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_sun, 0.84+-0.03 R_sun) in the Southern constellation Eridanus. Thanks to high-precision follow-up photometry and spectroscopy obtained by the telescopes TRAPPIST and Euler, the mass and size of this planet, WASP-50b, are well constrained to 1.47+-0.09 M_jup and 1.15+-0.05 R_jup, respectively. The transit ephemeris is 2455558.6120 (+-0.0002) + N x 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 (rho_star = 1.48+-0.10 rho_sun, Teff = 5400+-100 K, [Fe/H]= -0.12+-0.08) which favours 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 (Pont 2009; Hartman 2010). We measure a stellar inclination of 84 (-31,+6) deg, disfavouring a high stellar obliquity. Thanks to its large irradiation and the relatively small size of its host star, WASP-50b 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 proposed by Knutson et al. (2010).
Highlights
While the Kepler space mission is pursuing its pioneering search for habitable terrestrial planets transiting solar-type stars (Borucki et al 2011), ground-based, wide-field surveys continue to detect short-period transiting giant planets at an increasing rate
In order to confirm the origin of the transit signal on WASP-50 and to constrain thoroughly its parameters, three transits were observed at high-signal-to-noise ratios (SNR) with the new telescope TRAPPIST located at ESO La Silla Observatory in the Atacama Desert, Chile
Once WASP-50 was identified as a high priority candidate we gathered spectroscopic measurements with the CORALIE spectrograph mounted on Euler to confirm the planetary nature of the transiting body and obtain a mass measurement. 15 spectra were obtained from 2010 December 5 to 2011 January 5 with an exposure time of 30 min
Summary
While the Kepler space mission is pursuing its pioneering search for habitable terrestrial planets transiting solar-type stars (Borucki et al 2011), ground-based, wide-field surveys continue to detect short-period (i.e. a few days, or even less) transiting giant planets at an increasing rate. Hot Jupiters make it possible to study the physical response of a giant planet to an irradiation orders of magnitude larger than in solar system planets (Burrows et al 2008; Fortney et al 2010), and to the strong gravitational and magnetic fields so close to their host stars (Correia & Laskar 2010; Chang et al 2010) These planets could be able to influence the properties of their parent stars, notably by modifying their angular momentum budget and by inducing chromospheric activity (Lanza 2010). The atmospheres of hot Jupiters can be studied thoroughly Their generally large atmospheric scale heights and their frequent transits maximize the signal-to-noise ratios (SNR) achievable on short timescales in transit absorption spectrophotometry, while their large irradiation makes it possible to measure the thermal emission profile of their dayside through occultation-depth measurements at different wavelengths (Seager 2010).
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