Mark J. Pecaut, Eric E. Mamajek, Eric J. Bubar
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). 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 ~F4 and ~F2, respectively. An analysis of
the A-type stars shows US reaching the main sequence at about spectral type
~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 ~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, and the evolved M supergiant Antares, and estimate a
revised mean age for Upper Sco of 11+/-1+/-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. Specifically, we estimate the mass of 1RXS J1609-2105b to be
14^{+2}_{-3} Mjup, suggesting that it is a brown dwarf rather than a planet.
Finally, we find the fraction of F-type stars exhibiting Ha emission and/or a
K-band excess consistent with accretion to be 0/17 (<19%; 95% C.L.) in US at
~10 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 ~15 Myr. [Abridged]
View original:
http://arxiv.org/abs/1112.1695
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