We present an analysis of the intrinsic colors and temperatures of 5-30 Myr old pre-main sequence (pre-MS) stars using the F0 through M9 type members of nearby, negligibly reddened groups: η Cha cluster, TW Hydra Association, β Pic Moving Group, and Tucana-Horologium Association. To check the consistency of spectral types from the literature, we estimate new spectral types for 52 nearby pre-MS stars with spectral types F3 through M4 using optical spectra taken with the SMARTS 1.5m telescope. Combining these new types with published spectral types, and photometry from the literature (Johnson-Cousins BV I C , 2MASS JHK S and WISE W 1, W 2, W 3, and W 4), we derive a new empirical spectral type-color sequence for 5-30 Myr old pre-MS stars. Colors for pre-MS stars match dwarf colors for some spectral types and colors, but for other spectral types and colors, deviations can exceed 0.3 mag. We estimate effective temperatures (T eff ) and bolometric corrections (BCs) for our pre-MS star sample through comparing their photometry to synthetic photometry generated using the BT-Settl grid of model atmosphere spectra. We derive a new T eff and BC scale for pre-MS stars, which should be a more appropriate match for T Tauri stars than often-adopted dwarf star scales. While our new T eff scale for pre-MS stars is within ≃100 K of dwarfs at a given spectral type for stars

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 de Zeeuw et al. (1999), 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 ∼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, as well as 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 and find the inferred masses are typically ∼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 Jup , suggesting that it is a brown dwarf rather than a planet. Finally, we find the fraction of F-type stars exhibiting Hα emission and/or a K-band excess consistent with accretion to be 0/17 (< 19%; 95% C.L.) in US at ∼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 ∼16 Myr and ∼17 Myr, respectively.

We present results of a spectroscopic survey for new K-and M-type members of ScorpiusCentaurus (Sco-Cen), the nearest OB Association (∼100-200 pc). Using an X-ray, proper motion and color-magnitude selected sample, we obtained spectra for 361 stars, for which we report spectral classifications and Li and Hα equivalent widths. We identified 156 new members of Sco-Cen, and recovered 51 previously published members. We have combined these with previously known members to form a sample of 493 solar-mass (∼0.7-1.3 M ) members of Sco-Cen. We investigated the star-formation history of this sample, and re-assessed the ages of the massive main-sequence turn-off and the G-type members in all three subgroups. We performed a census for circumstellar disks in our sample using WISE infrared data and find a protoplanetary disk fraction for K-type stars of 4.4 +1.6 −0.9 % for Upper Centaurus-Lupus and Lower Centaurus-Crux at ∼16 Myr and 9.0 +4.0 −2.2 % for Upper Scorpius at ∼10 Myr. These data are consistent with a protoplanetary disk e-folding timescale of ∼4-5 Myr for ∼1 M stars, twice that previously quoted (Mamajek 2009), but consistent with the Bell et al. revised age scale of young clusters. Finally, we construct an age map of Scorpius-Centaurus which clearly reveals substructure consisting of concentrations of younger and older stars. We find evidence for strong age gradients within all three subgroups. None of the subgroups are consistent with being simple, coeval populations which formed in single bursts, but likely represents a multitude of smaller star formation episodes of hundreds to tens of stars each.

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The large relative sizes of circumstellar and circumplanetary disks imply that they might be seen in eclipse in stellar light curves. We estimate that a survey of ∼10 4 young (∼10 million year old) postaccretion pre-main sequence stars monitored for ∼10 years should yield at least a few deep eclipses from circumplanetary disks and disks surrounding low mass companion stars. We present photometric and spectroscopic data for a pre-main sequence K5 star (1SWASP J140747.93-394542.6 = ASAS J140748-3945.7), a newly discovered ∼0.9 M ⊙ member of the ∼16 Myr-old Upper Centaurus-Lupus subgroup of Sco-Cen at a kinematic distance of 128 ± 13 pc. This star exhibited a remarkably long, deep, and complex eclipse event centered on 29 April 2007 (as discovered in SuperWASP photometry, and with portions of the dimming confirmed by ASAS data). At least 5 multi-day dimming events of >0.5 mag are identified, with a >3.3 mag deep eclipse bracketed by two pairs of ∼1 mag eclipses symmetrically occurring ±12 days and ±26 days before and after. Hence, significant dimming of the star was taking place on and off over at least a ∼54 day period in 2007, and a strong >1 mag dimming event occurring over a ∼12 day span. We place a firm lower limit on the period of 850 days (i.e. the orbital radius of the eclipser must be >1.7 AU and orbital velocity must be <22 km/s). The shape of the light curve is similar to the lop-sided eclipses of the Be star EE Cep. We suspect that this new star is being eclipsed by a low-mass object orbited by a dense inner disk, further girded by at least 3 dusty rings of optical depths near unity. Between these rings are at least two annuli of near-zero optical depth (i.e. gaps), possibly cleared out by planets or moons, depending on the nature of the secondary. For possible periods in the range 2.33-200 yr, the estimated total ring mass is ∼8-0.4 M Moon (if the rings have optical opacity similar to Saturn's rings), and the edge of the outermost detected ring has orbital radius ∼0.4-0.09 AU. In the new era of time-domain astronomy opened by surveys like SuperWASP, ASAS, etc., and soon to be revolutionized by LSST, discovering and characterizing eclipses by circumplanetary and circumsecondary disks will provide us with observational constraints on the conditions which spawn satellite systems around gas giant planets and planetary systems around stars.