16751

The Vela supernova remnant (SNR) is an ideal laboratory for studying the physical processes associated with SNR evolution in a highly inhomogeneous interstellar medium (ISM). Previous UV and optical absorption-line studies have revealed the remarkable nature of the highly disturbed interstellar gas interacting with the Vela SNR. The most noteworthy features are (1) the high positive and negative velocity absorption components seen in numerous directions comma (2) the strong absorption from excited fine-structure levels of neutral carbon detected in low and high velocity gas comma and (3) the many different absorption components that exhibit significant ongoing temporal variations. However comma our understanding of the cloud-shock interactions taking place in the Vela region is far from complete. Here comma we propose the first large-scale systematic investigation of shocked gas in the Vela SNR to employ the high resolution comma high sensitivity comma and extensive wavelength coverage provided by HST-STIS. We will observe eight stars clustered along a ridge of high pressure gas near the western edge of the northern X-ray bright region in the Vela SNR. The high resolution and broad wavelength coverage of the STIS spectra will allow us to examine the radial velocities comma gas densities comma thermal pressures comma ionization fractions comma and dust depletions in individual clouds along each line of sight. Five of our targets have archival STIS and-or GHRS observations comma which will allow us to determine how the physical conditions have changed over time. The result will be the most comprehensive analysis to date of spatial and temporal variations in the physical conditions of clouds shocked by the Vela SNR.