A large sub-Neptune transiting the thick-disk M4 V TOI-2406
Gabor, P.; Queloz, D.; Henning, T.; Apai, D.; Chen, W. P.; Winn, J. N.; Delrez, L.; Collins, K. A.; Demory, B. -O.; Gillon, M.; Jenkins, J. M.; Seager, S.; Twicken, J. D.; Van Grootel, V.; Guerrero, C. A.; Burgasser, A. J.; Jehin, E.; Sabin, L.; Barkaoui, K.; Latham, D. W.; Ricker, G.; Schwarz, R. P.; Petrucci, R.; McCormac, J.; Furlan, E.; Giacalone, S.; Gnilka, C.; Stassun, K. G.; Gómez-Muñoz, M. A.; Günther, M. N.; de Wit, J.; Triaud, A. H. M. J.; Daylan, T.; Fausnaugh, M.; Guerrero, N.; Howell, S. B.; Dransfield, G.; Hesse, K.; Burdanov, A.; Thompson, S.; Murray, C. A.; Ducrot, E.; Gómez Maqueo Chew, Y.; Niraula, P.; Pozuelos, F. J.; Rackham, B. V.; Schanche, N.; Sebastian, D.; Timmermans, M.; Guillén, P. F.; Rodriguez, D. R.; Schroffenegger, U.; Ghachoui, M.; Wells, R. D.; Dévora-Pajares, M.; Gan, T.; Pedersen, P. P.; Theissen, C. A.; Dietrich, J.; Chinchilla, P.; Burn, R.; Plauchu-Frayn, I.; Landa, A.; Melgoza, G.; Montalvo, F.; Serrano, H.; Riesgo, H.; Bixel, A.; Garcia, L.; Gibbs, A. B.; Gore, R.; Lester, K.; Schroeder, A.; Scott, N.
Switzerland, United States, Argentina, Mexico, Germany, Belgium, United Kingdom, Morocco, Taiwan, Spain, China
Abstract
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.