The well-aligned orbit of WASP-84b: evidence for disc migration

Abstract

We report the sky-projected orbital obliquity (spin-orbit angle) of WASP-84b, a 0.70-$M_{\mathrm{Jup}}$ planet in a 8.52-day orbit around a G9V/K0V star, to be $\lambda =0.3\pm 1.7^{\circ }$. We obtain a true obliquity of $\psi =14.8\pm 8.0^{\circ }$ from a measurement of the inclination of the stellar spin axis with respect to the sky plane. Due to the young age and the weak tidal forcing of the system, we suggest that the orbit of WASP-84b is unlikely to have both realigned and circularised from the misaligned and/or eccentric orbit likely to have arisen from high-eccentricity migration. Therefore we conclude that the planet probably migrated via interaction with the protoplanetary disc. This would make it the first short-orbit, giant planet to have been shown to have migrated via this pathway. Further, we argue that the distribution of obliquities for planets orbiting cool stars ($T_{\mathrm{eff}}$ < 6250 K) suggests that high-eccentricity migration is an important pathway for the formation of short-orbit, giant planets.

6 pages, 3 figures, 1 table, submitted to ApJL

References in corpus (15)

• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• Formation of Hot Planets by a combination of planet scattering, tidal circularization, and Kozai mechanism
• Tidal dissipation in stars and giant planets
• Efficient identification of exoplanetary transit candidates from SuperWASP light curves
• Determining stellar macroturbulence using asteroseismic rotational velocities from Kepler
• Stellar activity of planetary host star HD 189733
• Alignment of the stellar spin with the orbits of a three-planet system
• Long Term Evolution of Close Planets Including the Effects of Secular Interactions
• Photometric and spectroscopic detection of the primary transit of the 111-day-period planet HD 80606 b
• WASP-14b: 7.3 Mj transiting planet in an eccentric orbit
• On the tidal origin of hot Jupiter stellar obliquity trends
• Tidal Dissipation and Obliquity Evolution in Hot Jupiter Systems
• The GAPS Programme with HARPS-N at TNG. III: The retrograde orbit of HAT-P-18b
• The Spin-Orbit Alignment of the HD17156 Transiting Eccentric Planetary System
• WASP-117b: a 10-day-period Saturn in an eccentric and misaligned orbit

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