Small-scale structure in the diffuse interstellar medium

We report the initial results of a study to probe the small-scale structure in the diffuse interstellar medium (ISM) through International Ultraviolet Explorer (IUE) and optical observations of interstellar absorption lines toward both components of resolvable binary stars. The binaries (κ CrA, 57 Aql, 59 And, HR 1609/10, 19 Lyn, and θ Ser) observed with IUE have projected linear separations ranging from 5700 to 700 AU. Except for κ CrA, the strengths of the interstellar absorption lines toward both components of these binaries agree to within 10%. In the case of κ CrA, the optically thin interstellar Mg I and Mn II lines are ≈50% stronger toward κ² CrA than κ¹ CrA. Higher resolution observations of interstellar Ca II show that this difference is concentrated in the main interstellar component at ν_LSR = 9 ± 2 km s^-1. Interestingly, this velocity corresponds to an intervening cloud that may be associated with the prominent Loop I shell in the local ISM. Given the separation (23″) and distance (120 pc) of κ CrA, the line strength variations indicate that this cloud has structure on scales of 2800 AU or less. The homogeneity in our other sightlines contrasts with several recent studies which find significant ISM structure on much smaller (≈25 AU) and much larger (≈0.1 pc) scales.