High Velocity , High Excitation Neutral

HD72089 is situated behind the Vela supernova remnant, and the interstellar absorption lines in the spectrum of this star are remarkable for two reasons. First, there are 6 distinct velocity components that span the (heliocentric) velocity range ?60 to +121 km s ?1 in the lines of Na I and Ca II. Second, two of the components at high velocity, one at +85 km s ?1 and another at +121.5 km s ?1 , have densities that are large enough to produce observable lines from neutral carbon. The gas moving at +121.5 km s ?1 has such a large pressure that the excited ne-structure levels of the ground electronic state of C I are collisionally populated nearly in proportion to their level degeneracies. This high velocity gas exhibits unusually low column densities of Mg I and Na I, compared to that of C I. We propose that the +121.5 km s ?1 component represents gas that has cooled and recombined in a zone that follows a shock driven into a cloud by the very recent passage of a SN blast wave. A representative preshock density of n H 13 cm ?3 and velocity v s = 100 km s ?1 is indicated by the strength of diiuse O III] emission lines seen in directions very near HD72089. The strong collisional population of excited C I and apparent absence of excited levels of O I give a most favorable t to the conditions 1000 < n H < 2900 cm ?3 over a temperature range 300 < T < 1000K. The fact that the compression is not substantially more than this indicates that the preshock gas may have had an embedded, transverse magnetic eld with a strength B > 1G. The large dynamical pressure of the supernova blast wave that would be needed to create the cloud shock that we describe implies that the energy of the supernova was 8 10 51 erg, if the Vela remnant is 500 pc away. We can bring this value much closer to typical supernova energies E < 10 51 erg if the distance to the remnant is revised downward by at least a factor of 2.