Diving Beneath the Sea of Stellar Activity: Chromatic Radial Velocities of the Young AU Mic Planetary System

Abstract

We present updated radial-velocity (RV) analyses of the AU Mic system. AU Mic is a young (22 Myr) early-M dwarf known to host two transiting planets—P b ∼ 8.46 days, R b = 4.38 − 0.18 + 0.18 R ⊕ , P c ∼ 18.86 days, R c = 3.51 − 0.16 + 0.16 R ⊕ . With visible RVs from Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical echelle Spectrographs (CARMENES)-VIS, CHIRON, HARPS, HIRES, Minerva-Australis, and Tillinghast Reflector Echelle Spectrograph, as well as near-infrared (NIR) RVs from CARMENES-NIR, CSHELL, IRD, iSHELL, NIRSPEC, and SPIRou, we provide a 5σ upper limit to the mass of AU Mic c of M c ≤ 20.13 M ⊕ and present a refined mass of AU Mic b of M b = 20.12 − 1.57 + 1.72 M ⊕ . Used in our analyses is a new RV modeling toolkit to exploit the wavelength dependence of stellar activity present in our RVs via wavelength-dependent Gaussian processes. By obtaining near-simultaneous visible and near-infrared RVs, we also compute the temporal evolution of RV “color” and introduce a regressional method to aid in isolating Keplerian from stellar activity signals when modeling RVs in future works. Using a multiwavelength Gaussian process model, we demonstrate the ability to recover injected planets at 5σ significance with semi-amplitudes down to ≈10 m s−1 with a known ephemeris, more than an order of magnitude below the stellar activity amplitude. However, we find that the accuracy of the recovered semi-amplitudes is ∼50% for such signals with our model.

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

“…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). Additionally, a 5𝜎-upper limit of 20.13 M ⊕ has been reported for AU Mic c in Cale et al (2021).…”

“…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). Additionally, a 5𝜎-upper limit of 20.13 M ⊕ has been reported for AU Mic c in Cale et al (2021).…”

“…As a consequence, only a handful young (< 100 Myr) close-in planetary systems have been revealed so far: most of them are transiting planets detected by the K2 and TESS space missions (David et al 2016;Mann et al 2016b,a;David et al 2019;Plavchan et al 2020;Rizzuto et al 2020;Mann et al 2021), and the remainder are non-transiting planets unveiled from the RV wobbles of their host star (Donati et al 2016;Yu et al 2017). So far, only two systems younger than 100 Myr, namely AU Mic and V1298 Tau, have planets with well-constrained mean bulk density measured (Klein et al 2021;Cale et al 2021;Suárez Mascareño et al 2021).…”

One year of AU Mic with HARPS: I -- measuring the masses of the two transiting planets

“…P20 also reported an additional transit-like event from a possible second transiting planet. TESS re-observed the system two years later, enabling Martioli et al (2021, hereafter M21) to confirm the detection of AU Mic c, with a radius of 3.24 ± 0.16 𝑅 ⊕ planet and a period of 18.859 d, though no more than a 5𝜎 upper limit of 20.13 𝑀 ⊕ has been reported for its mass (Cale et al 2021). Note that the existence of more than one transiting planet around AU Mic is not altogether surprising, given that AU Mic hosts an edge-on debris disc, and that observations of the transits of AU Mic b have shown its orbit to be aligned (including Addison et al 2021;Martioli et al 2020;Szabó et al 2021;Hirano et al 2020).…”

“…P20 combined data from several optical and near-infrared spectrographs, obtained over several years (starting long before the discovery of the transits) in an effort to constrain the planet's mass, but obtained only an upper limit. A more intensive observing campaign using the SPIRou near-infrared spectropolarimeter (Donati et al 2020), focusing on several consecutive rotational cycles, enabled Klein et al (2021, hereafter K21) to measure a mass of 17.1 +4.7 −4.5 𝑀 ⊕ for AU Mic b, recently refined at 20.1 +1.7 −1.6 𝑀 ⊕ in Cale et al (2021). P20 also reported an additional transit-like event from a possible second transiting planet.…”