Issue |
A&A
Volume 687, July 2024
|
|
---|---|---|
Article Number | A187 | |
Number of page(s) | 14 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/202348279 | |
Published online | 12 July 2024 |
Equilibrium tides and magnetic activity in stars with close-by massive planets
The intriguing case of WASP-18
INAF-Osservatorio Astrofisico di Catania,
Via S. Sofia 78,
95123
Catania,
Italy
e-mail: antonino.lanza@inaf.it; sylvain.breton@inaf.it
Received:
14
October
2023
Accepted:
20
March
2024
Aims. WASP-18 is an F6V star that hosts a planet with a mass of ~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.
Methods. 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 characterised by a very high Reynolds number (Re ~ 1010), 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.
Results. 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.
Key words: planet-star interactions / stars: activity / stars: magnetic field / planetary systems / stars: individual: WASP-18 / stars: solar-type
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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