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Session 51 - Interstellar Medium II.
Display session, Thursday, January 08
Exhibit Hall,
Observations of interstellar absorption lines toward both members of resolvable binary star systems provide a sensitive probe of small-scale structure in the diffuse ISM. Optical observations of \ionNa1 and \ionCa2 at very high spectral resolution \mbox(\Delta v=0.3 km s^-1) toward both members of the binary \mbox\mu Cru have revealed line strength variations in at least 4 velocity components, indicative of ISM structure on scales less than the projected binary separation of \mbox6600 AU (Meyer amp; Blades 1996, ApJL, 464, L179). We present new observations of the \mbox\ionNa1 D_2 line towards this binary taken with the Ultra-High Resolution Facility (UHRF) on the Anglo-Australian Telescope, as well as HST GHRS echelle observations of multiple lines of \ionC1, \ionMg1, \ionCr2, and \ionZn2 for both stars. The \mbox\ionNa1 D_2 measurements show no variations in the line profile in either star between observations taken 21 months apart. This constancy coupled with the cold, quiescent nature of the gas responsible for most of the variation suggests that this material is not circumstellar. Of particular interest is the narrow \mbox(b=0.4 km s^-1) absorption component observed at \mboxv_helio\sim-8.6 km s^-1 towards \mbox\mu^1 Cru, which is not seen in the spectrum of \mbox\mu^2 Cru. Preliminary profile fits to the GHRS data using the component structure from the higher-resolution \ionNa1 data yield column density ratios for the neutral species of \mboxN(\ionC1)/N(\ionNa1)\sim10 and \mboxN(\ionMg1)/N(\ionNa1)\sim6 for this component. These ratios differ from those typically seen in cold interstellar gas, and we explore a number of different models for the structure and physical conditions of the cloud in which this absorption arises.
