Computer Science
Scientific paper
Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10155w&link_type=abstract
HST Proposal ID #10155
Computer Science
Hst Proposal Id #10155 Ism
Scientific paper
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 a mini-survey of interstellar absorption lines toward 13 stars distributed throughout the LMC and SMC -- which would more than double the number of Magellanic Clouds sightlines with extensive and accurate interstellar abundance information. A single STIS E230M setting will include lines from Zn II, Si II, Fe II, Ti II, Cr II, Ni II, and several trace neutral species -- allowing the abundance/depletion pattern 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}.
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