The changing face of AU Mic b: stellar spots, spin-orbit commensurability, and transit timing variations as seen by CHEOPS and TESS

¹ ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9700 Szombathely, Szent Imre h. u. 112, Hungary
e-mail: szgy@gothard.hu
² MTA-ELTE Exoplanet Research Group, 9700 Szombathely, Szent Imre h. u. 112, Hungary
³ INAF, Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
⁵ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
⁶ Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia
⁷ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51 1290 Versoix, Switzerland
⁸ Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
⁹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
¹⁰ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, 1121 Budapest, Konkoly Thege Miklós út 15-17, Hungary
¹² ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
¹³ Sydney Institute for Astronomy, School of Physics A29, University of Sydney, NSW 2006, Australia
¹⁴ Institut d’astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis blvd. Arago, 75014 Paris, France
¹⁵ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
¹⁶ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, C/CanMagrans s/n, 08193 Bellaterra, Spain
¹⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Barcelona, Spain
¹⁸ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹⁹ Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain
²⁰ Admatis, Miskolc, Hungary
²¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²³ Depto. de Astrofísica, Centro de Astrobiologia (CSIC-INTA), ESAC campus, 28692 Villanueva de la Cãda (Madrid), Spain
²⁴ Center for Space and Habitability, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
²⁵ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²⁶ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²⁷ Institut de Physique du Globe de Paris (IPGP), 1 rue Jussieu, 75005 Paris, France
²⁸ School of Physics and Astronomy, Physical Science Building, North Haugh, St Andrews, UK
²⁹ ESTEC, European Space Agency, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
³⁰ Lund Observatory, Dept. of Astronomy and Theoreical Physics, Lund University, Box 43, 22100 Lund, Sweden
³¹ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du six Août 19C, 4000 Liège, Belgium
³² Astrobiology Research Unit, Université de Liège, Allée du six Août 19C, 4000 Liège, Belgium
³³ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
³⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
³⁵ Center for Astronomy and Astrophysics, Technical University Berlin, Hardenberstrasse 36, 10623 Berlin, Germany
³⁶ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
³⁷ Division Technique INSU, BP 330, 83507 La Seyne cedex, France
³⁸ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
³⁹ Dipartimento di Fisica e Astronomia “Galileo Galilei’, Universià degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴⁰ IMCEE, UMR8028 CNRS, Observatoire de Paris, PSL Univ., Sorbonne Univ., 77 av. Denfert-Rochereau, 75014 Paris, France
⁴¹ NCCR/PlanetS, Centre for Space & Habitability, University of Bern, Bern, Switzerland
⁴² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁴³ INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁴⁴ Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
⁴⁵ Astrophysics Group, Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0He, UK
⁴⁶ Institut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany
⁴⁷ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125, Torino, Italy

Received: 14 January 2021
Accepted: 28 July 2021

Abstract

AU Mic is a young planetary system with a resolved debris disc showing signs of planet formation and two transiting warm Neptunes near mean-motion resonances. Here we analyse three transits of AU Mic b observed with the CHaracterising ExOPlanet Satellite (CHEOPS), supplemented with sector 1 and 27 Transiting Exoplanet Survey Satellite (TESS) photometry, and the All-Sky Automated Survey from the ground. The refined orbital period of AU Mic b is 8.462995 ± 0.000003 d, whereas the stellar rotational period is P_rot = 4.8367 ± 0.0006 d. The two periods indicate a 7:4 spin–orbit commensurability at a precision of 0.1%. Therefore, all transits are observed in front of one of the four possible stellar central longitudes. This is strongly supported by the observation that the same complex star-spot pattern is seen in the second and third CHEOPS visits that were separated by four orbits (and seven stellar rotations). Using a bootstrap analysis we find that flares and star spots reduce the accuracy of transit parameters by up to 10% in the planet-to-star radius ratio and the accuracy on transit time by 3–4 min. Nevertheless, occulted stellar spot features independently confirm the presence of transit timing variations (TTVs) with an amplitude of at least 4 min. We find that the outer companion, AU Mic c, may cause the observed TTVs.