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Wednesday 20 Jan 2021Stellar magnetic field and close-in planet mass of the young AU Mic system with SPIRou

Baptiste Klein - IRAP, Toulouse

Remote seminar 14:00-15:00

Measuring the mean densities of close-in planets orbiting pre-main-sequence (PMS) stars is crucially needed by planet formation and evolution models. This requires to measure both planet radii, from the depth of transit light curves, and masses, from the radial velocity (RV) wobbles induced by the planet on its host star. However, PMS stars exhibit intense magnetic activity inducing fluctuations in both photometric and RV curves that overshadow planet signatures. As a result, no close-in planet younger than 25 Myr has a well-constrained bulk density. Observing in the near-infrared (nIR), where low-mass PMS stars emit most of their light, is crucial to disentangle planet signatures from stellar activity signals whose amplitude is expected to decrease with increasing wavelengths.

In this seminar, I will present the analysis of 27 nIR high-resolution spectra of the nearby active 22 Myr-old star AU Microscopii (AU Mic) collected with the spectropolarimeter SPIRou at the end 2019. A close-in transiting Neptune-sized planet was recently detected around AU Mic from TESS / SPITZER light-curves. However, tentative velocimetric follow-ups of the star in the optical domain resulted in no more than an upper limit for the planet mass due to the large dispersion induced by stellar activity in the radial velocity (RV) time-series.

We jointly model the planet and stellar activity RV components, resulting in a 3.9 sigma-detection of AU Mic b from our SPIRou data, implying a Neptune-like density for the planet. A consistent detection of the planet is independently obtained by simultaneously reconstructing the surface distribution of bright and dark inhomogeneities and estimating the planet parameters using Doppler imaging. Using Zeeman-Doppler Imaging, we invert our time-series of intensity and circularly-polarized line profiles into distributions of brightness and large-scale magnetic field at the surface of the star and explore how these distributions are sheared by latitudinal differential rotation. Finally, we investigate the magnetic activity of AU Mic by computing various indicators and found that the disk-integrated magnetic flux density correlates best with the stellar activity RV signal, in line with recent results obtained for the Sun.

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