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(-0.00000042)(+0.00000039) days) with a radius R-b = 1.325(-0.058)(+0.057) R-circle dot, a mass M-b = 2.46 +/- 0.19 M-circle plus, and thus a mean density rho(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)(+0.10) R-circle plus, M-c = 8.04(-0.48+0.50) M-circle plus, and thus rho(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 similar to 1.6 and similar to 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,R-valley/d log P = -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 (rho) over cap 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.