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D.M. Meyer (Northwestern Univ.)
High-resolution optical observations of the interstellar Na I absorption toward binary stars and star clusters are revealing a rich variety of subparsec-scale structure in diffuse clouds. At the ~103 to 104 AU scales probed by binary stars, individual Na I velocity components often change in strength, occasionally disappear entirely, and sometimes appear to shift in velocity. In a few cases, there is now evidence of temporal variations indicative of Na I structure on scales as small as ~10 AU. On larger scales of ~103 to 106 AU, integral field spectroscopic observations of extended sources such as globular clusters are beginning to provide fully-sampled, two-dimensional maps of the diffuse cloud Na I structure. Since Na I is typically not a dominant ion in diffuse clouds, a key question posed by all of these observations is whether the small-scale variations are due to differences in the total H column density or the Na ionization equilibrium. In the case of the former, the Na I is clearly tracing physical structures in the gas. In the case of the latter, the Na I is reflecting environmental variations in parameters such as the cloud temperature, pressure, electron density, and/or UV radiation field. High-resolution UV observations of other interstellar species with instruments such as STIS onboard HST can potentially resolve this issue. In particular, measurements of the C I fine-structure excitation can provide a direct test of the high densities inferred for some of the small-scale Na I structure.