Activity of low-mass stars in the light of spot signature in the Fourier domain

¹ Institut d’Astrophysique Spatiale (IAS), CNRS, Université Paris-Saclay, Orsay 91400, France
² LESIA, CNRS, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon Cedex, France
³ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

^⋆ Corresponding author; lucie.degott@universite-paris-saclay.fr

Received: 21 June 2024
Accepted: 17 February 2025

Abstract

Context. Magnetic fields exhibit a wide variety of behaviours in low-mass stars and further characterisation is required to understand these observations. Stellar photometry from space missions such as MOST, CoRoT, Kepler, and TESS (and in the near future PLATO) provide thousands of highly precise light curves (LCs) that can shed new light on stellar activity, in particular through the signature of transiting spots.

Aims. We study the impact of star spots on LCs in the Fourier domain, reducing the degeneracies encountered by direct spot modelling in the temporal domain. We use this new formulation to explore the spot properties from the available data.

Methods. We propose a model of LC power spectra at low frequency based on a description of spot transits that allows us to retrieve information about the amplitude of their photometric impact ℋ and about the spot mean lifetime over the observation τ_life when the power spectrum exibits rotation peaks. We validated this method with simulated LCs and then applied it to the Kepler data to extract global trends over a set of more than 37 755 stars.

Results. Our analysis leads to a classification of the sample into ‘peakless’ or ‘with peaks’ spectra and enables the identification of different activity regimes based on ℋ and τ_life for different Rossby number ranges. More specifically, we observed an intense regime of activity between Ro = 0.7 and Ro = 1, for stars with masses under 1 M_⊙.

Conclusions. This new systematic method can be used to provide new observational constraints on stellar activity (and possibly a link with stellar magnetism) when applied to large photometric datasets, such as those from the future PLATO mission.