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R. Sankrit, W.P. Blair, K.R. Sembach, R.L. Shelton (JHU)
We have used the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to obtain an observation of the Vela supernova remnant (SNR) in the southern Milky Way. At a distance of only ~250 pc, Vela suffers little foreground extinction and is an excellent far-UV target. Rather than observing a bright, optical filament as done in most previous far-UV studies, we observed a region based on bright soft X-ray surface brightness. The region observed is close to the so-called ``face-on shock" position that was observed by Raymond et al. (1997, ApJ, 482, 881) with the Hopkins Ultraviolet Telescope (HUT). The FUSE observation used a 30'' square aperture and a total integration of 11 ksec, primarily during orbital night. The filled aperture provides a diffuse source resolution of 0.3 Å, or ~100 \rm km ~ s-1.
The FUSE spectrum is dominated by strong O~VI \lambda\lambda1031.9, 1037.6 doublet emission lines. The lines are broadened only a small amount above the filled aperture resolution, with FWHM ~130 \rm km ~ s-1. The lines are non-gaussian, however, with weak, broad wings extending out to 500 \rm km ~ s-1 FW0I, which are especially visible on the \lambda1037.6 line. The short wavelength wing of the stronger \lambda1031.9 line appears to be weaker or missing, perhaps indicating some differential overlying absorption. The ratio of line intensities for the doublet is close to, but perhaps slightly below, the optically thin 2:1 ratio.
Also observed in the FUSE spectrum is C~III \lambda977.0, but at a lower flux level than seen in the nearby region observed with HUT. The overall velocity width is identical to O~VI \lambda1031.9, but with two peaks separated by 175 \rm km ~ s-1. This appearance could be due to two separate velocity components or, more likely, overlying self-absorption in C~III that reduces the flux in the central core of the line. Very weak S~VI \lambda933.4 is also detected.
We will discuss the implications of this new data set, including models of the expected emissions and overlying absorption. This work is supported by NASA Contract NAS5-32985 to the Johns Hopkins University.
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