AAS 200th meeting, Albuquerque, NM, June 2002
Session 71. Stellar Youth: Tomorrow's Degenerates
Display, Thursday, June 6, 2002, 9:20am-4:00pm, SW Exhibit Hall

## [71.14] The Star-Formation History and Accretion Disk Census of Pre-Main Sequence Stars in the LCC and UCL Subgroups of Sco-Cen

E.E. Mamajek, M.R. Meyer, J.W. Liebert (Steward Obs.)

We present results of a spectroscopic survey of an X-ray and proper motion-selected sample of late-type stars in the Lower Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL) subgroups of the nearest OB association: Scorpio-Centaurus. The primary goals of the survey are to determine the star-formation history of the OB subgroups, and to assess the fraction of actively accreting stars in a sample dominated by post-T Tauri'' pre-main sequence stars. We identify 110 new pre-MS stars, selected by their strong Li 6707 absorption, subgiant surface gravities, proper motions consistent with subgroup membership, and HRD positions above the ZAMS. We demonstrate that measuring the gravity-sensitive band-ratio of Sr II 4077 to Fe I 4071 is a valuable means of classifying Li-rich stars as either pre-MS or ZAMS in nature. We estimate the mean ages of the pre-MS populations to be 18 Myr (LCC) and 15 Myr (UCL). A reevaluation of the MS turn-off ages for the de Zeeuw et al. (1999; AJ 117, 354) early B-type Hipparcos-selected membership yields mean ages of 16 Myr (LCC) and 17 Myr (UCL). Contrary to previous studies, it appears that LCC is roughly coeval with UCL, or slightly older. The upper limit on the duration of the low-mass star-formation in each OB subgroup is approximately 6 Myr. The kinematic distances to the Sco-Cen pre-MS stars range from 85-215 pc, with 12 of the LCC members lying within 100 pc of the Sun. Only 1 out of 110 pre-MS solar-type stars in our sample shows both enhanced H\alpha emission and a K-band excess indicative of active accretion from a truncated circumstellar disk: the nearby (85 pc) CTT star PDS 66. The authors acknowledge support through NASA/JPL and a Sigma Xi grant.

Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.