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

¹ Univ. de Toulouse, CNRS, IRAP, 14 avenue Belin, 31400 Toulouse, France
² Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
³ Université de Montréal, Département de Physique, IREX, Montréal, QC H3C 3J7, Canada
⁴ Departamento de Física – ICEx – UFMG, Av. Antônio Carlos, 6627, 30270-901 Belo Horizonte, MG, Brazil
⁵ Department of Earth Sciences, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822 USA
⁶ Institute for Astrophysics, University of Vienna, 1180 Vienna, Austria
⁷ Dublin Institute for Advanced Studies, Astronomy & Astrophysics Section, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland

^★ Corresponding author: jean-francois.donati@irap.omp.eu

Received: 2 May 2025
Accepted: 1 July 2025

Abstract

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 M_b = 6.3_−1.8^+2.5 M_⊕ and M_c = 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 M_e = 21.1_−4.3^+5.4 M_⊕.