Spectroscopy of Thin Dust Shells in the Magellanic Clouds

Details Spectroscopy of Thin Dust Shells in the Magellanic Clouds Spectroscopy of Thin Dust Shells in the Magellanic Clouds

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006sptz.prop30332h&link_type=abstract

Spitzer Proposal ID #30332

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Scientific paper

This program will use the IRS to re-observe four optically thin oxygen-rich dust shells in the Magellanic Clouds first observed in the MC_DUST program, which was part of the IRS GTO allocation. Several sources in the MC_DUST program show either no dust emission or a dust excess too weak with respect to the noise to properly characterize with any confidence. We propose to follow-up four of these sources that show evidence for unusual emission with longer integrations, both in SL and LL. One source in the SMC, which we have confirmed to be an S star, shows unusual molecular absorption and possible emission at 13 and/or 14 microns. Two sources show evidence for pyroxene dust emission, which would be highly unusual for optically thin circumstllar dust shells. Another source is a candidate for alumina dust emission, which we have confirmed in only one other spectrum in our sample so far. Alumina is believed to be precursor to silicates as dust shells begin to form around evolved stars. Lower abundances of Al with respect to O, Mg, Si, and Fe would be a natural explanation for the apparent lack of alumina dust in the Magellanic Clouds, but a selection effect might also play a role, as most of the sources observed are bright and red and more likely to be surrounded by silicate dust. We need to obtain spectra with better sensitivity among the low-contrast dust sources before we can discount a selection effect.

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