Star-planet tidal interactions in the WASP-12 system

Star-planet tidal interactions in the WASP-12 system

Gracjan Maciejewski

To date, the exoplanet WASP-12 b is the only hot Jupiter for which the shortening of its orbital period was detected. A mechanism that drives this orbital decay remains a puzzle. Equilibrium stellar tides alone were found to be too weak to explain the observed rate of inspiral and dynamical stellar tides or planetary obliquity tides have been considered. Compared to other stars, WASP-12 is distinguished by its extremely high efficiency of the dissipation of the tidal energy, typical rather for sub-giants than for main sequence dwarfs. Therefore, it cannot be ruled out that the system is observed at a specific moment of the stellar evolution in which the star leaves the main sequence, heading towards the red giants branch. Using the new radial velocity (RV) measurements acquired with the HARPS-N instrument at the Telescopio Nazionale Galileo, we show that the observed orbital eccentricity of WASP-12 b is non-zero at a 5-sigma level, and the longitude of pericentre of this apparently eccentric orbit is close to 270 degrees. This orbital configuration is compatible with a solution containing a circular orbit and an RV signal induced by the tidal fluid flow in the star. The amplitude of the RV tides was found to be consistent with a value calculated using the equilibrium tide approximation. This finding opens new possibilities in probing the physical properties of stellar interiors.

Proceedings of the Polish Astronomical Society, vol. 10, 88-93 (2020)

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