Star-planet magnetic interaction and activity in late-type stars with close-in planets

TL;DR

This paper develops a theoretical model for magnetic interactions between late-type stars and close-in planets, estimating energy release mechanisms and comparing them with observed stellar activity, revealing limitations of current explanations for hot spots.

Contribution

The authors introduce a new theory for star-planet magnetic interactions focusing on energy dissipation via magnetic reconnection and helicity modulation, with quantitative estimates and application to observed systems.

Findings

Energy dissipation scales as v_r B_s^(4/3) B_p^(2/3) R_p^2.

Hot spots in phase with planets are only explained by axisymmetric stellar fields.

Observed hot spots cannot be fully explained by the modeled magnetic interactions.

Abstract

Late-type stars interact with their close-in planets through their coronal magnetic fields. We introduce a theory for the interaction between the stellar and planetary fields focussing on the processes that release magnetic energy in the stellar coronae. We consider the energy dissipated by the reconnection between the stellar and planetary magnetic fields as well as that made available by the modulation of the magnetic helicity of the coronal field produced by the orbital motion of the planet. We estimate the powers released by both processes in the case of axisymmetric and non-axisymmetric, linear and non-linear force-free coronal fields finding that they scale as v_r (B_s)^(4/3) (B_p)^(2/3) (R_p)^2, where v_r is the relative velocity between the stellar and planetary fields, B_s the mean stellar surface field, B_p the planetary field at the poles, and R_p the radius of the planet. A chromospheric hot spot or a flaring activity phased to the orbital motion of the planet are found only when the stellar field is axisymmetric. In the case of a non-axisymmetric field, the time modulation of the energy release is multiperiodic and can be easily confused with the intrinsic stellar variability. We apply our theory to the systems with some reported evidence of star-planet magnetic interaction finding a dissipated power at least one order of magnitude smaller than that emitted by the chromospheric hot spots. The phase lags between the planets and the hot spots are reproduced by our models in all the cases except for upsilon And. In conclusion, the chromospheric hot spots rotating in phase with the planets cannot be explained by the energy dissipation produced by the interaction between stellar and planetary fields as considered by our models and require a different mechanism.