Equilibrium tides and magnetic activity in stars with close-by massive planets. The intriguing case of WASP-18

TL;DR

This paper proposes a mechanism involving equilibrium tide-induced turbulence in stars with close massive planets, explaining their unexpectedly low magnetic activity despite rapid rotation.

Contribution

It introduces a novel theory that tidal interactions can suppress magnetic activity by altering internal stellar stratification and flux emergence.

Findings

Tidal turbulence can reduce magnetic flux emergence.

Suppressed starspot formation explains low X-ray activity.

Mechanism applies to WASP-18 and similar systems.

Abstract

WASP-18 is an F6V star that hosts a planet with a mass of about 10 Jupiter masses and an orbital period of 0.94 days. In spite of its relatively fast rotation and young age, the star remains undetected in X-rays, thus implying a very low level of magnetic activity. To account for such unexpected properties, we propose a mechanism that modifies the internal stratification and the photospheric magnetic activity of a late-type main sequence star with a close-by massive planet based on the action of the equilibrium tide. We speculate that the horizontal flow produced by the equilibrium tide may interact with the convective plumes in the overshoot layer below the stellar outer convective envelope. The interaction is characterized by a very high Reynolds number leading to the development of turbulent boundary layers at the surface of such structures, whereas turbulent wakes extend over most of the overshoot layer that they straddle. We propose that such a tidally induced turbulence can lead to a reduction of the filling factor of the downdrafts in the overshoot layer. As a consequence, the absolute value of the sub-adiabatic gradient increases in that layer hindering the emergence of magnetic flux tubes responsible for the formation of photospheric starspots. We conjecture that this process is occurring in WASP-18, thus providing a possible mechanism to account for the very low level of magnetic activity observed for such a planet host.