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

Gy M Szabo, D Gandolfi, A Brandeker, S Csizmadia, Z Garai, N Billot, C Broeg, D Ehrenreich, A Fortier, L Fossati, S Hoyer, L Kiss, A Lecavelier des Etangs, PFL Maxted, I Ribas, Y Alibert, R Alonso, GA Escude, T Barczy, SCC BarrosD Barrado, W Baumjohann, M Beck, T Beck, A Bekkelien, X Bonfils, W Benz, L Borsato, MD Busch, J Cabrera, S Charnoz, AC Cameron, CC Van Damme, MB Davies, L Delrez, M Deleuil, ODS Demangeon, BO Demory, A Erikson, M Fridlund, D Futyan, A García Muñoz, M Gillon, M Guedel, P Guterman, K Heng, KG Isaak, G Lacedelli, J Laskar, M Lendl, C Lovis, A Luntzer, D Magrin, V Nascimbeni, G Olofsson, HP Osborn, R Ottensamer, I Pagano, E Palle, G Peter, D Piazza, G Piotto, D Pollacco, D Queloz, R Ragazzoni, N Rando, H Rauer, NC Santos, G Scandariato, D Segransan, LM Serrano, D Sicilia, AE Simon, AMS Smith, SG Sousa, M Steller, N Thomas, S Udry, V Van Grootel, NA Walton, TG Wilson

Research output: Contribution to journalArticlepeer-review


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.
Original languageEnglish
Article numberA159
Number of pages15
JournalAstronomy & Astrophysics
Early online date26 Oct 2021
Publication statusPublished - Oct 2021


  • Planetary systems


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