http://arxiv.org/abs/2012.13238
AU Mic is a young, active star around which a transiting planet has been recently detected. Here we report an analysis of its TESS light curve where we model the quasi-periodic rotational modulation by starspots simultaneously to the flaring activity and planetary transits. We measured a flare occurrence rate of 6.35 flares-per-day for flares with amplitudes in the range $0.06\% < f_{\rm max} < 1.5\%$ of the star flux. We employ a Bayesian MCMC 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 radius ratio of $0.05345^{+0.00014}{-0.00015}$ implies a physical radius of $4.38\pm0.18$ M$\oplus$ and a planet density of $1.1\pm0.3$ g\,cm$^{-3}$, confirming that AU Mic b is a Neptune-size moderately inflated planet. Whereas 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 $18.858991\pm0.000010$ d and a planetary radius of $3.51\pm0.16$ R$\oplus$, which defines AU Mic c as a warm Neptune-size planet with an expected mass in the range 1.7 M$\oplus < M_{\rm c} < 27.7$ M$\oplus$. 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 TTV. Our non-detection of significant TTV in AU Mic b suggests an upper limit to the mass of AU Mic c of $<7$ M$\oplus$. Being 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 as well as of planet-planet and planet-disk dynamics at the early stages of planetary evolution.
E. Martioli, G. Hébrard, A. Correia, et. al.
Fri, 25 Dec 20
5/51
Comments: Draft submitted for publication to the Astronomy & Astrophysics on Dec 24, 2020
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