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arXiv · 2507.01746

Six-yr SPIRou monitoring of the young planet-host dwarf AU Mic

J. -F. Donati·P. I. Cristofari·C. Moutou·A. L'Heureux·N. J. Cook·E. Artigau·S. H. P. Alencar·E. Gaidos·A. Vidotto·P. Petit·A. Carmona·T. Ray·the SPIRou science team
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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 d 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. The magnetic modeling with Zeeman-Doppler imaging provides a consistent description of the global field of AU Mic that 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 was 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 yr. From velocimetric data, we derived improved mass estimates for the two transiting planets, respectively equal to M_b = 6.3+2.5-1.8 M_earth and M_c = 11.6+3.3-2.7 M_earth, yielding very contrasting densities of 0.32+0.13-0.10 and 2.9+1.1-0.8 g/cm3, and a new 90% confidence upper limit of 4.9 M_earth for candidate planet d (period 12.7 d) suspected to induce the transit-timing variations of b and c. We also confirm our claim regarding candidate planet e orbiting with a period of 33.11+-0.06 d, albeit with a smaller mass of M_e = 21.1+5.4-4.3 M_earth.

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J. -F. Donati, P. I. Cristofari, C. Moutou, A. L'Heureux, N. J. Cook, E. Artigau, S. H. P. Alencar, E. Gaidos, A. Vidotto, P. Petit, A. Carmona, T. Ray, the SPIRou science team. 2025-07-02. Six-yr SPIRou monitoring of the young planet-host dwarf AU Mic. https://arxiv.org/abs/2507.01746

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