Abstract
We report the discovery of Kepler-77b (alias KOI-127.01), a Saturn-mass transiting planet in a 3.6-day orbit around a metal-rich solar-like star. We combined the publicly available Kepler photometry (quarters 1-13) with high-resolution spectroscopy from the Sandiford@McDonald and FIES@NOT spectrographs. We derived the system parameters via a simultaneous joint fit to the photometric and radial velocity measurements. Our analysis is based on the Bayesian approach and is carried out by sampling the parameter posterior distributions using a Markov chain Monte Carlo simulation. Kepler-77b is a moderately inflated planet with a mass of Mp=0.430+/-0.032 Mjup, a radius of Rp=0.960+/-0.016 Rjup, and a bulk density of 0.603+/-0.055 g/cm^3. It orbits a slowly rotating (P=36+/-6 days) G5V star with M*=0.95+/-0.04 Msun, R*=0.99+/-0.02 Rsun, Teff=5520+/-60 K, [M/H]=0.20+/-0.05, that has an age of 7.5+/-2.0 Gyr. The lack of detectable planetary occultation with a depth higher than about 10 ppm implies a planet geometric and Bond albedo of Ag<0.087+/-0.008 and Ab<0.058+/-0.006, respectively, placing Kepler-77b among the gas-giant planets with the lowest albedo known so far. We found neither additional planetary transit signals nor transit-timing variations at a level of about 0.5 minutes, in accordance with the trend that close-in gas giant planets seem to belong to single-planet systems. The 106 transits observed in short-cadence mode by Kepler for nearly 1.2 years show no detectable signatures of the planet's passage in front of starspots. We explored the implications of the absence of detectable spot-crossing events for the inclination of the stellar spin-axis, the sky-projected spin-orbit obliquity, and the latitude of magnetically active regions.
Highlights
Space-based transit surveys are opening up a new exciting era in exoplanetary science, enlarging the known parameter space of planetary systems
Those objects have been accurately vetted for astrophysical false-positives using Kepler photometry, many of them − especially those where only a single planet is observed to transit − remain planetary candidates and deserve additional investigations, including high-resolution spectroscopy and radial velocity (RV) measurements
Ground-based observations (e.g., Colón et al 2012; Lillo-Box et al 2012; Santerne et al 2012) have recently proven that the false-positive rate of Kepler transiting candidates might be noticeably higher than the 5% previously claimed by Morton & Johnson (2011b), proving the need for more investigations to assess the real nature of these objects
Summary
Space-based transit surveys are opening up a new exciting era in exoplanetary science, enlarging the known parameter space of planetary systems. Based upon the analysis of the first sixteen months of Kepler photometry, Batalha et al (2013) have recently announced 2321 transiting planet candidates (Kepler object of interests, KOIs), almost doubling the number of candidates previously published by Borucki et al (2010). Those objects have been accurately vetted for astrophysical false-positives using Kepler photometry We combined Kepler public photometry with high-resolution spectroscopy carried out with Sandiford at McDonald-2.1 m and FIES at NOT to confirm the planetary nature of the transiting object and derive the system parameters.
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