Letter to the Editor

Spin-orbit alignment and magnetic activity in the young planetary system AU Mic^⋆

¹ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France
e-mail: martioli@iap.fr
² Laboratório Nacional de Astrofísica, Rua Estados Unidos 154, Itajubá, MG 37504-364, Brazil
³ Observatoire de Haute Provence, St Michel l’Observatoire, France
⁴ Univ. de Toulouse, CNRS, IRAP, 14 Avenue Belin, 31400 Toulouse, France
⁵ Université de Montréal, Département de Physique, IREX, Montréal, QC H3C 3J7, Canada
⁶ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
⁷ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ Canada-France-Hawaii Telescope, CNRS, Kamuela, HI 96743, USA
⁹ Department of Earth Sciences, University of Hawaii at Mänoa, Honolulu, HI 96822, USA
¹⁰ Université de Montpellier, CNRS, LUPM, 34095 Montpellier, France
¹¹ Mississippi State University, Department of Physics & Astronomy, Hilbun Hall, Starkville, MS, 39762, USA
¹² Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Received: 18 June 2020
Accepted: 7 August 2020

Abstract

We present high-resolution near-infrared spectropolarimetric observations using the SPIRou instrument at Canada-France-Hawaii Telescope (CFHT) during a transit of the recently detected young planet AU Mic b, with supporting spectroscopic data from iSHELL at NASA InfraRed Telescope Facility. We detect Zeeman signatures in the Stokes V profiles and measure a mean longitudinal magnetic field of ¯B_ℓ = 46.3 ± 0.7 G. Rotationally modulated magnetic spots likely cause long-term variations of the field with a slope of dB_ℓ/dt = −108.7 ± 7.7 G d⁻¹. We apply the cross-correlation technique to measure line profiles and obtain radial velocities through CCF template matching. We find an empirical linear relationship between radial velocity and B_ℓ, which allows us to estimate the radial-velocity induced by stellar activity through rotational modulation of spots for the five hours of continuous monitoring of AU Mic with SPIRou. We model the corrected radial velocities for the classical Rossiter-McLaughlin effect, using MCMC to sample the posterior distribution of the model parameters. This analysis shows that the orbit of AU Mic b is prograde and aligned with the stellar rotation axis with a sky-projected spin-orbit obliquity of λ = 0°_−15°^+18°. The aligned orbit of AU Mic b indicates that it formed in the protoplanetary disk that evolved into the current debris disk around AU Mic.