Unveiling Vela -- Variability of Interstellar Lines in the Direction of
  the Vela Supernova Remnant III. Na D and Ca II K
  High-resolution optical spectra were obtained in 2017-2019 with The Southern
African Large Telescope of fifteen stars in the direction of the Vela supernova
remnant. Interstellar Ca ii H and K and Na i D lines are discussed in this
paper. In particular, the line profiles are compared with profiles at a
comparable spectral resolution obtained in 1993-1996 by Cha & Sembach. Ten of
the lines of sight show changes to one or more of the components in that line
of sight. Changes include small changes (1-2 km/s) in radial velocity and/or
increases/decreases in equivalent width over the two decades between the
periods of observation. Changes are more obvious in the Ca K line than in the
Na D lines. These changes are attributed to gas disturbed by interactions
between the supernova ejecta and the surrounding interstellar medium. A
representative timescale may be 20-50 years. Small-scale variations in line
profiles across the face of the remnant suggest, as previously remarked, that a
linear scale for interactions is a small fraction of the 40 pc size of the
present remnant.
