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B. Zuckerman (UCLA), I. Song (Gemini Observatory), A.J. Weinberger (Carnegie Institution of Washington), E.E. Becklin (UCLA)
We report extraordinary mid-infrared excess emission around a solar-type star of estimated age a few hundred million years. Ground-based mid-IR observations confirm that a strong IRAS 12 and 25 micron source is in fact located on the star and that the 12 micron emission appears point-like down to the diffraction limit of the Keck telescope (~ 0."2). A simple blackbody fit to the excess indicates a ~ 420 K dusty disk located \lesssim 1 AU from the central star with a huge infrared fractional luminosity (tau) of ~5%. This is to be compared to tau (= L[IR]/L[star]) of main sequence Vega-like stars which are < 0.1%. In addition, at Vega-like stars, dust temperatures are typically < 100 K.
A Keck LWS 8-13 micron low resolution spectrum shows strong silicate emission at ~ 11.3 microns, confirmed by an R = 1,000 Gemini Michelle spectrum. The strength of the feature implies the typical grain size should not be much bigger than ~ 1 micron. Constraints imposed by the broadband spectrum from 3.5 to 60 microns support the small grain sizes deduced from the silicate feature. In addition, the IRAS 60 micron upper limit places a strong upper limit on the quantity of cold dust grains.
At only ~1 AU from the central star, small grains cannot survive long. The collisional grinding timescale is ~ 100 yrs and grains fractured to smaller than a few tenths of a micron will be quickly blown away by radiation pressure. The Poynting-Roberton drag timescale for 1 micron dust is only a few thousand years. Therefore, all grains in the system should be second generation.
What can generate such a unique -- high temperature and high tau -- dusty disk? Perhaps the most plausible explanation is recent and frequent collisions between numerous planetesimals (asteroids?) near 1 AU.
This research was supported in part by NASA's Astrobiology Institute.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.