WASP-6b Migrated Into Becoming a Inflated Hot Jupiter

Transits and starspots in the WASP-6 planetary system

Authors:

Tregloan-Reed et al

Abstract:

We present updates to \textsc{prism}, a photometric transit-starspot model, and \textsc{gemc}, a hybrid optimisation code combining MCMC and a genetic algorithm. We then present high-precision photometry of four transits in the WASP-6 planetary system, two of which contain a starspot anomaly. All four transits were modelled using \textsc{prism} and \textsc{gemc}, and the physical properties of the system calculated. We find the mass and radius of the host star to be 0.836±0.063M⊙ and 0.864±0.024R⊙, respectively. For the planet we find a mass of 0.485±0.027MJup, a radius of 1.230±0.035RJup and a density of 0.244±0.014ρJup. These values are consistent with those found in the literature. In the likely hypothesis that the two spot anomalies are caused by the same starspot or starspot complex, we measure the stars rotation period and velocity to be 23.80±0.15\,d and 1.78±0.20\,km\,s−1, respectively, at a co-latitude of 75.8∘. We find that the sky-projected angle between the stellar spin axis and the planetary orbital axis is λ=7.2∘±3.7∘, indicating axial alignment. Our results are consistent with and more precise than published spectroscopic measurements of the Rossiter-McLaughlin effect. These results suggest that WASP-6b formed at a much greater distance from its host star and suffered orbital decay through tidal interactions with the protoplanetary disc.