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KOI-13b
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Title: Rapid rotators revisited: absolute dimensions of KOI-13
Author: Ian D. Howarth, Giuseppe Morello

We analyse Kepler light-curves of the exoplanet KOI-13b transiting its moderately rapidly rotating (gravity-darkened) parent star. A physical model, with minimal ad hoc free parameters, reproduces the time-averaged light-curve at the ca. 10 parts per million level. We demonstrate that this Roche-model solution allows the absolute dimensions of the system to be determined from the star's projected equatorial rotation speed, v(e)sin(i), without any additional assumptions; we find a planetary radius 1.330.05 R(Jup), stellar polar radius 1.550.06 R(sun), combined mass M(*) + M(P) (\simeq M*) = 1.47 0.17 M(sun), and distance d \simeq 37025 pc, where the errors are dominated by uncertainties in relative flux contribution of the visual-binary companion KOI-13B. The implied stellar rotation period is within ca. 5% of the non-orbital, 25.43-hr signal found in the Kepler photometry. We show that the model accurately reproduces independent tomographic observations, and yields an offset between orbital and stellar-rotation angular-momentum vectors of 60.250.05 degrees.

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RE: KOI-13
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Title: A massive exoplanet candidate around KOI-13: Independent confirmation by ellipsoidal variations
Authors: D. Mislis, S. Hodgkin

We present an analysis of the KOI-13.01 candidate exoplanet system included in the September 2011 Kepler data release. The host star is a known and relatively bright (m_{KP} = 9.95) visual binary with a separation significantly smaller (0.8 arcsec) than the size of a Kepler pixel (4 arcsec per pixel). The Kepler light curve shows both primary and secondary eclipses, as well as significant out-of-eclipse light curve variations. We confirm that the transit occurs round the brighter of the two stars. We model the relative contributions from (i) thermal emission from the companion, (ii) planetary reflected light, (iii) Doppler beaming, and (iv) ellipsoidal variations in the host-star arising from the tidal distortion of the host star by its companion. Our analysis, based on the light curve alone, enables us to constrain the mass of the KOI-13.01 companion to be M_C = 8.3 1.25M_J and thus demonstrates that the transiting companion is a planet (rather than a brown dwarf which was recently proposed by {b7}). The high temperature of the host star (Spectral Type A5-7V, T_ eff = 8511-8020 K), combined with the proximity of its companion KOI-13.01, may make it one of the hottest exoplanets known, with a detectable thermal contribution to the light curve even in the Kepler optical passband. However, the single passband of the Kepler light curve does not enable us to unambiguously distinguish between the thermal and reflected components of the planetary emission. Infrared observations may help to break the degeneracy, while radial velocity follow-up with \sigma ~ 100 m s^{-1} precision should confirm the mass of the planet.

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Title: Kepler KOI-13.01 - Detection of beaming and ellipsoidal modulations pointing to a massive hot Jupiter
Authors: T. Mazeh, G. Nachmani, G. Sokol, S. Faigler, S. Zucker

KOI-13 was presented by the Kepler team as a candidate for having a giant planet - KOI-13.01, with orbital period of 1.7 d and transit depth of ~0.8%. We have analysed the Kepler Q2 data of KOI-13, which was publicly available at the time of the submission of this paper, and derived the amplitudes of the beaming, ellipsoidal and reflection modulations: 8.6 1.1, 66.8 1.6 and 72.0 1.5 ppm (parts per million), respectively. After the paper was submitted, Q3 data were released, so we repeated the analysis with the newly available light curve. The results of the two quarters were quite similar. From the amplitudes of the beaming and the ellipsoidal modulations we derived two independent estimates of the mass of the secondary. Both estimates, 6 3 M_Jup and 4 2 M_Jup, suggested that KOI-13.01 was a massive planet, with one of the largest known radii. We also found in the data a periodicity of unknown origin with a period of 1.0595 d and a peak-to-peak modulation of ~60 ppm. The light curve of Q3 revealed a few more small-amplitude periodicity with similar frequencies. It seemed as if the secondary occultation of KOI-13 was slightly deeper than the reflection peak-to- peak modulation by 16.8 4.5 ppm. If real, this small difference was a measure of the thermal emission from the night side of KOI-13.01. We estimated the effective temperature to be 2600 150 K, using a simplistic black-body emissivity approximation. We then derived the planetary geometrical and Bond albedos as a function of the day-side temperature. Our analysis suggested that the Bond albedo of KOI-13.01 might be substantially larger than the geometrical albedo.

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Title: Asymmetric transit curves as indication of orbital obliquity: clues from the brown dwarf companion in KOI-13
Authors: Gy. M. Szabo, R. Szabo, J. M. Benko, H. Lehmann, Gy. Mezo, A. E. Simon, Zs. Kovari, G. Hodosan, Zs. Regaly, L. L. Kiss

KOI-13.01, a planet-sized companion in an optical double star was announced as one of the 1235 Kepler planet candidates in February 2011. The transit curves show significant distortion that was stable over the ~130 days time-span of the data. Here we investigate the phenomenon via detailed analyses of the two components of the double star and a re-reduction of the Kepler data with pixel-level photometry. Our results indicate that KOI-13 is a common proper motion binary, with two rapidly rotating components (v sin i ~ 65--70 km/s). We identify the host star of KOI-13.01 and conclude that the transit curve asymmetry is consistent with a companion orbiting a rapidly rotating, possibly elongated star on an oblique orbit. After correcting the Kepler light curve to the second light of the optical companion star, we derive a radius of 2.2 R_J for the transiter, implying that the object is a hot brown dwarf. KOI-13 is the first example for detecting orbital obliquity for a substellar companion without measuring the Rossiter-McLaughlin effect from spectroscopy.

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