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.; WIN

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2021 vol. 653

0004-6361

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 ± 0.008Mo˙, R∗ = 0.202 ± 0.011Ro˙, 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.