Session 45 - Interstellar Medium I.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

## [45.04] Small-Scale ISM Structure: The Remarkable Sightline Toward \mu Cru

D. M. Meyer (Northwestern U.), J. C. Blades (STScI)

Observations of interstellar absorption lines toward both members of resolvable binary star systems can provide a sensitive probe of small-scale structure in the diffuse ISM\@. We have recently obtained very high resolution (\Deltav \approx 0.3 km s^-1) observations of the interstellar Na I D lines toward several binaries with the Ultra-High Resolution Facility (UHRF) on the 3.9 m Anglo-Australian Telescope. In the case of \mu Cru, which consists of two B stars separated by 38\farcs8 at a distance of 170 pc, these observations reveal line strength variations in at least four velocity components indicative of ISM structure on scales less than the projected binary separation of 6600 AU (0.03 pc). The most striking of these variations involves a Na I component seen at a heliocentric velocity of -9 km s^-1 toward \mu^1 Cru but not at all toward \mu^2 Cru. This component has an appreciable column density (N(Na I) = 7 x 10^10 cm^-2) and it clearly exhibits hyperfine splitting with a Na I line width (FWHM \approx 0.7 km s^-1) indicative of a cloud temperature below 200 K\@. We have also obtained UHRF spectra of the interstellar Ca II K line toward \mu Cru which reveal a N(Na I)/N(Ca II) ratio (\approx7) for the -9 km s^-1 component that is suggestive of some dust depletion but is much lower than that of typical diffuse clouds. The other obvious profile differences in this sightline include a pair of cold components near +4 km s^-1 that are much more pronounced toward \mu^2 Cru than \mu^1 Cru. Given the remarkable variations in multiple velocity components toward \mu Cru, it should not be too surprising that our high resolution observations of Na I toward other binaries reveal that such small-scale structure in the cold ISM is common. We will discuss the implications of these results for a better microphysical understanding of diffuse interstellar clouds.