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 ¯Bℓ = 46.3 ± 0.7 G. Rotationally modulated magnetic spots likely cause long-term variations of the field with a slope of dBℓ/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 Bℓ, 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.
Highlights
Detecting and characterizing planets around young stars is key to understanding the early stages of planetary evolution
Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated from the summit of Maunakea by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii
We first model the Spectro-Polarimètre Infra Rouge (SPIRou) radial velocities of AU Mic obtained from the median-filtered cross-correlation function (CCF), as the combination of its reflex orbital motion caused by planet b, assuming a circular orbit and the Plavchan et al (2020) orbital parameters, and the classical Rossiter-McLaughlin (RM) effect, with the stellar limb darkening accounted for as described in Ohta et al (2005)
Summary
Detecting and characterizing planets around young stars is key to understanding the early stages of planetary evolution. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated from the summit of Maunakea by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. AU Mic is a young and active M1 star with a spatially resolved edge-on debris disk (Kalas et al 2004), and is a member of the β Pictoris Moving Group (Torres et al 2006). Young systems with detected planets (e.g., V830 Tau b; Donati et al 2016), and especially those with either a remnant debris disk like β Pic b (Lagrange et al 2009) or transiting. AU Mic has both a disk and at least one transiting planet, and is unique among debris disk hosts for being an M star, the most numerous type of star in our Galaxy and the most promising spectral type to find habitable planets using current techniques
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