We present results of a spectroscopic survey of X-ray- and proper motion-selected samples of late-type stars in the Lower Centaurus-Crux ( LCC ) and Upper Centaurus-Lupus ( UCL ) subgroups of the nearest OB association : Scorpius-Centaurus .
The primary goals of the survey are to determine the star-formation history of the OB subgroups , and to assess the frequency of accreting stars in a sample dominated by “ post-T Tauri ” pre-main sequence ( pre-MS ) stars .
We investigate two samples : ( 1 ) proper motion candidates from the ACT and TRC astrometric catalogs ( Hoogerwerf 2000 ) with X-ray counterparts in the ROSAT All-Sky Survey ( RASS ) Bright Source Catalog , and ( 2 ) G and K-type stars in the Hipparcos catalog found to be candidate members by de Zeeuw et al .
( 1999 ) .
We obtained optical spectra of 130 candidates with the Siding Springs 2.3-m dual-beam spectrograph .
Pre-MS stars were identified by ( 1 ) strong Li \lambda 6707 absorption , ( 2 ) subgiant surface gravities , ( 3 ) proper motions consistent with Sco-Cen membership , and ( 4 ) HR diagram positions consistent with being pre-MS. We find 93 % of the RASS-ACT/TRC stars to be probable pre-MS members , compared to 73 % of the Hipparcos candidates .
We demonstrate that measuring the gravity-sensitive band-ratio of Sr ii \lambda 4077 to Fe i \lambda 4071 is a valuable means of discriminating pre-MS and zero-age main sequence ( ZAMS ) stars .
Using secular parallaxes , and Hipparcos , Tycho-2 , and 2MASS photometry , we construct an HR diagram .
Depending on the choice of published evolutionary tracks , we find the mean ages of the pre-MS populations to range between 17-23 Myr for LCC , and 15-22 Myr for UCL .
Taking into account observational errors , it appears that 95 % of the low-mass star-formation in each subgroup must have occurred in less than 8 Myr ( LCC ) and 12 Myr ( UCL ) .
Using the ( 6 ) tracks , we find main sequence turn-off ages for Hipparcos B-type members to be 16 \pm 1 Myr for LCC and 17 \pm 1 Myr for UCL .
Contrary to previous findings , it appears that LCC is coeval with , or slightly older than , UCL .
The secular parallaxes of the Sco-Cen pre-MS stars yield distances of 85-215 pc , with 12 of the LCC members lying within 100 pc of the Sun .
Only 1 out of 110 ( 0.9 ^ { +2.1 } _ { -0.8 } % ; 1 \sigma ) pre-MS solar-type stars in the sample with ages of 13 \pm 1 ( s.e . ) \pm 6 ( 1 \sigma ) Myr and masses of 1.3 \pm 0.2 ( 1 \sigma ) M _ { \odot } shows both enhanced H \alpha emission and a K-band excess indicative of accretion from a truncated circumstellar disk : the nearby ( d \simeq 86 pc ) classical T Tauri star PDS 66 .