Six-year SPIRou monitoring of the young planet-host AU Mic

In this paper we revisit our spectropolarimetric and velocimetric analysis of the young M dwarf AU Mic based on data collected with SPIRou at the Canada-France-Hawaii telescope, over a monitoring period of 2041 days from 2019 to 2024. The longitudinal magnetic field, the small-scale magnetic field, and the differential temperature of AU Mic, derived from the unpolarized and circularly polarized spectra, were clearly modulated with the stellar rotation period, with a pattern that evolved over time. Magnetic modeling with Zeeman-Doppler imaging provides a consistent description of the global field of AU Mic. This model agrees not only with the least-squares deconvolved profiles of the circularly polarized and unpolarized spectral lines, but also with the small-scale field measurements derived from the broadening of spectral lines, for each of the 11 subsets of the full data. We find that the large-scale field is mostly poloidal, with a dominant dipole component slightly tilted to the rotation axis which decreased from 1.4 to 1.1 kG before increasing at the end of the campaign. The average small-scale field followed a similar trend, decreasing from 2.8 to 2.6 kG, then rising. The long-term magnetic evolution we report for AU Mic suggests that, if cyclic, the cycle period is significantly longer than 6 years. From velocimetric data, we derive improved mass estimates for the two transiting planets, with respective masses of Mb = 6.3−1.8+2.5 M⊕ and Mc = 11.6−2.7+3.3 M⊕, yielding very contrasting densities of 0.32−0.10+0.13 and 2.9−0.8+1.1 g cm−3 and a new 90% confidence upper limit of 4.9 M⊕ for candidate planet d (period 12.7 days), suspected to induce the transit-timing variations of b and c. We also confirm our claim that candidate planet e orbits AU Mic with a period of 33.11 ± 0.06 d, albeit with a smaller mass of Me = 21.1−4.3+5.4 M⊕.