HIGH-VELOCITY GAS IN THE VELA SUPERNOVA REMNANT

We present high-resolution, high signal-to-noise spectra of the interstellar Na I and Ca II lines toward seven stars in the direction of the Vela supernova remnant. We show that high-velocity gas having a complex velocity structure exists toward these stars. A plot of the Log N(Ca II)/N(Na I) ratio as a function of velocity shows that the ratio increases monotonically as seen for local and low-density ISM sight lines, but there appears to be a decrease of about 0.5 dex in the ratio near ≈100 km s-1 in the direction of the Vela SNR and other high-velocity gas regions. Several of the highest-velocity components toward the Vela stars have unusually small values of N(Ca II)/N(Na I) compared to moderate velocity components and provide interesting counterexamples to the "Roulty-Spitzer" effect. We suggest that these components are sites of post-shocked recombining gas in which pre-existing dust grains have not yet been completely destroyed. For several components, we detect time variability in the Na I and Ca II absorption line strengths, and for the high-velocity (vLSR≈+190 km s-1) component toward HD 72997 we also see systematic velocity shifts in both the Na I and Ca II lines that imply the gas is being accelerated at the rate of ≈1-2 km s-1 yr-1. Similarities in the structure, strength, and velocities of the high-velocity gas absorption toward three stars separated by less than 6′ in IC 2395 suggest that some of the high-velocity gas in the Vela region may be accelerated by events other than the initial explosion that resulted in pulsar PSR 0833-045. © 1995 American Astronomical Society.