Revisiting the dynamical masses of the transiting planets in the young AU Mic system: Potential AU Mic b inflation at ~20 Myr

¹ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino bajo del castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
⁴ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK
⁵ Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Calle Gran Capital 2–4, 08034 Barcelona, Spain
⁷ Institut de Ciéncies de l’Espai (CSIC-IEEC), UAB Campus, Calle de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
⁸ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008, Granada, Spain
¹¹ Centro Astronómico Hispano en Andalucía (CAHA), Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Spain
¹² Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
¹³ Dipartimento di Fisica, Universita degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
¹⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
¹⁶ Departamento de Física de la Tierra y Astrofísica and IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁷ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁸ Steward Observatory and Department of Astronomy, The University of Arizona, Tucson, AZ 85721, USA

Received: 20 March 2024
Accepted: 5 July 2024

Abstract

Context. Understanding planet formation is important in the context of the origin of planetary systems in general and of the Solar System in particular, as well as to predict the likelihood of finding Jupiter, Neptune, and Earth analogues around other stars.

Aims. We aim to precisely determine the radii and dynamical masses of transiting planets orbiting the young M star AU Mic using public photometric and spectroscopic datasets.

Methods. We performed a joint fit analysis of the TESS and CHEOPS light curves and more than 400 high-resolution spectra collected with several telescopes and instruments. We characterise the stellar activity and physical properties (radius, mass, density) of the transiting planets in the young AU Mic system through joint transit and radial velocity fits with Gaussian processes.

Results. We determine a radius of R_p^b = 4.79 ± 0.29 R_⊕, a mass of M_p^b = 9.0 ± 2.7 M_⊕, and a bulk density of ρ_p^b = 0.49 ± 0.16 g cm⁻³ for the innermost transiting planet AU Mic b. For the second known transiting planet, AU Mic c, we infer a radius of R_p^c = 2.79 ± 0.18 R_⊕, a mass of M_p^c = 14.5 ± 3.4 M_⊕, and a bulk density of ρ_p^c = 3.90 ± 1.17 g cm⁻³. According to theoretical models, AU Mic b may harbour an H₂ envelope larger than 5% by mass, with a fraction of rock and a fraction of water. AU Mic c could be made of rock and/or water and may have an H₂ atmosphere comprising at most 5% of its mass. AU Mic b has retained most of its atmosphere but might lose it over tens of millions of years due to the strong stellar radiation, while AU Mic c likely suffers much less photo-evaporation because it lies at a larger separation from its host. Using all the datasets in hand, we determine a 3σ upper mass limit of M_p^[d] sin i = 8.6 M_⊕ for the AU Mic’d’ TTV-candidate. In addition, we do not confirm the recently proposed existence of the planet candidate AU Mic ’e’ with an orbital period of 33.4 days. We investigated the level of the radial velocity variations and show that it is lower at longer wavelength with smaller changes from one observational campaign to another.