High-velocity interstellar absorption associated with the supernova
  remnant W28
  We present an analysis of moderately high resolution optical spectra obtained
for the sight line to CD-23 13777, an O9 supergiant that probes high velocity
interstellar gas associated with the supernova remnant W28. Absorption
components at both high positive and high negative velocity are seen in the
interstellar Na I D and Ca II H and K lines toward CD-23 13777. The high
velocity components exhibit low Na I/Ca II ratios, suggesting efficient grain
destruction by shock sputtering. High column densities of CH+, and high CH+/CH
ratios, for the components seen at lower velocity may be indicative of enhanced
turbulence in the clouds interacting with W28. The highest positive and
negative velocities of the components seen in Na I and Ca II absorption toward
CD-23 13777 imply that the velocity of the blast wave associated with W28 is at
least 150 km/s, a value that is significantly higher than most previous
estimates. The line of sight to CD-23 13777 passes very close to a well-known
site of interaction between the SNR and a molecular cloud to the northeast. The
northeast molecular cloud exhibits broad molecular line emission, OH maser
emission from numerous locations, and bright extended GeV and TeV gamma-ray
emission. The sight line to CD-23 13777 is thus a unique and valuable probe of
the interaction between W28 and dense molecular gas in its environs. Future
observations at UV and visible wavelengths will help to better constrain the
abundances, kinematics, and physical conditions in the shocked and quiescent
gas along this line of sight.
