Full Stokes magnetometry of the active M dwarfs AU Mic and EV Lac with SPIRou

TL;DR

This study uses SPIRou spectropolarimetry to map and analyze the magnetic fields of active M dwarfs AU Mic and EV Lac, revealing differences in their magnetic topology and the importance of linear polarization data.

Contribution

First detailed magnetic topology maps of AU Mic and EV Lac using near-infrared spectropolarimetry and Zeeman-Doppler imaging, highlighting the role of linear polarization in complex field reconstruction.

Findings

AU Mic has a mainly poloidal, axisymmetric magnetic field.

EV Lac exhibits a more complex, stronger, and less axisymmetric magnetic field.

Linear polarization (Stokes QU) improves magnetic field reconstruction, especially for complex fields.

Abstract

We report in this paper circularly and linearly polarized observations of the young active M dwarfs AU Mic and EV Lac with the near-infrared SPIRou spectropolarimeter at the Canada-France-Hawaii Telescope, collected from August to October 2023 over a few rotation cycles of both stars. Applying Least-Squares Deconvolution (LSD) to our spectra, we detected Zeeman signatures in circular (Stokes V) and linear (Stokes QU) polarization, and Zeeman broadening in unpolarized (Stokes I) LSD profiles, all exhibiting clear rotational modulation. Using the stellar surface tomographic technique of Zeeman-Doppler imaging on our sets of observations, along with a simple parametric description of how the small-scale and large-scale fields relate to each other, we recovered the magnetic topologies of AU Mic and EV Lac successively from LSD Stokes V, Stokes IV and Stokes IVQU profiles, to investigate how the reconstructed maps evolve as we provide more information, and ultimately infer reliable magnetic maps of both stars. We find that AU Mic hosts a fairly simple and mostly poloidal large-scale field aligned with the rotation axis within about 10deg, whereas that of EV Lac is more complex, stronger and less axisymmetric. Both stars feature intense small-scale fields, of about 4 kG for AU Mic and 6 kG for EV Lac when averaged over the whole stellar surface. Stokes QU Zeeman signatures allow one to reconstruct stellar magnetic fields more reliably, and are especially useful for stars with more complex fields and low vsini's like EV Lac.