New constraints on the planetary system around the young active star AU Mic
Eder Martioli, G. Hébrard, A. C. M. Correia, J. Laskar, A. Lecavelier des Étangs
Abstract
AU Microscopii (AU Mic) is a young, active star whose transiting planet was recently detected. Here, we report our analysis of its TESS light curve, where we modeled the BY Draconis type quasi-periodic rotational modulation by starspots simultaneously to the flaring activity and planetary transits. We measured a flare occurrence rate in AU Mic of 6.35 flares per day for flares with amplitudes in the range of 0.06% < f max < 1.5% of the star flux. We employed a Bayesian Markov chain Monte Carlo analysis to model the five transits of AU Mic b observed by TESS, improving the constraints on the planetary parameters. The measured planet-to-star effective radius ratio of R p ∕ R ⋆ = 0.0496 ± 0.0007 implies a physical radius of 4.07 ± 0.17 R ⊕ and a planet density of 1.4 ± 0.4 g cm −3 , confirming that AU Mic b is a Neptune-size moderately inflated planet. While a single feature possibly due to a second planet was previously reported in the former TESS data, we report the detection of two additional transit-like events in the new TESS observations of July 2020. This represents substantial evidence for a second planet (AU Mic c) in the system. We analyzed its three available transits and obtained an orbital period of 18.859019 ± 0.000016 d and a planetary radius of 3.24 ± 0.16 R ⊕ , which defines AU Mic c as a warm Neptune-size planet with an expected mass in the range of 2.2 M ⊕ < M c < 25.0 M ⊕ , estimated from the population of exoplanets of similar sizes. The two planets in the AU Mic system are in near 9:4 mean-motion resonance. We show that this configuration is dynamically stable and should produce transit-timing variations (TTV). Our non-detection of significant TTV in AU Mic b suggests an upper limit for the mass of AU Mic c of <7 M ⊕ , indicating that this planet is also likely to be inflated. As a young multi-planet system with at least two transiting planets, AU Mic becomes a key system for the study of atmospheres of infant planets and of planet-planet and planet-disk dynamics at the early stages of planetary evolution.