WASP-50 b: a hot Jupiter transiting a moderately active solar-type star

Abstract

We report the discovery by the WASP transit survey of a giant planet in a close orbit (0.0295 ± 0.0009 AU) around a moderately bright (V = 11.6, K = 10) G9 dwarf (0.89 ± 0.08 M , 0.84 ± 0.03 R ) in the Southern constellation Eridanus. Thanks to highprecision follow-up photometry and spectroscopy obtained by the telescopes TRAPPIST and Euler, the mass and size of this planet, WASP-50 b, are well constrained to 1.47 ± 0.09 M Jup and 1.15 ± 0.05 R Jup , respectively. The transit ephemeris is 2 455 558.6120 (±0.0002) + N × 1.955096 (±0.000005) HJD UTC . The size of the planet is consistent with basic models of irradiated giant planets. The chromospheric activity (log R HK = −4.67) and rotational period (P rot = 16.3 ± 0.5 days) of the host star suggest an age of 0.8 ± 0.4 Gy that is discrepant with a stellar-evolution estimate based on the measured stellar parameters (ρ * = 1.48 ± 0.10 ρ , T eff = 5400 ± 100 K, [Fe/H] = −0.12 ± 0.08) which favors an age of 7 ± 3.5 Gy. This discrepancy could be explained by the tidal and magnetic influence of the planet on the star, in good agreement with the observations that stars hosting hot Jupiters tend to show faster rotation and magnetic activity. We measure a stellar inclination of 84 +6 −31 deg, disfavoring a high stellar obliquity. Thanks to its large irradiation and the relatively small size of its host star, WASP-50 b is a good target for occultation spectrophotometry, making it able to constrain the relationship between hot Jupiters' atmospheric thermal profiles and the chromospheric activity of their host stars.

Comparison of gyrochronological and isochronal age estimates for transiting exoplanet host stars

“…comm. ), rather than the lower and more accurate value that we have used here based on an analysis of the spectrum from Gillon et al (2011). From a comparison of the gyrochronological and isochronal age estimates, Brown notes that of these eight stars, two show "an age difference of a few Gyr".…”

The TRAPPIST survey of southern transiting planets

“…Using as input values the resulting stellar density and values for T eff and [Fe/H] drawn from their prior distributions, a modified version of the stellar mass calibration law deduced by Torres et al (2010) from well-constrained detached binary systems (see Gillon et al 2011b, for details) is used to derive the stellar mass. The stellar radius is then derived from the stellar density and mass.…”

“…If measurements for the rotational period of the star and for its projected rotational velocity are available, they can be used in addition to the stellar radius values deduced at each step of the MCMC to derive a posterior PDF for the inclination of the star (Watson et al 2010;Gillon et al 2011b). We did not use this option here despite that the rotational period of the star was determined from WASP photometry to be 15.6 ± 0.4 days (H11), because the V sin I * measurement presented by H11 (4.0 ± 0.4 km s −1 ) was presented by these authors as probably affected by a systematic error due to additional broadening of the lines.…”

The discoveries of WASP-91b, WASP-105b and WASP-107b: Two warm Jupiters and a planet in the transition region between ice giants and gas giants

“…We routinely investigate the promising transit signals that we find in WASP lightcurves with the EulerCam imager and the CORALIE spectrograph, both of which are mounted on the 1.2-m Euler-Swiss telescope, and the 0.6-m TRAPPIST imager and (Lendl et al 2012;Queloz et al 2000;Gillon et al 2011;Jehin et al 2011). We provide a summary of our observations of the three target stars in Table 1.…”