Aaron C. Rizzuto

The Mass?Radius Relation of Young Stars. I. USco 5, an M4.5 Eclipsing Binary in Upper Scorpius Observed by K2

Evolutionary models of pre-main sequence stars remain largely uncalibrated, especially for masses below that of the Sun, making each new dynamical mass and radius measurement a valuable test of theoretical models. Stellar mass dependent features of star formation (such as disk evolution, planet formation, and even the IMF) are fundamentally tied to these models, which implies a systematic uncertainty that can only be improved with precise measurements of calibrator stars. We present the discovery that UScoCTIO 5, a known spectroscopic binary (P = 34 days, Mtot sin(i) = 0.64 Msun), is an eclipsing system with both primary and secondary eclipses apparent in K2 light curves obtained during Campaign 2. We have simultaneously fit the eclipse profiles from the K2 light curves and the existing RV data to demonstrate that UScoCTIO 5 consists of a pair of nearly identical M4.5 stars with M_A = 0.329 +/- 0.002 Msun, R_A = 0.834 +/- 0.006 Rsun, M_B = 0.317 +/- 0.002 Msun, and R_B = 0.810 +/- 0.006 Rsun. The radii are broadly consistent with pre-main sequence ages predicted by stellar evolutionary models, but none agree to within the uncertainties. All models predict systematically incorrect masses at the 25--50% level for the HR diagram position of these mid-M dwarfs, suggesting significant modifications for any trend of other properties that vary with stellar mass. The form of the discrepancy for most model sets is not that they predict luminosities that are too low, but rather that they predict temperatures that are too high, suggesting that the models do not fully encompass the physics of energy transport (via convection and/or missing opacities) and/or a miscalibration of the SpT-Teff scale. The simplest modification to the models (changing Teff to match observations) would yield an older age for this system, in line with the recently proposed older age of Upper Scorpius (~11 Myr).

New pre-main-sequence stars in the Upper Scorpius subgroup of Sco–Cen

We present 237 new spectroscopically confirmed pre-main-sequence K and M-type stars in the young Upper Scorpius subgroup of the Sco-Cen association, the near- est region of recent massive star formation. Using the Wide-Field Spectrograph at the Australian National University 2.3 m telescope at Siding Spring, we observed 397 kinematically and photometrically selected candidate members of Upper Scorpius, and identified new members by the presence of Lithium absorption. The HR-diagram of the new members shows a spread of ages, ranging from ~3-20 Myr, which broadly agrees with the current age estimates of ~5-10 Myr. We find a significant range of Li 6708 equivalent widths among the members, and a minor dependence of HR-diagram position on the measured equivalent width of the Li 6708A line, with members that appear younger having more Lithium. This could indicate the presence of either popu- lations of different age, or a spread of ages in Upper Scorpius. We also use Wide-Field Infrared Survey Explorer data to infer circumstellar disk presence in 25 of the members on the basis of infrared excesses, including two candidate transition disks. We find that 11.2+-3.4% of the M0-M2 spectral type (0.4-0.8 M_sun) Upper Sco stars display an excess that indicates the presence of a gaseous disk.

Zodiacal Exoplanets in Time (Zeit) Iii: A Short-Period Planet Orbiting a Pre-Main-Sequence Star in the Upper Scorpius Ob Association

We confirm and characterize a close-in (

DYNAMICAL MASSES of YOUNG STARS. I. DISCORDANT MODEL AGES of UPPER SCORPIUS

P_{\rm{orb}}

High-Precision Radio and Infrared Astrometry of Lspm J1314+1320ab. Ii. Testing Pre-Main-Sequence Models at the Lithium Depletion Boundary With Dynamical Masses

= 5.425 days), super-Neptune sized (

MAPPING THE SHORES OF THE BROWN DWARF DESERT. IV. OPHIUCHUS

5.04^{+0.34}_{-0.37}

Long-baseline interferometric multiplicity survey of the Sco-Cen OB association

Earth radii) planet transiting K2-33 (2MASS J16101473-1919095), a late-type (M3) pre-main sequence (11 Myr-old) star in the Upper Scorpius subgroup of the Scorpius-Centaurus OB association. The host star has the kinematics of a member of the Upper Scorpius OB association, and its spectrum contains lithium absorption, an unambiguous sign of youth (<20 Myr) in late-type dwarfs. We combine photometry from K2 and the ground-based MEarth project to refine the planets properties and constrain the host stars density. We determine \names bolometric flux and effective temperature from moderate resolution spectra. By utilizing isochrones that include the effects of magnetic fields, we derive a precise radius (6-7%) and mass (16%) for the host star, and a stellar age consistent with the established value for Upper Scorpius. Follow-up high-resolution imaging and Doppler spectroscopy confirm that the transiting object is not a stellar companion or a background eclipsing binary blended with the target. The shape of the transit, the constancy of the transit depth and periodicity over 1.5 years, and the independence with wavelength rules out stellar variability, or a dust cloud or debris disk partially occulting the star as the source of the signal; we conclude it must instead be planetary in origin. The existence of K2-33b suggests close-in planets can form in situ or migrate within

Dippers and Dusty Disks Edges: A Unified Model

\sim 10

The Greater Taurus–Auriga Ecosystem. I. There is a Distributed Older Population

Myr, e.g., via interactions with a disk, and that long-timescale dynamical migration such as by Lidov-Kozai or planet-planet scattering is not responsible for all short-period planets.

High-Precision Radio and Infrared Astrometry of Lspm J1314+1320ab. I. Parallax, Proper Motions, and Limits on Planets

We present the results of a long term orbit monitoring program, using sparse aperture masking observations taken with NIRC2 on the Keck-II telescope, of seven G to M-type members of the Upper Scorpius subgroup of the Sco-Cen OB association. We present astrometry and derived orbital elements of the binary systems we have monitored, and also determine the age, component masses, distance and reddening for each system using the orbital solutions and multi-band photometry, including Hubble Space Telescope photometry, and a Bayesian fitting procedure. We find that the models can be forced into agreement with any individual system by assuming an age, but that age is not consistent across the mass range of our sample. The G-type binary systems in our sample have model ages of ~11.5 Myr, which is consistent with the latest age estimates for Upper Scorpius, while the M-type binary systems have significantly younger model ages of ~7 Myr. Based on our fits, this age discrepancy in the models corresponds to a luminosity under-prediction of 0.8-0.15 dex, or equivalently an effective temperature over-prediction of 100-300 K for M-type stars at a given premain-sequence age. We also find that the M-type binary system RXJ 1550.0-2312 has an age (~16 Myr) and distance (~90 pc) indicating that it is either a nearby young binary system or a member of the Upper-Centaurus-Lupus subgroup with a 57% probability of membership.