The Magnetic Field of the Active Planet-hosting M Dwarf AU Mic

AU Mic is a young, very active M dwarf star with a debris disk and at least one transiting Neptune-size planet. Here we present a detailed analysis of the magnetic field of AU Mic based on previously unpublished high-resolution optical and near-infrared spectropolarimetric observations. We report a systematic detection of circular and linear polarization signatures in the stellar photospheric lines. Tentative Zeeman Doppler imaging modeling of the former data suggests a nonaxisymmetric global field with a surface-averaged strength of about 90 G. At the same time, linear polarization observations indicate the presence of a much stronger 2 kG axisymmetric dipolar field, which contributes no circular polarization signal due to the equator-on orientation of AU Mic. A separate Zeeman broadening and intensification analysis allowed us to determine a mean field modulus of 2.3 and 2.1 kG from the Y- and K-band atomic lines, respectively. These magnetic field measurements are essential for understanding environmental conditions within the AU Mic planetary system.