The changing face of AU Mic b: stellar spots, spin-orbit commensurability, and Transit Timing Variations as seen by CHEOPS and TESS

TL;DR

This study analyzes multiple transit observations of AU Mic b to reveal stellar activity, spin-orbit resonance, and transit timing variations, suggesting an outer companion influences the system's dynamics.

Contribution

It provides the first detailed analysis of stellar spot effects, spin-orbit commensurability, and TTVs in the AU Mic system using CHEOPS, TESS, and ground data.

Findings

Refined orbital period of AU Mic b is 8.462995 days.

Stellar rotation period is 4.8367 days, indicating a 7:4 spin-orbit resonance.

Transit timing variations of at least 4 minutes are confirmed, possibly caused by AU Mic c.

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 (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.