Effects of magnetic fields on the center-to-limb variation in solar-type stars

¹ Landessternwarte – Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
e-mail: H.Ludwig@lsw.uni-heidelberg.de
² Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
³ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden

Received: 1 May 2023
Accepted: 3 August 2023

Abstract

Context. High-precision photometry of exoplanet transits obtained with the Kepler satellite allows one to derive information on the center-to-limb variation (CLV) of the host stars. Recent analyses indicate a small but systematic discrepancy between observations and theoretical expectations based on detailed multidimensional model atmospheres. It has been hypothesized that the discrepancy is related to the neglect of magnetic fields in the models.

Aims. Our goal is to test the above hypothesis for solar-like stars. We further intend to quantify the consequences for interferometry, and the possibility of extracting information at the level of stellar magnetic activity from the CLV.

Methods. We constructed a sequence of multidimensional models including magnetic fields of varying strengths. We derived theoretical predictions on the CLV, taking into account factors like the observational passband, stellar sphericity, the methodology of the light curve analysis, and interstellar extinction.

Results. The models predict a relative brightening of the stellar limb with increasing magnetic field strength, which qualitatively goes in the direction of reducing the mismatch between observation and theory. Quantitatively, however, the mismatch is not fully eliminated. Interstellar extinction on a level A_V ≲ 1 mag has little impact on the CLV and is largely degenerate with the influence of magnetic fields. Global magnetic activity at field strengths ≲300 G influences interferometric radius measurements to ≲1%. We emphasize that our results refer to measurements taken in the Kepler passband.

Conclusions. The presence of magnetic activity appears to be a plausible explanation for the present discrepancy between observation and theory. The still-present partial mismatch needs to be understood. To this end, we point to improvements in modeling and wishes for more observational data of active stars, including spectral information.