The Star-formation History and Accretion Disk Fraction of the Scorpius-Centaurus OB Association

We present a study of the star-formation history and accretion disk fraction of ~0.6-1.8 Msun 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;=142 pc) and Lower Centaurus-Crux (LCC; 16±3 Myr; =118 pc) are consistent with previous results. However, our results for Upper Scorpius (US; 10±3 Myr; =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

A Spitzer MIPS Study of 2.5-2.0 M\odot Stars in Scorpius-Centaurus

We have obtained Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) 24 {\mu}m and 70 {\mu}m observations of 215 nearby, Hipparcos B- and A-type common proper motion single and binary systems in the nearest OB association, Scorpius-Centaurus. Combining our MIPS observations with those of other ScoCen stars in the literature, we estimate 24 {\mu}m B+A-type disk fractions of 17/67 (25+6%), 36/131 (27+4%), and 23/95 (24+5%) for Upper Scorpius (\sim11 Myr), Upper Centaurus Lupus (\sim15 Myr), and Lower Centaurus Crux (\sim17 Myr), respectively, somewhat smaller disk fractions than previously obtained for F- and G-type members. We confirm previous IRAS excess detections and present new discoveries of 51 protoplanetary and debris disk systems, with fractional infrared luminosities ranging from LIR/L\ast = 1e-6 to 1e-2 and grain temperatures ranging from Tgr = 40 - 300 K. In addition, we confirm that the 24 {\mu}m and 70 {\mu}m excesses (or fractional infrared luminosities) around B+A type stars are smaller than those measured toward F+G type stars and hypothesize that the observed disk property dependence on stellar mass may be the result of a higher stellar companion fraction around B- and A-type stars at 10 - 200 AU and/or the presence of Jupiter-mass companions in the disks around F- and G- type stars. Finally, we note that the majority of the ScoCen 24 {\mu}m excess sources also possess 12 {\mu}m excess, indicating that Earth-like planets may be forming via collisions in the terrestrial planet zone at \sim10 - 100 Myr.Comment: 62 pages, 17 figures including 6 colo

Does the Debris Disk around HD 32297 Contain Cometary Grains?

We present an adaptive optics imaging detection of the HD 32297 debris disk at L' (3.8 \microns) obtained with the LBTI/LMIRcam infrared instrument at the LBT. The disk is detected at signal-to-noise per resolution element ~ 3-7.5 from ~ 0.3-1.1" (30-120 AU). The disk at L' is bowed, as was seen at shorter wavelengths. This likely indicates the disk is not perfectly edge-on and contains highly forward scattering grains. Interior to ~ 50 AU, the surface brightness at L' rises sharply on both sides of the disk, which was also previously seen at Ks band. This evidence together points to the disk containing a second inner component located at $\lesssim$ 50 AU. Comparing the color of the outer (50 $< r$/AU $< 120$) portion of the disk at L' with archival HST/NICMOS images of the disk at 1-2 \microns allows us to test the recently proposed cometary grains model of Donaldson et al. 2013. We find that the model fails to match the disk's surface brightness and spectrum simultaneously (reduced chi-square = 17.9). When we modify the density distribution of the model disk, we obtain a better overall fit (reduced chi-square = 2.9). The best fit to all of the data is a pure water ice model (reduced chi-square = 1.06), but additional resolved imaging at 3.1 \microns is necessary to constrain how much (if any) water ice exists in the disk, which can then help refine the originally proposed cometary grains model.Comment: Accepted to ApJ January 13, 2014. 12 pages (emulateapj style), 9 figures, 1 tabl

HD 101088, An Accreting 14 AU Binary in Lower Centaurus Crux With Very Little Circumstellar Dust

We present high resolution (R=55,000) optical spectra obtained with MIKE on the 6.5 m Magellan Clay Telescope as well as Spitzer MIPS photometry and IRS low resolution (R~60) spectroscopy of the close (14 AU separation) binary, HD 101088, a member of the ~12 Myr old southern region of the Lower Centaurus Crux (LCC) subgroup of the Scorpius-Centaurus OB association. We find that the primary and/or secondary is accreting from a tenuous circumprimary and/or circumsecondary disk despite the apparent lack of a massive circumbinary disk. We estimate a lower limit to the accretion rate of > 1x10^-9 solar masses per year, which our multiple observation epochs show varies over a timescale of months. The upper limit on the 70 micron flux allows us to place an upper limit on the mass of dust grains smaller than several microns present in a circumbinary disk of 0.16 moon masses. We conclude that the classification of disks into either protoplanetary or debris disks based on fractional infrared luminosity alone may be misleading.Comment: 8 pages, 2 figures, ApJ accepte