The interstellar clouds of Adams and Blaauw revisited : an HI absorption study - II

In the preceding paper (paper I), we presented HI absorption spectra towards radio sources very close to the lines of sight towards 25 bright stars against which optical absorption spectra had been obtained earlier. In this paper we analyse the results and draw some conclusions. To summarize briefly, in most cases we found HI absorption at velocities corresponding to the optical absorption features provided one restricted oneself to velocities < ∼ 10 km s −1. At higher velocities we did not detect any HI absorption down to an optical depth limit of 0.1 (except in four cases which we attribute to gas in systematic motion rather than clouds in random motion). After discussing various scenarios, we suggest that this trend should perhaps be understood in terms of the high velocity interstellar clouds being accelerated, heated and ablated by expanding supernova remnants.