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A. J. Weinberger, E. E. Becklin, B. Zuckerman, I. Song (Carnegie Institution of Washington)
The Beta Pictoris Association (Zuckerman et al. 2001; Song, Zuckerman & Bessell 2003) is a very close (~40 pc) group of about 30 stars all of age ~12 Myr that have common Galactic space motion indicating a common birthplace. Five of these stars, including Beta Pic itself, have infrared excesses detected by IRAS and another two have excesses detected by ISO. To search for previously undetected, tenuous, warm disks around stars in the Beta Pictoris Association, we used the Long Wavelength Spectrograph at the W. M. Keck Observatory to survey 19 stars with sensitive, 3.8, 12 and 18~\micron\ photometry.
The survey could detect Zodiacal-like dust, with temperature ~200 K, at the level observed for Beta Pic itself. In general, disks with such a warm temperature appear to be rare. Only a handful of other disks with 12 \mum excess have been identified, including HR 7012 and HR 7329 which are also in the Beta Pic Association.
Mid-infrared observations from ground-based telescopes can be much more sensitive than IRAS (typically 40 mJy, 1\sigma) at 10--20 \mum and thus detect more tenuous warm disks. Using characteristic integration times on-source of 120 and 400 s at 12 and 18 \mum, our 1\sigma sensitivity limits were approximately 4 mJy and 10 mJy, respectively.
At the average distance of the Beta Pic Association, IRAS had insufficient sensitivity to detect the photospheres of stars with spectral types later than A, whereas we detected photospheres down to M3. Preliminary analysis suggests that no significant infrared excesses were detected around the sample stars, although hints of excess appear in a few. It thus appears unusual for warm circumstellar dust, at temperatures characteristic of the region of the terrestrial planets, to persist around late-type stars even to 12 Myr.
We acknowledge support by the NASA Origins of Solar Systems program and the NASA Astrobiology Institute.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.