Ultraviolet Absorption Lines from High-Velocity Gas in the Vela Supernova Remnant: New Insights from Space Telescope Imaging Spectrograph Echelle Observations of HD 72089

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Abstract
The star HD 72089 is located behind the Vela supernova remnant and shows a complex array of high- and low-velocity interstellar absorption features arising from shocked clouds. A spectrum of this star was recorded over the wavelength range 1196.4-1397.2 Å at a resolving power of λ/Δλ = 110,000 and a signal-to-noise ratio of 32 by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We have identified seven narrow components of C I and have measured their relative populations in excited fine-structure levels. Broader features at heliocentric velocities ranging from -70 to 130 km s-1 are seen in C II, N I, O I, Si II, S II, and Ni II. In the high-velocity components, the unusually low abundances of N I and O I, relative to S II and Si II, suggest that these elements may be preferentially ionized to higher stages by radiation from hot gas immediately behind the shock fronts.
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