Evidence of Interstellar N[CLC]a[/CLC] [CSC]i[/CSC] 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 coudé 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 (~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 ~20-200 cm -3 , significantly lower than the densities inferred in past H I 21 cm and Na I studies.