Mark J. Pecaut

INTRINSIC COLORS, TEMPERATURES, AND BOLOMETRIC CORRECTIONS OF PRE-MAIN-SEQUENCE STARS

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 IC, 2MASS JHKS and WISE W1, W2, W3, and W4), 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 (Teff) 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 Teff 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 Teff scale for pre-MS stars is within ≃100 K of dwarfs at a given spectral type for stars <G5, for G5 through K6, the pre-MS stars are ∼250 K cooler than their main sequence counterparts. Lastly, we present (1) a modern Teff, optical/IR color, and bolometric correction sequence for O9V-M9V MS stars based on an extensive literature survey, (2) a revised Qmethod relation for dereddening UBV photometry of OB-type stars, and (3) introduce two candidate spectral standard stars as representatives of spectral types K8V and K9V. Subject headings: open clusters and associations: individual(η Cha cluster, TW Hydra Association, β Pic Moving Group, Tucana-Horologium Association); — stars: pre-main sequence — stars: fundamental parameters (colors, temperatures)

PLANETARY CONSTRUCTION ZONES IN OCCULTATION: DISCOVERY OF AN EXTRASOLAR RING SYSTEM TRANSITING A YOUNG SUN-LIKE STAR AND FUTURE PROSPECTS FOR DETECTING ECLIPSES BY CIRCUMSECONDARY AND CIRCUMPLANETARY DISKS

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 ~104 young (~10 million year old) post-accretion 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 2007 April 29 (as discovered in Super Wide Angle Search for Planets (SuperWASP) photometry, and with portions of the dimming confirmed by All Sky Automated Survey (ASAS) data). At least five 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?1). The shape of the light curve is similar to the lopsided 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 three 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 Saturns 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 Large Synoptic Survey Telescope, discovering and characterizing eclipses by circumplanetary and circumsecondary disks will provide us with observational constraints on the conditions that spawn satellite systems around gas giant planets and planetary systems around stars.

The star formation history and accretion-disc fraction among the K-type members of the Scorpius–Centaurus OB association

We present results of a spectroscopic survey for new K- and M-type members of Scorpius-Centaurus (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 Halpha 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 Msun) 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

A SPITZER MIPS STUDY OF 2.5-2.0 M ☉ STARS IN SCORPIUS-CENTAURUS

^{+1.6}_{-0.9}

HD 101088, AN ACCRETING 14 AU BINARY IN LOWER CENTAURUS CRUX WITH VERY LITTLE CIRCUMSTELLAR DUST ∗

% for Upper Centaurus-Lupus and Lower Centaurus-Crux at ~16 Myr and 9.0

Modeling Transiting Circumstellar Disks: Characterizing the Newly Discovered Eclipsing Disk System OGLE LMC-ECL-11893

^{+4.0}_{-2.2}

New Young Stars and Brown Dwarfs in the Upper Scorpius Association

% for Upper Scorpius at ~10 Myr. These data are consistent with a protoplanetary disk e-folding timescale of ~4-5 Myr for ~1 Msun 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.

Anomalous Spectral Types and Intrinsic Colors of Young Stars

We have obtained Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) 24 μm and 70 μm 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 μm B+A-type disk fractions of 17/67 (25+6 – 5%), 36/131 (27+4 – 4%), and 23/95 (24+5 – 4%) for Upper Scorpius (~11 Myr), Upper Centaurus Lupus (~15 Myr), and Lower Centaurus Crux (~17 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 L IR/L * = 10–6 to 10–2 and grain temperatures ranging from T gr = 40 to 300 K. In addition, we confirm that the 24 μm and 70 μm 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. Finally, we note that the majority of the ScoCen 24 μm excess sources also possess 12 μm excess, indicating that Earth-like planets may be forming via collisions in the terrestrial planet zone at ~10-100 Myr.

A REVISED AGE FOR UPPER SCORPIUS AND THE STAR-FORMATION HISTORY AMONG THE F-TYPE MEMBERS OF THE SCORPIUS-CENTAURUS OB ASSOCIATION

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 Infrared Spectrometer 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 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 1\times 10^{-9} \;M_{\odot } \;{\rm yr}^{-1}

A MAGELLAN MIKE AND SPITZER MIPS STUDY OF 1.5-1.0 M ☉ STARS IN SCORPIUS-CENTAURUS

SRC=http://ej.iop.org/images/0004-637X/714/2/1542/apj342600ieqn1.gif/>, which our multiple observation epochs show varies over a timescale of months. The upper limit on the 70 μm 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 M moon. We conclude that the classification of disks into either protoplanetary or debris disks based on fractional infrared luminosity alone may be misleading.