A disk census for the nearest group of young stars:: Mid-infrared observations of the TW Hydrae association

A group of young, active stars in the vicinity of TW Hydrae has recently been identified as a possible physical association with a common origin. Given its proximity (similar to 50 pc), age (similar to 10 Myr), and abundance of binary systems, the TW Hya association is ideally suited to studies of the diversity and evolution of circumstellar disks. Here we present mid-infrared observations of 15 candidate members of the group, 11 of which have no previous flux measurements at wavelengths longer than 2 mu m. We report the discovery of a possible 10 mu m excess in CD -33 degrees 7795, which may be due to a circumstellar disk or a faint and as yet undetected binary companion. Of the other stars, only TW Hya, HD 98800, Hen 3-600A, and HR 4796A-all of which were detected by IRAS-show excess thermal emission. Our 10 mu m flux measurements for the remaining members of the association are consistent with photospheric emission, allowing us to rule out dusty inner disks. In light of these findings, we discuss the origin and age of the TW Hya association as well as the implications for disk evolution timescales.