The changing face of AU Mic b: stellar spots, spin-orbit commensurability, and Transit Timing Variations as seen by CHEOPS and TESS
Creators: name: Szabó, Gy. M.
name: Gandolfi, D.
name: Brandeker, A.
name: Csizmadia, Sz.
name: Garai, Z.
name: Billot, N.
name: Broeg, C.
name: Ehrenreich, D.
name: Fortier, A.
name: Fossati, L.
name: Hoyer, S.
name: Kiss, L.
name: Etangs, A. Lecavelier des
name: Maxted, P. F. L.
name: Ribas, I.
name: Alibert, Y.
name: Alonso, R.
name: Escudé, G. Anglada
name: Bárczy, T.
name: Barros, S. C. C.
name: Barrado, D.
name: Baumjohann, W.
name: Beck, M.
name: Beck, T.
name: Bekkelien, A.
name: Bonfils, X.
name: Benz, W.
name: Borsato, L.
name: Busch, M-D.
name: Cabrera, J.
name: Charnoz, S.
name: Cameron, A. Collier
name: Van Damme, C. Corral
name: Davies, M. B.
name: Delrez, L.
name: Deleuil, M.
name: Demangeon, O. D. S.
name: Demory, B. -O.
name: Erikson, A.
name: Fridlund, M.
name: Futyan, D.
name: Muñoz, A. García
name: Gillon, M.
name: Guedel, M.
name: Guterman, P.
name: Heng, K.
name: Isaak, K. G.
name: Lacedelli, G.
name: Laskar, J.
name: Lend, M.
name: Lovis, C.
name: Luntzer, A.
name: Magrin, D.
name: Nascimbeni, V.
name: Olofsson, G.
name: Osborn, H. P.
name: Ottensamer, R.
name: Pagano, I.
name: Pallé, E.
name: Peter, G.
name: Piazza, D.
name: Piotto, G.
name: Pollacco, D.
name: Queloz, D.
name: Ragazzoni, R.
name: Rando, N.
name: Rauer, H.
name: Santos, N. C.
name: Scandariato, G.
name: Ségransan, D.
name: Serrano, L. M.
name: Sicilia, D.
name: Simon, A. E.
name: Smith, A. M. S.
name: Sousa, S. G.
name: Steller, M.
name: Thomas, N.
name: Udry, S.
name: Van Grootel, V.
name: Walton, N. A.
name: Wilson, T. G.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 (ASAS) from the ground. The refined orbital period of AU Mic b is 8.462995 \pm 0.000003 d, whereas the stellar rotational period is P_{rot}=4.8367 \pm 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 minutes. Nevertheless, occulted stellar spot features independently confirm the presence of transit timing variations (TTVs) with an amplitude of at least 4 minutes. We find that the outer companion, AU Mic c may cause the observed TTVs.
description: 16 pages, 11 figures, 7 tables; accepted by Astronomy and Astrophysics