Observations of Ubiquitous Small-Scale Structure in the Diffuse Interstellar Medium

Abstract

We present results of a study of the Na I D interstellar absorption toward 17 binary and/or common proper-motion systems (including two triples). The stars range in spectral type from O6 to A5, lie at distances between 85 and 1200 pc, and have stellar separations between 480 and 29,000 AU. We compare the Na I absorption present in each of the two (or three) lines of sight and find that the line strength and/or profile varies for all 17 of the systems examined. We infer that small-scale structure in clouds containing Na I is ubiquitous and discuss the implications of that conclusion.