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

Gy. M. Szabó, Melvyn B Davies, T. G. Wilson, et al.

    Research output: Contribution to journalArticlepeer-review

    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. © ESO 2021.
    Original languageEnglish
    Article numberA159
    JournalAstronomy & Astrophysics
    Volume654
    DOIs
    Publication statusPublished - 2021 Oct 1

    Subject classification (UKÄ)

    • Astronomy, Astrophysics and Cosmology

    Free keywords

    • Planetary systems
    • Extrasolar planets
    • Gold
    • Orbits
    • Surveys
    • Timing circuits
    • Debris disk
    • Exo-planets
    • Neptune
    • Planet formation
    • Planetary system
    • Spin orbits
    • Star spots
    • Stellars
    • Timing variations
    • Transit timing
    • Stars

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