AAS 202nd Meeting, May 2003
Session 34 When Do Planets Form?
Topical Associated Poster, Tuesday, May 27, 2003, 10:00am-6:30pm, West Exhbit Hall

## [34.01] A Search for Warm Circumstellar Disks in the TW Hydra Association

A. J. Weinberger (Carnegie Institution of Washington, Department of Terrestrial Magnetism), E. E. Becklin, B. Zuckerman, I. Song (University of California Los Angeles)

The TW Hydrae association (TWA) is a collection of about two dozen star systems within ~40 pc of each other. The TWA is one of the closest (~60 pc) sites of recent star formation to Earth. Primary star members range in spectral type from A0 to M3 and have common ages of 5-10 Myr as determined from pre-main sequence evolutionary tracks, lithium abundances, or both. The IRAS Faint Source Catalog had insufficient sensitivity to probe fully for circumstellar disks in TWA. At the distance to and age of TWA, 12 micron stellar photospheric emission was detectable by IRAS for spectral types earlier than K0. Three cooler TWA stars were detected only from their extremely large mid-far-infrared excesses.

Mid-infrared observations from ground-based telescopes can improve greatly over IRAS sensitivity and thus detect warm tenuous disks around late-type stars. We have performed a ground based search for circumstellar disks around eighteen members of the TWA using the Long Wavelength Spectrograph at the W. M. Keck Observatory. Our survey was in three mid-infrared bands centered at 3.8, 12, and 18 micron with characteristic integration times on-source of 100, 200, and 400 s, respectively. Our 1 \sigma sensitivity limits in these filters were approximately 3 mJy, 4 mJy, and 20 mJy respectively. All of the stars were detected easily at 3.8 microns (L-band). Fifteen of the eighteen were detected at 12 microns, and none was detected at 18 microns. No significant infrared excesses were detected around the sample stars. We do not confirm the previously reported possible excess around TWA 5. 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 for 10 Myr.

Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.