The magnetic field and multiple planets of the young dwarf AU~Mic

Abstract

In this paper we present an analysis of near-infrared spectropolarimetric and velocimetric data of the young M dwarf AU Mic, collected with SPIRou at the Canada-France-Hawaii telescope from 2019 to 2022, mostly within the SPIRou Legacy Survey. With these data, we study the large- and small-scale magnetic field of AU Mic, detected through the unpolarized and circularly-polarized Zeeman signatures of spectral lines. We find that both are modulated with the stellar rotation period (4.86 d), and evolve on a timescale of months under differential rotation and intrinsic variability. The small-scale field, estimated from the broadening of spectral lines, reaches $2.61\pm 0.05$ kG. The large-scale field, inferred with Zeeman-Doppler imaging from Least-Squares Deconvolved profiles of circularly-polarized and unpolarized spectral lines, is mostly poloidal and axisymmetric, with an average intensity of $550\pm 30$ G. We also find that surface differential rotation, as derived from the large-scale field, is $\simeq$30% weaker than that of the Sun. We detect the radial velocity (RV) signatures of transiting planets b and c, although dwarfed by activity, and put an upper limit on that of candidate planet d, putatively causing the transit-timing variations of b and c. We also report the detection of the RV signature of a new candidate planet (e) orbiting further out with a period of $33.39\pm 0.10$ d, i.e., near the 4:1 resonance with b. The RV signature of e is detected at 6.5$\ddot{I}ƒ$ while those of b and c show up at $\simeq$4$\ddot{I}ƒ$, yielding masses of $10.2_{-2.7}^{+3.9}$ and $14.2_{-3.5}^{+4.8}$ Earth masses for b and c, and a minimum mass of $35.2_{-5.4}^{+6.7}$ Earth masses for e.

MNRAS, in press (20 pages and 12 figures + 9 pages of supplementary material)