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 Spectrograph and multiepoch KPNO coude feed observations of multiple interstellar lines toward the binary system HD 32039/40. Comparisons of the Na I profiles observed over a period of 4.25 yr reveal significant temporal variation in the Na I column in at least one component, implying that there is structure at scales on the order of the proper motion (similar to 15-21 AU). Large differences are also observed among multiple components in the profiles of the neutral species between the binary sight lines (5060 AU separation). In addition, significant differences in the S II profile are seen between 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 variable 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.