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

¹ Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94720, USA e-mail: bwelsh@ssl.berkeley.edu
² Service d'Aéronomie du CNRS, 91371 Verrières-le-Buisson, France

Received: 19 May 2008
Accepted: 4 September 2008

Abstract

Aims. We present far ultraviolet (FUV: 990-1082 Å) spectroscopic observations of 17 early-type stars located within 200 pc of the Sun using the NASA FUSE satellite in order to search for interstellar absorption from the important high ion of OVI (1032 Å).

Methods. These spectral data were fit with a local stellar continuum such that measurement of, or upper limits to, the equivalent width of the OVI absorption line could be made. The resultant line profiles were then fit with an absorption model to determine the OVI column density along each sight-line.

Results. OVI was detected towards 5 of the 17 targets, all towards stars with distances >80 pc. The average volumic density, n_OVI, along these 5 sight-lines was found to be 5.2  10^-8 cm^-3. This value, in agreement with measurements of OVI in the local interstellar medium (ISM) by other authors, is measurably higher than that typically found throughout the general ISM. The location of these 5 stars places them just within, or just at, the neutral boundary to the Local Cavity. If this cavity contains a hot million degree K gas, then it seems likely that the presently observed OVI absorption is formed in conductive interfaces within a transition layer between hot and cooler gas. We provide tentative evidence that the spatial distribution of OVI absorbers and emitters exhibits a loose correlation with that of the million degree K soft X-ray background (SXRB) emission at galactic latitudes of +30° < b < -30°. In such a scenario very little SXRB emission is required in the galactic plane, in agreement with current theoretical models that invoke solar wind charge exchange as the origin for the majority of this background signal. Alternately, if the Local Cavity is an old SNR of age ~3 million yr, it is possible that the central regions may no longer contain soft X-ray emitting gas, with appreciable amounts of OVI ions lying within warm (T ~ 10⁶ K) gas at the periphery of the bubble cavity.