A large sub-Neptune transiting the thick-disk M4 V TOI-2406

Wells, R. D.; Rackham, B. V.; Schanche, N.; Petrucci, R.; Gómez Maqueo Chew, Y.; Demory, B. -O.; Burgasser, A. J.; Burn, R.; Pozuelos, F. J.; Günther, M. N.; Sabin, L.; Schroffenegger, U.; Gómez-Muñoz, M. A.; Stassun, K. G.; Van Grootel, V.; Howell, S. B.; Sebastian, D.; Triaud, A. H. M. J.; Apai, D.; Plauchu-Frayn, I.; Guerrero, C. A.; Guillén, P. F.; Landa, A.; Melgoza, G.; Montalvo, F.; Serrano, H.; Riesgo, H.; Barkaoui, K.; Bixel, A.; Burdanov, A.; Chen, W. P.; Chinchilla, P.; Collins, K. A.; Daylan, T.; de Wit, J.; Delrez, L.; Dévora-Pajares, M.; Dietrich, J.; Dransfield, G.; Ducrot, E.; Fausnaugh, M.; Furlan, E.; Gabor, P.; Gan, T.; Garcia, L.; Ghachoui, M.; Giacalone, S.; Gibbs, A. B.; Gillon, M.; Gnilka, C.; Gore, R.; Guerrero, N.; Henning, T.; Hesse, K.; Jehin, E.; Jenkins, J. M.; Latham, D. W.; Lester, K.; McCormac, J.; Murray, C. A.; Niraula, P.; Pedersen, P. P.; Queloz, D.; Ricker, G.; Rodriguez, D. R.; Schroeder, A.; Schwarz, R. P.; Scott, N.; Seager, S.; Theissen, C. A.; Thompson, S.; Timmermans, M.; Twicken, J. D.; Winn, J. N.
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Astronomy and Astrophysics

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Context. Large sub-Neptunes are uncommon around the coolest stars in the Galaxy and are rarer still around those that are metal-poor. However, owing to the large planet-to-star radius ratio, these planets are highly suitable for atmospheric study via transmission spectroscopy in the infrared, such as with JWST.
Aims: Here we report the discovery and validation of a sub-Neptune orbiting the thick-disk, mid-M dwarf star TOI-2406. The star's low metallicity and the relatively large size and short period of the planet make TOI-2406 b an unusual outcome of planet formation, and its characterisation provides an important observational constraint for formation models.
Methods: We first infer properties of the host star by analysing the star's near-infrared spectrum, spectral energy distribution, and Gaia parallax. We use multi-band photometry to confirm that the transit event is on-target and achromatic, and we statistically validate the TESS signal as a transiting exoplanet. We then determine physical properties of the planet through global transit modelling of the TESS and ground-based time-series data.
Results: We determine the host to be a metal-poor M4 V star, located at a distance of 56 pc, with properties Teff = 3100 ± 75 K, M* = 0.162 ± 0.008M⊙, R* = 0.202 ± 0.011R⊙, and [Fe∕H] = −0.38 ± 0.07, and a member of the thick disk. The planet is a relatively large sub-Neptune for the M-dwarf planet population, with Rp = 2.94 ± 0.17R⊕ and P= 3.077 d, producing transits of 2% depth. We note the orbit has a non-zero eccentricity to 3σ, prompting questions about the dynamical history of the system.
Conclusions: This system is an interesting outcome of planet formation and presents a benchmark for large-planet formation around metal-poor, low-mass stars. The system warrants further study, in particular radial velocity follow-up to determine the planet mass and constrain possible bound companions. Furthermore, TOI-2406 b is a good target for future atmospheric study through transmission spectroscopy. Although the planet's mass remains to be constrained, we estimate the S/N using amass-radius relationship, ranking the system fifth in the population of large sub-Neptunes, with TOI-2406 b having a much lower equilibrium temperature than other spectroscopically accessible members of this population.
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