2024A&A...682A..66B

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-732b is an ultrashort-period planet (P = 0.76837931_{–0.00000042}^+0.0000039 days) with a radius R_b = 1.325_–0.058^+0.057R_⊕, a mass M_b = 2.46 ± 0.19 M_⊕, and thus a mean density ρ_b = 5.8_–0.8^+1.0 g cm^–3, while the outer planet at P = 12.252284 ± 0.000013 days has R_c = 2.39_–0.11^+0.10R_⊕, M_c = 8.04_–0.48^+0.50M_⊕, and thus ρ_c = 3.24_–0.43^+0.55 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 logP = -0.065_–0.013^+0.024, 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 ρ^_valley / d log P = -0.02_–0.04^+0.12.
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.