10692(  1) - 04/05/05 12:41  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      10692
                                                       Version: 1
                                                       Check-in Date: 05-Apr-2005
                                                                      16:41:20

1.Proposal Title:
An Archival Survey of Interstellar Abundances in the Magellanic Clouds
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2. Proposal For  3. Cycle  4. Parallel Pointing Tolerance
AR               14        <none>
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5. Investigators
                                                                                     Contact?
    PI: Dr. Daniel E. Welty               University of Chicago
   CoI: Dr. James T. Lauroesch            Northwestern University                       N
   CoI: Dr. Donald G. York                University of Chicago                         N

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6. Abstract

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).
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                                                10692(  1) - 04/05/05 12:41  - [  2]

            Summary Form for Proposal  10692

  Item                         Used in this proposal
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  Proposal Category            AR
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