Probing the magnetic fields of starspots with transit mapping

Context. Starspots, regions of strong magnetic fields, serve as indicators of stellar activity and the dynamo mechanism at play in the interior of stars. The magnetic fields of main-sequence stars play a crucial role in driving stellar activity. An effective approach to better understanding stellar magnetic fields and activity lies in the detailed characterisation of starspot properties.Aims. We propose a new method for estimating the magnetic fields of starspots that employs modelling techniques of planetary transit mapping, which provides estimates of the size, intensity, and location of spots on the stellar photosphere.Methods. A starspot’s maximum magnetic field was calculated using the linear relationship with the spot flux deficit, ΔF_{spot (the spot’s brightness times its area) and the well-characterised relation for sunspots determined in this work, Bspot = 1170 + 844log Δ​Fspot (G).Results. Applying this relationship to previously mapped spots on the photospheres of 14 FGK and M stars yields spot maximum magnetic fields ranging from 2700 G to 4600 G, with an overall average of 3900 ± 400 G. We looked for correlations between starspot magnetic fields and stellar properties. We did not find any correlation between a spot’s mean extreme magnetic field and effective temperature, nor the differential shear. However, a weak anti-correlation is seen between the spots’ magnetic field and stellar age as well as between the magnetic field and the rotation period.Conclusions. When compared with previous results of small-scale magnetic field measurements, the B values obtained here are basically constant and near the saturation limit found for rapid rotators. This implies that it is not the intensity of the magnetic field of starspots that decreases with age but rather the filling factor. This result offers a unique window into the magnetic dynamo of stars.}