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

Abstract

Context. Late-type stars interact with their close-in planets through their coronal magnetic fields. Aims. 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. Methods. 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 B 4/3 0 B 2/3 p0 R 2 p v rel , where B 0 is the mean stellar surface field, B p0 the planetary field at the poles, R p the radius of the planet, and v rel the relative velocity between the stellar and the planetary fields. Results. 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 υ And. Conclusions. 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.

Time-resolved UVES observations of a stellar flare on the planet host HD 189733 during primary transit

“…1, Pillitteri et al (2010Pillitteri et al ( , 2011Pillitteri et al ( , 2014 caused by the orbiting exoplanet. Their findings are supported by the models of Lanza (2012), who predict a modulation of coronal flaring activity with the orbital phase of the planet.…”

On the correlation between stellar chromospheric flux and the surface gravity of close-in planets

“…Lanza (2009Lanza ( , 2012 introduced coronal field models and described their properties. Linear force-free fields represent the minimum energy configuration for a given total magnetic helicity that is attained thanks to the continuous energy dissipation associated with the reconnection between the coronal and the planetary fields as the planet orbits the star.…”

The GAPS programme with HARPS-N at TNG

“…A peculiar characteristic of this star is the optically variable modulation of the light curve that is probably due to photospheric spots that persisted at fixed longitudes for a few hundred days, as observed by the MOST satellite . SPMI is a long-debated issue, with the best evidence coming from a modulation of chromospheric activity tracers phased with the planetary orbital period rather than with the stellar rotation period (Shkolnik et al 2005(Shkolnik et al , 2008Lanza 2009Lanza , 2012.…”