We present an analysis of the ages and star-formation history of the F-type stars in the Upper Scorpius ( US ) , Upper Centaurus-Lupus ( UCL ) and Lower Centaurus-Crux ( LCC ) subgroups of Scorpius-Centaurus ( Sco-Cen ) , the nearest OB association .
Our parent sample is the kinematically-selected Hipparcos sample of , restricted to the 138 F-type members .
We have obtained classification-resolution optical spectra and have also determined the spectroscopic accretion disk fraction .
With Hipparcos and 2MASS photometry , we estimate the reddening and extinction for each star and place the candidate members on a theoretical H-R diagram .
For each subgroup we construct empirical isochrones and compare to published evolutionary tracks .
We find that 1 ) our empirical isochrones are consistent with the previously published age-rank of the Sco-Cen subgroups , 2 ) subgroups LCC and UCL appear to reach the main sequence turn-on at spectral types \sim F4 and \sim F2 , respectively .
An analysis of the A-type stars shows US reaching the main sequence at about spectral type \sim A3 .
3 ) The median ages for the pre-main sequence members of UCL and LCC are 16 Myr and 17 Myr , respectively , in agreement with previous studies , however we find that 4 ) Upper Sco is much older than previously thought .
The luminosities of the F-type stars in US are typically a factor of \sim 2.5 less luminous than predicted for a 5 Myr old population for four sets of evolutionary tracks .
We re-examine the evolutionary state and isochronal ages for the B- , A- , and G-type Upper Sco members , as well as the evolved M supergiant Antares , and estimate a revised mean age for Upper Sco of 11 \pm 1 \pm 2 Myr ( statistical , systematic ) .
Using radial velocities and Hipparcos parallaxes we calculate a lower limit on the kinematic expansion age for Upper Sco of > 10.5 Myr ( 99 % confidence ) .
However , the data are statistically consistent with no expansion .
We reevaluate the inferred masses for the known substellar companions in Upper Sco using the revised age and find the inferred masses are typically \sim 20–70 % higher than the original estimates which had assumed a much younger age ; specifically , we estimate the mass of 1RXS J1609-2105b to be 14 ^ { +2 } _ { -3 } M _ { \mathrm { Jup } } , suggesting that it is a brown dwarf rather than a planet .
Finally , we find the fraction of F-type stars exhibiting H \alpha emission and/or a K-band excess consistent with accretion to be 0/17 ( < 19 % ; 95 % C.L . )
in US at \sim 11 Myr , while UCL has 1/41 ( 2 ^ { +5 } _ { -1 } % ; 68 % C.L . )
accretors and LCC has 1/50 ( 2 ^ { +4 } _ { -1 } % ; 68 % C.L . )
accretors at \sim 16 Myr and \sim 17 Myr , respectively .