A window on exoplanet dynamical histories: Rossiter–McLaughlin observations of WASP-13b and WASP-32b

We present Rossiter-McLaughlin observations of WASP-13b and WASP-32b and determine the sky-projected angle between the normal of the planetary orbit and the stellar rotation axis ( ). WASP-13b and WASP-32b both have prograde orbits and are consistent with alignment with measured sky-projected angles of = 8 +13 12 and = 2 +17 19 , respectively. Both WASP-13 and WASP-32 have Te < 6250K and therefore these systems support the general trend that aligned planetary systems are preferentially found orbiting cool host stars. A Lomb-Scargle periodogram analysis was carried out on archival SuperWASP data for both systems. A statistically significant stellar rotation period detection (above 99.9% confidence) was identified for the WASP-32 system with Prot = 11:6 1:0 days. This rotation period is in agreement with the predicted stellar rotation period calculated from the stellar radius, R?, and v sini if a stellar inclination ofi? = 90 is assumed. With the determined rotation period, the true 3D angle between the stellar rotation axis and the planetary orbit, , was found to be = 11 14. We conclude with a discussion on the alignment of systems around cool host stars with Te < 6150K by calculating the tidal dissipation timescale. We find that systems with short tidal dissipation timescales are preferentially aligned and systems with long tidal dissipation timescales have a broad range of obliquities.