The magnetic properties of the planet host star Kepler-78

TL;DR

This study investigates the magnetic and rotational properties of Kepler-78, revealing a complex magnetic topology, a star cycle of approximately 1,300 days, and differential rotation, with implications for star-planet interaction models.

Contribution

The paper provides detailed measurements of Kepler-78's magnetic field, topology, and activity cycle, enhancing understanding of star-planet magnetic interactions.

Findings

Average magnetic field amplitude of 16 G

Magnetic topology with 60% poloidal and 40% toroidal energy

Detected differential rotation rate of 0.105+-0.039 rad/d

Abstract

Kepler-78 is host to a transiting 8.5-hour orbit super-Earth. In this paper, the rotation and magnetic properties of the planet host star are studied. We first revisit the Kepler photometric data for a detailed description of the rotation properties of Kepler-78, showing that the star seems to undergo a cycle in the spot pattern of ~1,300 day duration. We then use spectropolarimetric observations with CFHT/ESPaDOnS to measure the circular polarization in the line profile of the star during its rotation cycle, as well as spectroscopic proxies of the chromospheric activity. The average field has an amplitude of 16 G. The magnetic topology is characterized by a poloidal and a toroidal component, encompassing 60% and 40% of the magnetic energy, respectively. Differential rotation is detected with an estimated rate of 0.105+-0.039 rad/d. Activity tracers vary with the rotation cycle of the star; there is no hint that a residual activity level is related to the planetary orbit at the precision of our data. The description of the star magnetic field's characteristics then may serve as input for models of interactions between the star and its close-by planet, e.g., Ohmic dissipation and unipolar induction.