The Physical Characteristics of the Small-Scale Interstellar Structure toward μ Crucis

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Abstract
We present Hubble Space Telescope Goddard High Resolution Spectrograph echelle observations of multiple interstellar lines of C I, Mg I, Cr II, and Zn II toward both stars in the μ Crucis binary system. Despite large differences in the profiles of the neutral species, no significant variations between the stars are seen in the Cr II and Zn II line profiles. In particular, the Zn II absorption observed at -8.6 km s-1 toward μ Cru is constant despite greatly enhanced columns of the neutral species at this velocity toward μ1 Cru. An analysis of the fine-structure excitation of C I in this cloud implies that the density is n−3. From the lack of variation in the (optical) Na I D2 line profiles toward μ1 and μ2 Cru in spectra taken 21 months apart, we can place a lower limit to the size of the structures of ~10 AU. These results are discussed in the context of recent radio and optical studies of apparently pervasive high-density small-scale interstellar structure.
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