Rotation Periods of Young Brown Dwarfs: K2 Survey in Upper Scorpius

We report rotational periods for 16 young brown dwarfs in the nearby Upper Scorpius association, based on 72 days of high-cadence, high-precision photometry from the Kepler space telescope's K2 mission. The periods range from a few hours to two days (plus one outlier at 5 days), with a median just above one day, confirming that brown dwarfs, except at the very youngest ages, are fast rotators. Interestingly, four of the slowest rotators in our sample exhibit mid-infrared excess emission from disks; at least two also show signs of disk eclipses and accretion in the lightcurves. Comparing these new periods with those for two other young clusters and simple angular momentum evolution tracks, we find little or no rotational braking in brown dwarfs between 1-10 Myr, in contrast to low-mass stars. Our findings show that disk braking, while still at work, is inefficient in the substellar regime, thus provide an important constraint on the mass dependence of the braking mechanism.Comment: 9 pages, 4 figures, accepted for publication in ApJ

Probing the Evolution of the Dark Energy Density with Future Supernova Surveys

The time dependence of the dark energy density can be an important clue to the nature of dark energy in the universe. We show that future supernova data from dedicated telescopes (such as SNAP), when combined with data of nearby supernovae, can be used to determine how the dark energy density $\rho_X(z)$ depends on redshift, if $\rho_X(z)$ is not too close to a constant. For quantitative comparison, we have done an extensive study of a number of dark energy models. Based on these models we have simulated data sets in order to show that we can indeed reconstruct the correct sign of the time dependence of the dark energy density, outside of a degeneracy region centered on $1+w_0 = -w_1 z_{max}/3$ (where $z_{max}$ is the maximum redshift of the survey, e.g., $z_{max}=1.7$ for SNAP). We emphasize that, given the same data, one can obtain much more information about the dark energy density directly (and its time dependence) than about its equation of state.Comment: submitted to PR

Discovery and characterization of transiting circumbinary planets from NASA's Kepler mission

Planets with two suns have long fascinated our imagination yet it was only recently that astronomers were able to provide direct evidence of their existence. Several candidates have been proposed since 2003, based on measured timing variations in binary stellar systems, but it was not until 2011 and the launch of NASA’s Kepler mission that circumbinary planets were unambiguously detected through their transits. At the time of writing, the peerless-quality data from Kepler has enabled the confirmation of eight planets orbiting both members of seven gravitationally bound, eclipsing binaries (one of the systems has two circumbinary planets). This thesis presents our contribution to the field in terms of discovery and characterization of three of these transiting circumbinary planetary systems, specifically Kepler-47, Kepler-64, Kepler-413. As predicted by theoretical models, the planets we discovered are smaller than Jupiter, have orbits close to the limit for dynamical stability, and are nearly co-planar to their host binaries (although the circumbinary system Kepler-413 is sufficiently misaligned that, due to fast orbital precession, the planet does not transit at every inferior conjunction). The results of our work deliver important insight into the nature of this remarkable new class of objects, and provide deeper understanding of a) the type of binary stars that can support circumbinary planets; b) the orbital and physical properties of these fascinating systems (e.g. sizes, masses, orbital eccentricities, inclinations, precession rates); and c) planet formation and evolution in multiple stellar systems. Adding new members to the still small family of circumbinary planets has direct relevance for estimating the occurrence frequency of planets around binary stars in particular, and the Galactic planetary census in general, and for the extension of the concept of habitability to binary stellar systems. Here we describe the unique observational signatures of transiting circumbinary planets, the detection method and analysis tools we developed to find and characterize these systems, and the theoretical implications of our discoveries. Specifically, we present the custom-built algorithm we invented to search for individual transit signatures in a light curve. We applied the algorithm to the light curves of ~800 eclipsing binaries from Kepler, and discovered the aperiodic planetary transits in the three circumbinary systems mentioned above. To capitalize on these transits, we developed an analytical model that uses their measured depths and durations to constrain the properties of the host binary star. In addition, we present our ground-based spectroscopic and photometric observations that allowed us to measure the radial velocities of the circumbinary host stars and to constrain photometric contamination from unresolved sources. Finally, we discuss the photometric-dynamic model we developed and applied for the complete characterization of the transiting circumbinary systems Kepler-64 and Kepler-413

The Dartmouth Stellar Evolution Database

The ever-expanding depth and quality of photometric and spectroscopic observations of stellar populations increase the need for theoretical models in regions of age-composition parameter space that are largely unexplored at present. Stellar evolution models that employ the most advanced physics and cover a wide range of compositions are needed to extract the most information from current observations of both resolved and unresolved stellar populations. The Dartmouth Stellar Evolution Database is a collection of stellar evolution tracks and isochrones that spans a range of [Fe/H] from –2.5 to +0.5, [α/Fe] from –0.2 to +0.8 (for [Fe/H] ≤ 0) or +0.2 (for [Fe/H] \u3e 0), and initial He mass fractions from Y = 0.245 to 0.40. Stellar evolution tracks were computed for masses between 0.1 and 4 M☉, allowing isochrones to be generated for ages as young as 250 Myr. For the range in masses where the core He flash occurs, separate He-burning tracks were computed starting from the zero age horizontal branch. The tracks and isochrones have been transformed to the observational plane in a variety of photometric systems including standard UBV(RI)C, Stromgren uvby, SDSS ugriz, 2MASS JHKs, and HST ACS/WFC and WFPC2

A Gas Giant Circumbinary Planet Transiting the F Star Primary of the Eclipsing Binary Star KIC 4862625 and the Independent Discovery and Characterization of the two transiting planets in the Kepler-47 System

We report the discovery of a transiting, gas giant circumbinary planet orbiting the eclipsing binary KIC 4862625 and describe our independent discovery of the two transiting planets orbiting Kepler-47 (Orosz et al. 2012). We describe a simple and semi-automated procedure for identifying individual transits in light curves and present our follow-up measurements of the two circumbinary systems. For the KIC 4862625 system, the 0.52+/-0.018 RJup radius planet revolves every ~138 days and occults the 1.47+/-0.08 MSun, 1.7 +/-0.06 RSun F8 IV primary star producing aperiodic transits of variable durations commensurate with the configuration of the eclipsing binary star. Our best-fit model indicates the orbit has a semi-major axis of 0.64 AU and is slightly eccentric, e=0.1. For the Kepler-47 system, we confirm the results of Orosz et al. (2012). Modulations in the radial velocity of KIC 4862625A are measured both spectroscopically and photometrically, i.e. via Doppler boosting, and produce similar results.Comment: 40 pages, 17 figure