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

Brothwell, R. D.; Watson, C. A.; Hebrard, G.; Triaud, A. H. M. J.; Cegla, H. M.; Santerne, A.; Hebrard, E.; Anderson, D. R.; Pollacco, D.; Simpson, E. K.; Bouchy, F.; Brown, D. J. A.; Chew, Y. G. M.; Cameron, A. C.; Armstrong, D. J.; Barros, S. C. C.; Bento, J.; Bochinski, J.; Burwitz, V.; Busuttil, R.; Delrez, L.; Doyle, A. P.; Faedi, F.; Fumel, A.; Gillon, M.; Haswell, C. A.; Hellier, C.; Jehin, E.; Kolb, U.; Lendl, M.; Liebig, C.; Maxted, P. F. L.; McCormac, J.; Miller, G. R. M.; Norton, A. J.; Pepe, F.; Queloz, D.; Rodriguez, J.; Segransan, D.; Skillen, I.; Smalley, B.; Stassun, K. G.; Udry, S.; West, R. G. and Wheatley, P. J. (2014). A window on exoplanet dynamical histories: Rossiter-McLaughlin observations of WASP-13b and WASP-32b. Monthly Notices of the Royal Astronomical Society, 440(4) pp. 3392–3401.

DOI: https://doi.org/10.1093/mnras/stu520

Abstract

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 (λ). WASP-13b and WASP-32b both have prograde orbits and are consistent with alignment with measured sky-projected angles of λ =8°⁺¹³_-12 and λ =-2°⁺¹⁷_-19, respectively. Both WASP-13 and WASP-32 have T_eff < 6250 K, 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 per cent confidence) was identified for the WASP-32 system with P_rot = 11.6 ± 1.0 days. This rotation period is in agreement with the predicted stellar rotation period calculated from the stellar radius, R*, and vsin i if a stellar inclination of i* = 90° is assumed. With the determined rotation period, the true 3D angle between the stellar rotation axis and the planetary orbit, ψ, was found to be ψ = 11° ± 14°. We conclude with a discussion on the alignment of systems around cool host stars with T_eff < 6150 K by calculating the tidal dissipation time-scale. We find that systems with short tidal dissipation time-scales are preferentially aligned and systems with long tidal dissipation time-scales have a broad range of obliquities.

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