Magalie Deleuil

We present new CHEOPS transit observations of AU Mic b and AU Mic c obtained between June and September 2024, extending the baseline of transit-timing measurements of this young planetary system. For AU Mic b, the timing signal is well established, with a semi-amplitude (10 ± 3 min) and a characteristic modulation timescale (1168 ± 20 d) consistent with previous determinations. By contrast, the new CHEOPS data show that the large transit-timing deviation of AU Mic c reported previously was not sustained. After the steadily increasing timing trend observed in 2022 and 2023, the 2024 timings returned closer to the zero point of the observed-minus-calculated diagram, indicating a reversal of the previously reported behavior. For AU Mic c, both the transit-timing semi-amplitude (46 ± 26 min) and the characteristic modulation timescale (2150 ± 110 d) remain tentative. These results highlight the importance of continued long-term monitoring of the AU Mic system.

The K2-138 system consists of a K-dwarf transited by six planets in the sub-Neptune regime, with radii ranging from 1.5 to 3.4 R_⊕ and orbital periods of between 2.3 and 42 days. The five innermost planets form a chain of zero-order three-body resonances, while the outermost planet is suspected to be linked to the chain through a first-order three-body resonances. The transit timing variations induced by the resonant configuration enable a precise determination of the system's architecture. The fine-tuning and fragility of K2-138 and other resonant chains ensure that no significant scattering or collision event has taken place since the formation and migration of the planets in the protoplanetary disc, and hence provide important anchors for planet formation models. We aim to improve the characterisation of the architecture of this system, in particular the planetary masses and its resonant configuration. In addition, we place this system in context with other resonant chains, and expand on the recent finding that resonant sub-Neptunes are puffier than non-resonant ones by deriving a mass-radius relationship for each population. We performed a global analysis of all available photometry and radial velocity data for K2-138 using photo-dynamical modelling of the light curve. We also tried different sets of priors on the masses and eccentricity, as well as different stellar activity models, to study their effects on the masses estimated by transit timing variations and radial velocities. We show that our joint photo-dynamical and radial velocity analysis resulted in a robust mass determination for planets b to f, with a precision of ∼ 15% for the mass of planet f, and better than 10% for planets b to e. We also show that K2-138 is indeed currently locked in the resonant configuration, the five inner planets librating around an equilibrium of the chain; however, existing data are not sufficient to verify if the outer triplet is librating in a first-order three-body resonance. Finally, we demonstrate that the mass-radius relationship of resonant sub-Neptunes is well described by M=2.54,R^0.84, which is less dense than the non-resonant population, for which we found M=2.78,R^1.32. We also found that the resonant population has a smaller spread around its mass-radius relationship than the non-resonant population, pointing towards an inter-system similarity in the composition of the resonant sub-Neptunes.

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way’s thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material.

Context . The aim of the European Space Agency’s PLAnetary Transits and Oscillations of Stars (PLATO) mission is to detect planets orbiting around dwarfs and subgiant stars with spectral type F5 or later, including M dwarfs. The PLATO Input Catalogue (PIC) contains all PLATO targets available for observation by the PLATO nominal science. The latest version, PIC2.1.0.1, focuses on the Southern PLATO field, named LOPS2, selected as the first long observation field, and includes the P4 sample, one of the four target samples outlined in the Science Requirement Document. P4 includes the M dwarfs with magnitudes brighter than V=16 located within LOPS2. Aims . A characterisation of the M dwarfs in the PIC is essential for assessing their potentiality to host exoplanets, and eventually for estimating the hosted planet(s) properties. The purpose of this paper is to describe how we selected the P4 M dwarf targets, and obtained their fundamental parameters and properties. Methods . Measuring stellar parameters is a challenging task. Interferometry provides direct estimates of radii, whereas alternative approaches relying on theoretical assumptions are still affected by significant uncertainties. In this work, we introduce the P4 sample and detail the methodologies, all based on photometric criteria, adopted for the measurement of their stellar parameters. Results . Based on a statistical analysis of the P4 sample, we assess both the photometric and volume completeness, and classify the stellar populations according to their Galactic spatial-velocity components. The adopted stellar parameters are validated by comparison with independent methods from the literature used to estimate stellar radii. Conclusions . We conclude that the P4 sample is compliant with the PLATO science requirements. Being magnitude limited, its volume completeness decreases going towards distances larger than 30 pc, where late-type targets are progressively less covered. The observed large spread in the colour-magnitude diagram is likely due to the combination of several effects such as metallicity, age, binarity, and activity. The strategy we adopted for deriving stellar parameters provides results consistent with those obtained in the literature with different and independent methods.

Aims. We aim to refine the fundamental parameters of the TOI-2141 planetary system, which includes a transiting sub-Neptune orbiting a Sun-like star on a relatively long orbit of 18.26 days, by combining new photometric and spectroscopic observations. Methods. We analyzed new space-based photometry from TESS and CHEOPS, as well as 61 radial velocity measurements from HARPS-N. We performed individual and joint photometric and RV analyses using several modeling tools within a Bayesian model comparison framework. Results. We refined the radius and mass of the transiting planet TOI-2141 b to 3.15 ± 0.04 R _⊕ and 20.1 ± 1.6 M _⊕ to levels that are five and two times more precise, respectively, than the previously reported values. Our radial velocity analysis reveals two additional non-transiting companions with orbital periods of 5.46 and 60.45 days. Despite the innermost planet’s high geometric transit probability, we found no evidence of transits in the photometric data. Conclusions. The bulk properties of TOI-2141 b suggest a significant volatile envelope atop an Earth-like core. The modeling indicates a hydrogen-rich atmosphere that could have experienced mild photoevaporation over the system’s history. Planets b and c are believed to exhibit a modest mutual inclination of at least ~2.4°.

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