The changing face of AU Mic b : stellar spots, spin-orbit commensurability, and transit timing variations as seen by CHEOPS and TESS
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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.
Szabo , G M , Gandolfi , D , Brandeker , A , Csizmadia , S , Garai , Z , Billot , N , Broeg , C , Ehrenreich , D , Fortier , A , Fossati , L , Hoyer , S , Kiss , L , Lecavelier des Etangs , A , Maxted , PFL , Ribas , I , Alibert , Y , Alonso , R , Escude , GA , Barczy , T , Barros , SCC , Barrado , D , Baumjohann , W , Beck , M , Beck , T , Bekkelien , A , Bonfils , X , Benz , W , Borsato , L , Busch , MD , Cabrera , J , Charnoz , S , Cameron , AC , Van Damme , CC , Davies , MB , Delrez , L , Deleuil , M , Demangeon , ODS , Demory , BO , Erikson , A , Fridlund , M , Futyan , D , Muñoz , A G , Gillon , M , Guedel , M , Guterman , P , Heng , K , Isaak , KG , Lacedelli , G , Laskar , J , Lendl , M , Lovis , C , Luntzer , A , Magrin , D , Nascimbeni , V , Olofsson , G , Osborn , HP , Ottensamer , R , Pagano , I , Palle , E , Peter , G , Piazza , D , Piotto , G , Pollacco , D , Queloz , D , Ragazzoni , R , Rando , N , Rauer , H , Santos , NC , Scandariato , G , Segransan , D , Serrano , LM , Sicilia , D , Simon , AE , Smith , AMS , Sousa , SG , Steller , M , Thomas , N , Udry , S , Van Grootel , V , Walton , NA & Wilson , TG 2021 , ' The changing face of AU Mic b : stellar spots, spin-orbit commensurability, and transit timing variations as seen by CHEOPS and TESS ' , Astronomy & Astrophysics , vol. 654 , A159 . https://doi.org/10.1051/0004-6361/202140345
Astronomy & Astrophysics
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DescriptionA.C.C. and T.W. acknowledge support from STFC consolidated grant number ST/M001296/1.
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