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

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% < fmax < 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 Rp∕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⊕ < Mc < 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.

One year of AU Mic with HARPS – II. Stellar activity and star–planet interaction

“…Note that DI reconstructions are performed assuming a stellar inclination of i DI = 80 paper I † To gain some space in the table, we use aliases for the references. G18, A19, W19, P20, P09, M14, paper I, Kl21 and M21 stand respectively for Gaia Collaboration et al (2018), Afram & Berdyugina (2019), White et al (2019), Plavchan et al (2020), Plavchan et al (2009), Mamajek & Bell (2014), Zicher et al (2022), Klein et al (2021) and Martioli et al (2021).…”

“…GJ 803) is the second closest PMS star (D = 9.7248 pc;Gaia Collaboration et al 2018). This system has been intensively monitored over the past 50 years for its intense magnetic activity notably inducing ∼ 0.1-mag quasi-periodic photometric variations (e.g., Torres & Ferraz Mello 1973;Rodono et al 1986;Plavchan et al 2020;Martioli et al 2021), and frequent flaring events of variable intensity (e.g., Robinson et al 2001;Hebb et al 2007;MacGregor et al 2020;Martioli et al 2021). The detection of two transiting Neptune-sized planets from Spitzer and TESS light curves (TESS sectors 1 and 27; Plavchan et al 2020;Martioli et al 2021) has increased the already high interest of the scientific community in the system which has been intensively monitored with various state-of-the-art spectrometers since then.…”

“…This system has been intensively monitored over the past 50 years for its intense magnetic activity notably inducing ∼ 0.1-mag quasi-periodic photometric variations (e.g., Torres & Ferraz Mello 1973;Rodono et al 1986;Plavchan et al 2020;Martioli et al 2021), and frequent flaring events of variable intensity (e.g., Robinson et al 2001;Hebb et al 2007;MacGregor et al 2020;Martioli et al 2021). The detection of two transiting Neptune-sized planets from Spitzer and TESS light curves (TESS sectors 1 and 27; Plavchan et al 2020;Martioli et al 2021) has increased the already high interest of the scientific community in the system which has been intensively monitored with various state-of-the-art spectrometers since then. These follow-up observations made it possible to measure a zero-compatible spin-orbit angle (Addison et al 2020;Hirano et al 2020;Martioli et al 2020;Palle et al 2020), and a bulk density of 1.32 ± 0.2 g cm −3 for the close-in planet AU Mic b (Klein et al 2021;Cale et al 2021).…”

Rave: A non-parametric method for recovering the surface brightness and height profiles of edge-on debris disks

“…Optical and far-IR imaging also revealed the presence of an extended halo surrounding the disk (Matthews et al 2015). The system is also thought to host two close-in, Neptune-sized planets as suggested by TESS light curves (Plavchan et al 2020;Martioli et al 2021).…”

Searching for flaring star-planet interactions in AU Mic TESS observations

“…Recently, AU Mic, an M0-M1 (Pecaut & Mamajek 2013;Gaidos et al 2014) pre-main sequence dwarf star, and a member of the β Pic young moving group, which is 16 − 29 Myr old (Malo et al 2014;Binks & Jeffries 2014;Mamajek & Bell 2014;Bell et al 2015;Binks & Jeffries 2016;Shkolnik et al 2017;Miret-Roig et al 2020), was discovered to be a favorable candidate for magnetic star-planet interactions (Kavanagh et al 2021). The system hosts two planets in close orbits near a 2:1 mean-motion resonance, AU Mic b and AU Mic c (Plavchan et al 2020;Martioli et al 2021), with masses M b = 11.7 ± 5.0M ⊕ and M c = 22.2 ± 6.7M ⊕ (Zicher et al 2022). Several papers reported system parameters of AU Mic based on photometry from the Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2014), the CHEOPS mission (Benz et al 2021), and HARPS (Mayor et al 2003) spectroscopy (Plavchan et al 2020;Cale et al 2021;Gilbert et al 2022;Zicher et al 2022), which are mutually compatible within uncertainties.…”