Transits and starspots in the WASP-6 planetary system

Abstract

We present updates to \\textsc{prism}, a photometric transit-starspot model,\nand \\textsc{gemc}, a hybrid optimisation code combining MCMC and a genetic\nalgorithm. We then present high-precision photometry of four transits in the\nWASP-6 planetary system, two of which contain a starspot anomaly. All four\ntransits were modelled using \\textsc{prism} and \\textsc{gemc}, and the physical\nproperties of the system calculated. We find the mass and radius of the host\nstar to be $0.836\\pm 0.063\\,{\\rm M}_\\odot$ and $0.864\\pm0.024\\,{\\rm R}_\\odot$,\nrespectively. For the planet we find a mass of $0.485\\pm 0.027\\,{\\rm M}_{\\rm\nJup}$, a radius of $1.230\\pm0.035\\,{\\rm R}_{\\rm Jup}$ and a density of\n$0.244\\pm0.014\\,\\rho_{\\rm Jup}$. These values are consistent with those found\nin the literature. In the likely hypothesis that the two spot anomalies are\ncaused by the same starspot or starspot complex, we measure the stars rotation\nperiod and velocity to be $23.80 \\pm 0.15$\\,d and $1.78 \\pm\n0.20$\\,km\\,s$^{-1}$, respectively, at a co-latitude of 75.8$^\\circ$. We find\nthat the sky-projected angle between the stellar spin axis and the planetary\norbital axis is $\\lambda = 7.2^{\\circ} \\pm 3.7^{\\circ}$, indicating axial\nalignment. Our results are consistent with and more precise than published\nspectroscopic measurements of the Rossiter-McLaughlin effect. These results\nsuggest that WASP-6\\,b formed at a much greater distance from its host star and\nsuffered orbital decay through tidal interactions with the protoplanetary disc.\n