Post–T Tauri Stars in the Nearest OB Association

We present results of a spectroscopic survey of X-ray- and propermotion-selected samples of late-type stars in the Lower Cen-Cru (LCC) and UpperCen-Lup (UCL) subgroups of the nearest OB association: Sco-Cen. The primarygoals of the survey are to determine the star-formation history of the OBsubgroups, and to assess the frequency of accreting stars in a sample dominatedby ``post-T Tauri'' pre-MS stars. We investigate two samples: (1) ACT and TRCproper motion candidates with X-ray counterparts in the ROSAT All-Sky SurveyBSC, and (2) G and K-type Hipparcos candidate members (de Zeeuw et al. 1999).We obtained optical spectra of 130 candidates with the MSSSO 2.3-m DBS. Pre-MSstars were identified by (1) strong Li 6707A absorption, (2) subgiant surfacegravities, (3) proper motions consistent with Sco-Cen membership, and (4) HRDpositions consistent with being pre-MS. We demonstrate that measuring thegravity-sensitive band-ratio of Sr II 4077A to Fe I 4071A is a valuable meansof discriminating pre-MS and ZAMS stars. Depending on the choice ofevolutionary tracks, we find the mean pre-MS ages to range between 17-23 Myr(LCC), and 15-22 Myr (UCL). Accounting for observational uncertainties, itappears that 95% of the low-mass star-formation in each subgroup occurred inless than 8 Myr (LCC) and 12 Myr (UCL). We find main sequence turn-off ages forHipparcos B-type members to be 16+-1 Myr (LCC) and 17+-1 Myr (UCL). Contrary toprevious findings, it appears that LCC is coeval with, or slightly older than,UCL. Only 1 out of 110 (0.9^+2.1_-0.8 %) pre-MS solar-type stars in the samplewith ages of 13+-1 (s.e.)+-6 (1sigma) Myr and masses of 1.3+-0.2 Msun showsboth enhanced Halpha emission and a K-band excess indicative of accretion froma truncated circumstellar disk. (abridged)