The changing face of AU Mic b: stellar spots, spin-orbit commensurability, and transit timing variations as seen by CHEOPS and TESS
Guterman, P.; Alonso, R.; Deleuil, M.; Erikson, A.; Fridlund, M.; Queloz, D.; Rauer, H.; Olofsson, G.; Pallé, E.; Ribas, I.; Heng, K.; Isaak, K. G.; Scandariato, G.; Pagano, I.; Steller, M.; Thomas, N.; Baumjohann, W.; Gandolfi, D.; Fossati, L.; Csizmadia, Sz.; Smith, A. M. S.; Cabrera, J.; Ségransan, D.; Walton, N. A.; Ragazzoni, R.; Piotto, G.; Serrano, L. M.; Benz, W.; Collier Cameron, A.; Alibert, Y.; Delrez, L.; Fortier, A.; Hoyer, S.; Sousa, S. G.; Wilson, T. G.; Anglada Escudé, G.; Bárczy, T.; Barrado, D.; Barros, S. C. C.; Beck, M.; Beck, T.; Billot, N.; Bonfils, X.; Borsato, L.; Brandeker, A.; Broeg, C.; Charnoz, S.; Davies, M. B.; Demangeon, O. D. S.; Demory, B. -O.; Ehrenreich, D.; Gillon, M.; Laskar, J.; Lecavelier des Etangs, A.; Lendl, M.; Lovis, C.; Magrin, D.; Maxted, P. F. L.; Nascimbeni, V.; Osborn, H. P.; Ottensamer, R.; Peter, G.; Pollacco, D.; Rando, N.; Santos, N. C.; Simon, A. E.; Szabó, Gy. M.; Udry, S.; Van Grootel, V.; Lacedelli, G.; Bekkelien, A.; Futyan, D.; Kiss, L.; Luntzer, A.; García Muñoz, A.; Piazza, D.; Busch, M. -D.; Guedel, M.; Garai, Z.; Sicilia, D.; Van Damme, C. Corral
Hungary, Italy, Sweden, Germany, Slovakia, Switzerland, Austria, France, Australia, United Kingdom, Spain, Portugal, Netherlands, Belgium, United States
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 Prot = 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.