Transits and starspots in the WASP-6 planetary system

• Main Authors: Tregloan-Reed, Jeremy, Southworth, John, Burgdorf, M, Novati, S. Calchi, Dominik, M, Finet, F, Jørgensen, U. G, Maier, G, Mancini, L, Proft, S, Ricci, D, Snodgrass, C, Bozza, V, Browne, P, Dodds, P, Gerner, T, Harpsøe, K, Hinse, T. C, Hundertmark, M, Kains, N, Kerins, E, Liebig, C, Penny, M. T, Rahvar, S, Sahu, K, Scarpetta, G, Schäfer, S, Schönebeck, F, Skottfelt, J, Surdej, J
• Format: Journal Article
• Language: English
• Published: 31-03-2015
• Subjects: Physics - Earth and Planetary Astrophysics
• DOI: 10.48550/arxiv.1503.09184

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 $0.836\pm 0.063\,{\rm M}_\odot$ and $0.864\pm0.024\,{\rm R}_\odot$,
respectively. For the planet we find a mass of $0.485\pm 0.027\,{\rm M}_{\rm
Jup}$, a radius of $1.230\pm0.035\,{\rm R}_{\rm Jup}$ and a density of
$0.244\pm0.014\,\rho_{\rm Jup}$. 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 $23.80 \pm 0.15$\,d and $1.78 \pm
0.20$\,km\,s$^{-1}$, respectively, at a co-latitude of 75.8$^\circ$. We find
that the sky-projected angle between the stellar spin axis and the planetary
orbital axis is $\lambda = 7.2^{\circ} \pm 3.7^{\circ}$, 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-6\,b formed at a much greater distance from its host star and
suffered orbital decay through tidal interactions with the protoplanetary disc.