Discovery of a rapidly evolving global magnetic field in the M-dwarf YZ Cet and constraints on the magnetic field of its planet YZ Cet b

Abstract

We present a spectropolarimetric study of the nearby M4.5V exoplanet host star YZ Cet, based on near-infrared observations obtained with the SpectroPolarim\`etre InfraRouge (SPIRou) at the Canada--France--Hawaii Telescope. We detect striking changes in the large-scale magnetic field strength and geometry over the course of just a few stellar rotations, a level of short-term global magnetic field evolution rarely reported in M dwarfs. We modeled the temporal variation of the longitudinal magnetic field using a Gaussian regression process, which allowed us to robustly determine the stellar rotation period and quantify the evolution timescale of the magnetic field. Independent Zeeman Doppler Imaging reconstructions of the two epochs confirm a significant reconfiguration of the star's global magnetic strength and topology. The detection of a weaker, complex, axisymmetric magnetic field (mean $|B| \sim 201$~G), which changes into a stronger, non-axisymmetric, dipole-dominated field (mean $|B| \sim 276$~G) over a few rotation cycles, is in contrast to results from similar fully convective M-dwarf stars. YZ Cet is known to exhibit polarized radio bursts potentially driven by auroral radio emission from star--planet interaction (SPI). By combining our magnetic maps with recent radio observations, we refine the constraints on the magnetic field strength of the innermost planet, YZ Cet b. These results underscore the importance of monitoring stellar magnetic variability to interpret multi-wavelength SPI signatures and to characterize the magnetospheres of potentially habitable exoplanets.