Evidence of interstellar NaI structure at scales down to 15 AU in low-density gas

We present high-resolution Hubble Space Telescope/Space Telescope Imaging S pectrograph and multiepoch KPNO coude feed observations of multiple interst ellar lines toward the binary system HD 32039/40. Comparisons of the Na I p rofiles observed over a period of 4.25 yr reveal significant temporal varia tion in the Na I column in at least one component, implying that there is s tructure at scales on the order of the proper motion (similar to 15-21 AU). Large differences are also observed among multiple components in the profi les of the neutral species between the binary sight lines (5060 AU separati on). In addition, significant differences in the S II profile are seen betw een the stars-the first optical/UV detection of small-scale variations in a dominant ion. Measurements of the C I fine-structure excitation in the var iable components suggest that the densities in these components are similar to 20-200 cm(-3), significantly lower than the densities inferred in past H I 21 cm and Na I studies.