The physical characteristics of the small-scale interstellar structure toward mu Crucis

We present Hubble Space Telescope Goddard High Resolution Spectrograph eche lle observations of multiple interstellar lines of C I, Mg I, Cr II, and Zn II toward both stars in the mu Crucis binary system. Despite large differe nces in the profiles of the neutral species, no significant variations betw een 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 mu Cru is constant d espite greatly enhanced columns of the neutral species at this velocity tow ard mu(1) Cru. An analysis of the finestructure excitation of C I in this c loud implies that the density is n(H) < 250 cm(-3). From the lack of variat ion in the (optical) Na I D2 line profiles toward mu(1) and mu(2) Cru in sp ectra taken 21 months apart, we can place a lower limit to the size of the structures of similar to 10 AU. These results are discussed in the context of recent radio and optical studies of apparently pervasive high-density sm all-scale interstellar structure.