EDP Sciences
Free access
Volume 394, Number 2, November I 2002
Page(s) 691 - 699
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20021165

A&A 394, 691-699 (2002)
DOI: 10.1051/0004-6361:20021165

Far Ultraviolet Spectroscopic Explorer ( $\it FUSE$) observations of emitting and absorbing gas in the Local Interstellar Chimney

B. Y. Welsh1, 2, S. Sallmen1, D. Sfeir3, R. L. Shelton4 and R. Lallement5

1  Experimental Astrophysics Group, Space Sciences Laboratory, UC Berkeley, Berkeley, CA 94720, USA
2  Eureka Scientific, 2452 Delmer Street, Oakland, CA 94602-3017, USA
3  Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, USA
4  Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
5  Service d'Aéronomie du CNRS, 91371 Verrières-le-Buisson, France

(Received 12 March 2002 / Accepted 7 August 2002 )

We present Far Ultraviolet Spectroscopic Explorer ( $\it FUSE$) satellite measurements of the absorption and emission characteristics of interstellar gas associated with the Local Interstellar Chimney, which is an extension of the rarefied Local Bubble cavity that extends outward from the galactic disk towards the lower galactic halo. Far ultraviolet (FUV) diffuse background emission has been detected in the high ionization line of O VI ( $\lambda$1032 Å) for two lines-of-sight ( $l = 162.7^{\circ}$, $b
= +57.0^{\circ}$ ) and ( $l = 156.3^{\circ}$, $b
= +57.8^{\circ}$ ) at emission levels of $2500\pm700$ photons cm -2 s -1 sr -1 (LU) and $3300\pm1100$ LU respectively. These levels of O VI emission are very similar to those found for four other lines-of-sight sampled thus far by the $\it FUSE$ satellite, implying a fairly constant level of average O VI surface brightness emission at high galactic latitudes of about 2700 LU with a standard deviation of 450 LU.

These emission-line data are supplemented by FUV interstellar absorption line measurements taken towards the hot DA white dwarf star, REJ 1032+532 ( $l = 157.5^{\circ}$, $ b
= +53.2^{\circ}$ ), whose distance of 116 pc places it within the Local Bubble region. No high ionization interstellar O VI $\lambda$1032 Å absorption has been detected $(N({\rm O~VI}) < 13.0$ cm -2), which is consistent with the non-detections of interstellar C IV and Si IV absorption reported towards this star by Holberg et al. (1999a). Taken together, our FUV absorption and emission data may be explained by a scenario in which the O VI emission and absorption lines are $\it both$ formed at the conductive interface of the neutral boundary to the Local Bubble. For the presently sampled sight-lines we have found no correlation between the OVI emission line intensity and the associated 0.25 keV soft X-ray background flux as measured in the R1 and R2 bands by the $\it ROSAT$ satellite. The OVI line intensities also show no correlation with the soft X-ray background flux attributable to emission from the million degree K gas of the Local Hot Bubble as modeled by Kuntz & Snowden (2000). Any (new) model of the Local Bubble must now be able to explain (i) the low levels of variability in both the O VI emission-line intensity and the associated soft X-ray background flux for galactic sight-lines > |40|°, (ii) the observed pressure of $P/k\sim 10\,000$ cm -3 K for the local hot interstellar gas, and (iii) the paucity of high ionization absorption lines observed within the local ISM and the sudden increase in their measured column density for distances beyond the Local Bubble neutral boundary.

Key words: ISM: atoms -- ISM: bubbles -- Galaxy: solar neighbourhood

Offprint request: B. Y. Welsh, bwelsh@ssl.berkeley.edu

© ESO 2002