Investigating the young AU Mic system with SPIRou: large-scale stellar magnetic field and close-in planet mass

Abstract

We present a velocimetric and spectropolarimetric analysis of 27 observations of the 22-Myr M1 star AU Microscopii (AU Mic) collected with the high-resolution YJHK (0.98–2.35 μm) spectropolarimeter SPIRou from 2019 September 18 to November 14. Our radial velocity (RV) time-series exhibits activity-induced fluctuations of 45 m s−1 rms, ∼3 times smaller than those measured in the optical domain, that we filter using Gaussian Process Regression. We report a 3.9σ detection of the recently discovered 8.46 -d transiting planet AU Mic b, with an estimated mass of 17.1$^{+4.7}_{-4.5}$ M⊕ and a bulk density of 1.3 ± 0.4 g cm−3, inducing an RV signature of semi-amplitude K = 8.5$^{+2.3}_{-2.2}$ m s−1 in the spectrum of its host star. A consistent detection is independently obtained when we simultaneously image stellar surface inhomogeneities and estimate the planet parameters with Zeeman–Doppler imaging (ZDI). Using ZDI, we invert the time-series of unpolarized and circularly polarized spectra into surface brightness and large-scale magnetic maps. We find a mainly poloidal and axisymmetric field of 475 G, featuring, in particular, a dipole of 450 G tilted at 19° to the rotation axis. Moreover, we detect a strong differential rotation of dΩ = 0.167 ± 0.009 rad d−1 shearing the large-scale field, about twice stronger than that shearing the brightness distribution, suggesting that both observables probe different layers of the convective zone. Even though we caution that more RV measurements are needed to accurately pin down the planet mass, AU Mic b already appears as a prime target for constraining planet formation models, studying the interactions with the surrounding debris disc, and characterizing its atmosphere with upcoming space- and ground-based missions.

Dynamical masses of two infant giant planets

“…Figure 2 shows the position of the planets orbiting V1298 Tau in a mass-radius diagram compared to the known population of exoplanets. Similar to the case of AU Mic b 9 -the only other exoplanet of similar age with a mass measurement -the mass-radius relation of the planets orbiting V1298 Tau resembles that of the planets of our Solar System and of the general population of known transiting exoplanets.…”

“…These theoretical expectations remain untested to date, despite the increasing number of exoplanetary discoveries, as the detection and characterisation of very young planets is extremely challenging due to the intense stellar activity of their host stars 3, 4 . However, the recent discoveries of young planetary transiting systems allow to place initial constraints on evolutionary models [5][6][7][8][9] . With an estimated age of 20 million years, V1298 Tau is one of the youngest solar-type stars known to host transiting planets: it harbours a multiple system composed of two Neptune-sized, one Saturn-sized, and one Jupiter-sized planets 10,11 .Here we report the dynamical masses of two of the four planets.…”

New constraints on the planetary system around the young active star AU Mic

“…for f spot being the spot filling factor estimated at 20% based on magnetic activity (Klein et al 2021) and F spot /F phot being the flux fraction between the spots and the photosphere. The latter can be estimated assuming a blackbody flux for each of the two components, where the photospheric temperature is T phot = 3700 K and the spots' temperature is estimated at 86% of the photospheric value in M1 stars (Rackham et al 2018).…”

“…This Neptune-size planet is in a 8.5-d prograde orbit aligned with the stellar rotation axis (Martioli et al 2020;Hirano et al 2020;Palle et al 2020). While Plavchan et al (2020) only reported an upper limit on the mass of AU Mic b, Klein et al (2021) measured 17.1 +4.7 −4.5 M ⊕ thanks to infrared observations secured with the SPIRou spectropolarimeter (Donati et al 2020). Plavchan et al (2020) has also reported the detection of an isolated transit event of a second possible candidate planet, hereafter referred to as AU Mic c.…”

Detection Limits of Low-mass, Long-period Exoplanets Using Gaussian Processes Applied to HARPS-N Solar Radial Velocities

“…The orbital period is generally recovered with a high degree of confidence but the semi-amplitude is only recovered at the 5σ level for planets with semi-amplitude above 1 m s −1 . young pre-main-sequence stars V830 Tau (Donati et al 2016) and AU Mic (Klein et al 2020). In both studies, they were able to detect and characterize planetary signals with amplitudes five times smaller than the intrinsic RV variations.…”