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

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.

One year of AU Mic with HARPS – I. Measuring the masses of the two transiting planets

“…13 M ⊕ has been reported for its mass (Cale et al 2021 ). Note that the existence of more than one transiting planet around AU Mic is not altogether surprising, given that AU Mic hosts an edge-on debris disc, and that observations of the transits of AU Mic b have shown its orbit to be aligned (including Hirano et al 2020 ;Martioli et al 2020 ;Addison et al 2021 ;Szab ó et al 2021 ). The parameters of AU Mic and its two transiting planets that are rele v ant in this work, as reported in the literature at the time of writing, are listed in Table 1 .…”

“…As AU Mic b and c are close to a 2:1 mean-motion resonance, transit timing variations (TTVs) can be used to constrain their masses and eccentricities. Szab ó et al ( 2021 ) and Gilbert et al ( 2021 ) report tentative TTVs for AU Mic b at the level of 3-4 min and 80 s, respectively, using TESS and CHEOPS observations. Although the measurement of transit times is some what dif ficult because of A U Mic' s frequent flaring, these results indicate that strong TTVs, with amplitudes in excess of ∼10 min, are not present in the system.…”

A pair of Sub-Neptunes transiting the bright K-dwarf TOI-1064 characterised with CHEOPS

“…The first scientific results from CHEOPS show the range of science that can be achieved with such a high-precision instrument (Lendl et al 2020;Borsato et al 2021;Delrez et al 2021;Leleu et al 2021;Morris et al 2021;Szabó et al 2021;Van Grootel et al 2021;Maxted et al accepted). One such study reports on the improvement in precision of exoplanet sizes in the HD 108236 system (Bonfanti et al 2021), that highlights a key scientific goal of the CHEOPS mission: the refinement of exoplanet radii to decrease bulk density uncertainties, and thereby allowing internal structure and atmospheric evolution modelling.…”

A model for spin-orbit commensurability and synchronous starspot activity in stars with close-by planets

“…The star has a high obliquity and the orbit is eccentric as in the case of Kepler-63 for which P rot /P orb = (4.01 ± 0.01):7 according to Sanchis-Ojeda et al (2013). We include also WASP-107, that showed a ratio P rot /P orb = (3.0 ± 0.15) and has a high obliquity measured by means of spot occultations (Dai & Winn 2017); AU Mic, which has a high level of magnetic activity and a fast rotation corresponding to its very young age of ∼ 22 Myr (Szabó et al 2021); and τ Boo, where the mean stellar rotation is synchronized with the orbital motion of the planet.…”

“…2 revealing that the integers appearing in the commensurability ratio can be remarkably different which has led to regard such a phenomenon as a bare coincidence in the vast ensemble of transiting planetary systems known to date. Yet, the closeness of the ratio to that of rather small integers (≤ 8) has suggested the possibility that the commensurability is the result of an unknown physical process occurring in some systems -see, e.g., the discussion on Kepler-17 and HAT-P-11 by Béky et al (2014) or the case of AU Mic recently investigated by Szabó et al (2021). This work is dedicated to propose a possible physical mechanism to account for the spin-orbit commensurability.…”