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E. L. N. Jensen (Swarthmore College), B. A. Biller (CfA), D. W. Koerner (Univ.\ of Pennsylvania), R. S. Whitaker (Swarthmore College), N. R. Bonaventura (Univ.\ of Pennsylvania), A. Dullighan (M.I.T.)
We have conducted a systematic survey of the southern sky to search for nearby, low-mass, pre--main-sequence stars unassociated with known regions of star formation. A small number of such stars are known, such as TW Hya. However, it is presently unclear how prevalent this sort of star may be. The primary goal of our survey is to search for similar young stars in a way that is not biased with respect to the presence or absence of circumstellar disks. This allows us to probe disk evolution between the ages of T Tauri stars at ~1 Myr (when most stars have disks) and the zero-age main sequence at ~100 Myr (when most stars no longer have dusty disks). Using the Hipparcos catalog and the ROSAT Bright Source Catalog, we selected stars with spectral types of G5 or later that lie above the zero-age main sequence by at least a factor of two and show strong x-ray emission (L\rm X/L\rm bol \ge 10-3). By selecting stars only on the basis of x-ray emission and HR diagram position, not infrared excess or H\alpha emission, we are neither favoring nor excluding stars with disks. We then performed follow-up optical spectroscopy to look for strong Li absorption, a signature of youth. We find that: 1) a small fraction of our sample are indeed nearby pre--main-sequence stars, but that such stars are not common in our sample; 2) few of the young stars show evidence for circumstellar material as measured by IRAS infrared fluxes. We hope to survey these stars with SIRTF in order to place much more sensitive limits on the level of circumstellar material around low-mass stars at ages of ~10 Myr. This work is supported in part by the National Science Foundation.
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