Characterising TOI-732 b and c: New insights into the M-dwarf radius and density valley^★,★★

Context. TOI-732 is an M dwarf hosting two transiting planets that are located on the two opposite sides of the radius valley. Inferring a reliable demographics for this type of systems is key to understanding their formation and evolution mechanisms. Aims. By doubling the number of available space-based observations and increasing the number of radial velocity (RV) measurements, we aim at refining the parameters of TOI-732 b and c. We also use the results to study the slope of the radius valley and the density valley for a well-characterised sample of M-dwarf exoplanets. Methods. We performed a global Markov chain Monte Carlo analysis by jointly modelling ground-based light curves and CHEOPS and TESS observations, along with RV time series both taken from the literature and obtained with the MAROON-X spectrograph. The slopes of the M-dwarf valleys were quantified via a support vector machine (SVM) procedure. Results. TOI-732 b is an ultrashort-period planet (P = 0.76837931₋⁺₀⁰_00000042^00000039 days) with a radius R_b = 1.325⁺₋⁰₀⁰⁵⁷₀₅₈ R_☉, a mass M_b = 2.46 ± 0.19 M_☉, and thus a mean density ρ_b = 5.8⁺₋¹₀⁰₈ g cm⁻³, while the outer planet at P = 12.252284 ± 0.000013 days has R_c = 2.39⁺₋⁰₀¹⁰₁₁ R_☉, M_c = 8.04⁺₋⁰₀⁵⁰₄₈ M_☉, and thus ρ_c = 3.24⁺₋⁰₀⁵⁵₄₃ g cm⁻³. Even with respect to the most recently reported values, this work yields uncertainties on the transit depths and on the RV semi-amplitudes that are smaller up to a factor of ∼1.6 and ∼2.4 for TOI-732 b and c, respectively. Our calculations for the interior structure and the location of the planets in the mass-radius diagram lead us to classify TOI-732 b as a super-Earth and TOI-732 c as a mini-Neptune. Following the SVM approach, we quantified d log R_p,valley/d log P = −0.065⁺₋⁰₀⁰²⁴₀₁₃, which is flatter than for Sun-like stars. In line with former analyses, we note that the radius valley for M-dwarf planets is more densely populated, and we further quantify the slope of the density valley as d log ρ̂_valley/d log P = −0.02⁺₋⁰₀¹²₀₄. Conclusions. Compared to FGK stars, the weaker dependence of the position of the radius valley on the orbital period might indicate that the formation shapes the radius valley around M dwarfs more strongly than the evolution mechanisms.