  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.
