Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2013
We present a study of the star-formation history and accretion disk fraction of ∼0.6-
1.8 M⊙ stars in the nearest OB Association, Scorpius-Centaurus (Sco-Cen; 10-20
Myr; 100-200 pc). We have performed a low-resolution spectroscopic survey for new,
low-mass K- and M-type members of all three subgroups – Upper Scorpius (US), Upper
Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC). We find that young,
pre-main sequence stars have different intrinsic colors for a given spectral type than
their main-sequence (MS) counterparts and therefore MS colors and temperatures
are unsuitable for de-reddening the low-mass members of Sco-Cen and placing them
on an H-R diagram. Using nearby, pre-MS, unreddened moving groups, we derive a
spectral type–intrinsic color sequence appropriate for 5-30 Myr old pre-MS stars, and
use synthetic spectral energy distribution fits to infer the proper temperature and
bolometric correction scale for these young stars. We use this new pre-MS intrinsic
color and temperature calibration to place our ∼150 newly identified members of Sco-
Cen on an H-R diagram. We derive isochronal ages for the B-type MS turn-off and
the pre-MS F-type, G-Type members and the K- and M-type members of Sco-Cen.
We find a Teff -dependent age trend in the K/M-type stars, similar to previous studies
of other nearby star-forming regions. Our F- and G-type isochronal ages for Upper
Centaurus-Lupus (UCL; 16±2 Myr; < d >=142 pc) and Lower Centaurus-Crux
(LCC; 16±3 Myr; < d >=118 pc) are consistent with previous results. However, our
results for Upper Scorpius (US; 10±3 Myr; < d >=145 pc) indicate it is a factor of
two older than previously thought. Using ∼650 of the pre-MS members of Sco-Cen,
we construct an age map, which reveals regions which are systematically younger
or older than the mean Sco-Cen age, suggesting that the star-formation history of
the three subgroups is more complex than the simple division into three subgroups
would imply. Finally, we find a primordial disk fraction for US, UCL and LCC of
9+4
−2%, 5+2
−1% and 3+3
−1%, respectively, for K-type stars decreasing to <19% (95% CL),
2+5
−1%, and 2+4
−1%, respectively, for F-type stars at ∼10 Myr, ∼16 Myr, and ∼16 Myr,
respectively.
