Diffuse Media and Their Application to the Lower Halo of the Milky Way. I. The High-Stage Ions , 1998 .

[22]  R. Lallement,et al.  The heliospheric soft X-ray emission pattern during the ROSAT survey: Inferences on Local Bubble hot gas , 2004 .

[23]  E. Jenkins,et al.  A survey with Copernicus of interstellar O VI absorption , 1974 .

[24]  Martin A. Barstow,et al.  RE 1738 + 665: the hottest DA white dwarf detected by ROSAT , 1994 .

[25]  Jeffrey L. Linsky,et al.  The Structure of the Local Interstellar Medium. IV. Dynamics, Morphology, Physical Properties, and Implications of Cloud-Cloud Interactions , 2007, 0709.4480.

[26]  R. Lallement,et al.  New 3D gas density maps of NaI and CaII interstellar absorption within 300 pc , 2009, 0912.3040.

[27]  Thomas E. Cravens,et al.  Comet Hyakutake x‐ray source: Charge transfer of solar wind heavy ions , 1997 .

[28]  D. Cox,et al.  Flux-Tube Dynamics and a Model for the Origin of the Local Fluff , 2003 .

[29]  M. A. Barstow,et al.  Hubble Space Telescope spectroscopy of the Balmer lines in Sirius B , 2005, astro-ph/0506600.

[30]  Paul P. Plucinsky,et al.  Progress on Establishing the Spatial Distribution of Material Responsible for the 1/4 keV Soft X-Ray Diffuse Background Local and Halo Components , 1998 .

[31]  B. Savage,et al.  Properties of O VI Absorption in the Local Interstellar Medium , 2005, astro-ph/0509458.

[32]  M. Barstow,et al.  Far-Ultraviolet Space Telescope Imaging Spectrograph Spectra of the White Dwarf REJ 1032+532. II. Stellar Spectrum , 1999 .

[33]  Lunar,et al.  Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs , 2005, astro-ph/0507638.

[34]  M. Burleigh,et al.  RE J0317 – 853: the hottest known highly magnetic DA white dwarf , 1995 .

[35]  R. Shelton.  Simulations of Supernova Remnants in Diffuse Media. II. Three Remnants and Their X-Ray Emission , 1999, astro-ph/9904392.

[36]  I. Crawford,et al.  High-resolution observations of interstellar Na I and Ca II towards the southern opening of the 'Local Interstellar Chimney': probing the disc-halo connection , 2002 .

[37]  S. Sallmen,et al.  Probing the inner halo and IVC gas through the Local Interstellar Chimney , 2004 .

[38]  M. Burleigh,et al.  Heavy‐element abundance patterns in hot DA white dwarfs , 2003, astro-ph/0301519.

[39]  Peter Thejll,et al.  Hot White Dwarfs in the Extreme Ultraviolet Explorer Survey. II. Mass Distribution, Space Density, and Population Age , 1997 .

[40]  W. Landsman,et al.  THE HOT WHITE DWARF COMPANIONS OF HR 1608, HR 8210, AND HD 15638 , 1993 .

[41]  M. J. Mumma,et al.  Discovery of X-ray and Extreme Ultraviolet Emission from Comet C/Hyakutake 1996 B2 , 1996, Science.

[42]  David S. Finley,et al.  The Temperature Scale and Mass Distribution of Hot DA White Dwarfs , 1997 .

[43]  W. Oegerle,et al.  A Survey of O VI Absorption in the Local Interstellar Medium , 2004, astro-ph/0411065.

[44]  Interaction of the local bubble with its environment , 1998 .

[45]  A. Dalgarno,et al.  The Solar Wind Charge-eXchange Contribution to the Local Soft X-ray Background , 2008, 0805.3212.

[46]  David A. H. Buckley,et al.  An EUV-selected sample of DA white dwarfs from the ROSAT All-Sky Survey — I. Optically derived stellar parameters , 1997 .

[47]  Edward B. Jenkins,et al.  Spectrophotometric Results from the Copernicus Satellite. III. Ionization and Composition of the Intercloud Medium , 1973 .

[48]  Dan McCammon,et al.  First Maps of the Soft X-Ray Diffuse Background from the ROSAT XRT/PSPC All-Sky Survey , 1995 .

[49]  A. Levan,et al.  A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations , 2003, astro-ph/0306077.

[50]  P. Bergeron,et al.  PRECISE ESTIMATES OF THE SYNTHETIC SPECTRAL DISTANCES TO DA WHITE DWARFS , 2008 .

[51]  J. Raymond,et al.  THE SOLAR WIND CHARGE-TRANSFER X-RAY EMISSION IN THE 1/4 keV ENERGY RANGE: INFERENCES ON LOCAL BUBBLE HOT GAS AT LOW Z , 2008, 0812.3823.

[52]  Martin A. Barstow,et al.  THE EUVE AND IUE SPECTRA OF THE HOT DA WHITE-DWARF GD-659 , 1995 .

[53]  E. B. Jenkins,et al.  Coronal gas in the Galaxy. I - A new survey of interstellar O VI , 1978 .

[54]  Space Telescope Science Institute,et al.  CalFUSE v3: A Data-Reduction Pipeline for the Far Ultraviolet Spectroscopic Explorer , 2007, 0704.0899.

[55]  U. Michigan,et al.  An extended fuse survey of diffuse O VI emission in the interstellar medium , 2006, astro-ph/0604408.

[56]  Nigel Bannister,et al.  Circumstellar features in hot DA white dwarfs , 2003 .

[57]  Edward M. Sion,et al.  A Catalog of Spectroscopically Identified White Dwarfs , 1987 .

[58]  Martin A. Barstow,et al.  A High-Dispersion Spectroscopic Survey of the Hot White Dwarfs: The IUE NEWSIPS SWP Echelle Data Set , 1998 .

[59]  J. Thorstensen,et al.  Discovery of a White Dwarf Companion (MS0354.6-3650 = EUVE J0356-366) to a G2V Star , 1996 .