Other
Scientific paper
Apr 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009sptz.prop60119f&link_type=abstract
Spitzer Proposal ID #60119
Other
Scientific paper
We propose IRAC photometry for a sample of 11 highly metal-enriched white dwarfs, to confirm or rule out infrared excess due to warm circumstellar dust. Our ongoing Spitzer work has been highly successful in the identification of white dwarfs that are, simultaneously, externally polluted by metals and have closely orbiting circumstellar dust. Both the orbiting material and the photospheric heavy element abundances are refractory-rich and volatile-poor, implying these stars are polluted by rocky materialE In this way we continue to build a target list on which to perform spectroscopy of extrasolar rocky planetesimals, via their heavy element signatures in the otherwise pristine photospheres of white dwarfs. No other currently available technique can observe the bulk composition of extrasolar, terrestrial, planetary material -- this is the singular, enormous advantage offered by metal-rich white dwarfs. Our model invokes the tidal destruction of a single, large asteroid to produce circumstellar dust, while multiple, smaller asteroids are invoked to explain stars that are dust-free. In the latter case, orbiting dust is readily destroyed via collisions and sputtering as additional asteroids enter a pre-existing, closely orbiting disk at slightly different inclinations, resulting in a gaseous disk. In both cases, the white dwarf accretes, and becomes polluted by, material which is rich in heavy elements. Therefore, identification of warm circumstellar dust implies pollution by a single body, whose heavy element abundances should reflect an idiosyncratic pattern of an extrasolar analog to a large asteroid like Ceres. On the other hand, a lack of orbiting dust implies the metal abundance pattern reflects pollution by an ensemble of smaller extrasolar asteroids, and closer to an average chemical composition. Discriminating between these two cases is critical to the interpretation of optical and ultraviolet spectroscopy of the photospheric heavy elements seen in polluted white dwarfs.
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