Ultra-high-resolution observations of interstellar Na I and K I towards the Scorpius OB1 association

I present ultra-high-resolution ≈ observations of interstellar Na I and K I absorption lines towards three members of the Scorpius OB1 association (HD 152235, 152236 and 152249). These observations have, for the first time, resolved the intrinsic linewidths of most of the discrete absorption components present along these complex sightlines. The aims of the project were twofold: (i) to constrain the physical conditions prevailing in the highly blueshifted Sco OB1 shell components, and especially to search for evidence of active shocks within them; and (ii) to further constrain the low-velocity structure, where the Na I spectra are fully saturated but the unsaturated K I lines allow a clear resolution of the individual velocity components. The results of these analyses are discussed. Perhaps the most surprising result is the lack of any obvious correlation between the velocity dispersion of a velocity component (b-value) and its velocity. Specifically, the high-velocity shell components are generally found to be no broader than the low-velocity components attributed to foreground (often molecular) clouds, and cannot therefore be any hotter or more turbulent. Thus, with the possible exception of the most blueshifted component towards HD 152236, there is no evidence for active shocks in the shell components at present. However, consideration of the relative time-scales for post-shock cooling and grain surface adsorption indicates that shock processing in the past may still account for the low Na I/Ca II ratios of these components found in previous work