Characterising TOI-732 b and c: New insights into the M-dwarf radius and density valley
Authors: Bonfanti A. , Brady M. , Wilson T. G. , Venturini J. , Egger J. A. , Brandeker A. , Sousa S. G. , Lendl M. , Simon A. E. , Queloz D. , Olofsson G. , Adibekyan V. , Alibert Y. , Fossati L. , Hooton M. J. , Kubyshkina D. , Luque R. , Murgas F. , Mustill A. J. , Santos N. C. , Van Grootel V. , Alonso R. , Asquier J. , Bandy T. , Bárczy T. , Barrado Navascues D. , Barros S. C. C. , Baumjohann W. , Bean J. , Beck M. , Beck T. , Benz W. , Bergomi M. , Billot N. , Borsato L. , Broeg C. , Collier Cameron A. , Csizmadia Sz. , Cubillos P. E. , Davies M. B. , Deleuil M. , Deline A. , Delrez L. , Demangeon O. D. S. , Demory B.-O. , Ehrenreich D. , Erikson A. , Fortier A. , Fridlund M. , Gandolfi D. , Gillon M. , Güdel M. , Günther M. N. , Heitzmann A. , Helling Ch. , Hoyer S. , Isaak K. G. , Kasper D. , Kiss L. L. , Lam K. W. F. , Laskar J. , Lecavelier des Etangs A. , Magrin D. , Maxted P. F. L. , Mordasini C. , Nascimbeni V. , Ottensamer R. , Pagano I. , Pallé E. , Peter G. , Piotto G. , Pollacco D. , Ragazzoni R. , Rando N. , Rauer H. , Ribas I. , Scandariato G. , Ségransan D. , Seifahrt A. , Smith A. M. S. , Stalport M. , Stefánsson G. , Steinberger M. , Stürmer J. , Szabó Gy. M. , Thomas N. , Udry S. , Villaver E. , Walton N. A. , Westerdorff K. , Zingales T. 
Published: November 2023

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 Rb = 1.325-0.058+0.057R⊕, a mass Mb = 2.46 ± 0.19 M⊕, and thus a mean density ρb = 5.8-0.8+1.0 g cm-3, while the outer planet at P = 12.252284 ± 0.000013 days has Rc = 2.39-0.11+0.10R⊕, Mc = 8.04-0.48+0.50M⊕, and thus ρc = 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 Rp,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.