Assigned ID: 536 ----- Submission 1 (Fri Jan 21 23:31:35 GMT 2005) ----- Studies of the interstellar medium in the Magellanic Clouds explore somewhat different environmental conditions from those typically probed in our own Galactic ISM. Apart from a few studies of individual sightlines, however, little is known about the abundances and depletions in the ISM of the LMC and SMC. HST spectra of three SMC stars indicate that Si and Mg (generally thought to be major dust constituents) are essentially undepleted in the SMC gas --- even for components with severe depletions of Fe and Ni. Similar "anomalous" Si depletions have now been seen in cycle 12 STIS spectra of one LMC star, though "normal" Si depletions are seen toward a second. Intriguingly, the "anomalous" Si depletions are all found for sightlines which probe regions where the 2175 A extinction bump is either absent (most of SMC) or very weak (LMC2). We therefore propose an archival survey of interstellar absorption lines toward ~40 stars distributed throughout the LMC and SMC, using STIS E140M and E230M spectra originally obtained for stellar studies. The spectra include lines from species such as Mg II, Si II, S II, Cr II, Fe II, Ni II, Zn II, and several trace neutral species --- allowing the abundance/depletion patterns to be determined at many locations in the two galaxies. The analysis of these spectra will have significant implications for (1) making models of interstellar dust grains (which currently rely heavily on silicates); (2) understanding the relationships between depletions, dust, and H_2 (which may be somewhat different in the Milky Way, LMC, and SMC); and (3) interpreting the gas-phase abundances observed for more distant low-metallicity systems, such as the QSO absorption-line systems (which exhibit some similar properties).

ISM AND CIRCUMSTELLAR MATTER 12 50000.00 false 0.0 false false false false false SUPPORTED