A large sub-Neptune transiting the thick-disk M4V TOI-2406
Creators: name: Wells, R. D.
name: Rackham, B. V.
name: Schanche, N.
name: Petrucci, R.
name: Chew, Y. Gomez Maqueo
name: Demory, B. -O.
name: Burgasser, A. J.
name: Burn, R.
name: Pozuelos, F. J.
name: Gunther, M. N.
name: Sabin, L.
name: Schroffenegger, U.
name: Gomez-Munoz, M. A.
name: Stassun, K. G.
name: Van Grootel, V.
name: Howell, S. B.
name: Sebastian, D.
name: Triaud, A. H. M. J.
name: Apai, D.
name: Plauchu-Frayn, I.
name: Guerrero, C. A.
name: Guillen, P. F.
name: Landa, A.
name: Melgoza, G.
name: Montalvo, F.
name: Serrano, H.
name: Riesgo, H.
name: Barkaoui, K.
name: Bixel, A.
name: Burdanov, A.
name: Chen, W. P.
name: Chinchilla, P.
name: Collins, K. A.
name: Daylan, T.
name: de Wit, J.
name: Delrez, L.
name: Devora-Pajares, M.
name: Dietrich, J.
name: Dransfield, G.
name: Ducrot, E.
name: Fausnaugh, M.
name: Furlan, E.
name: Gabor, P.
name: Gan, T.
name: Garcia, L.
name: Ghachoui, M.
name: Giacalone, S.
name: Gibbs, A. B.
name: Gillon, M.
name: Gnilka, C.
name: Gore, R.
name: Guerrero, N.
name: Henning, T.
name: Hesse, K.
name: Jehin, E.
name: Jenkins, J. M.
name: Latham, D. W.
name: Lester, K.
name: McCormac, J.
name: Murray, C. A.
name: Niraula, P.
name: Pedersen, P. P.
name: Queloz, D.
name: Ricker, G.
name: Rodriguez, D. R.
name: Schroeder, A.
name: Schwarz, R. P.
name: Scott, N.
name: Seager, S.
name: Theissen, C. A.
name: Thompson, S.
name: Timmermans, M.
name: Twicken, J. D.
name: Winn, J. N.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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. Here we report the discovery and validation of a sub-Neptune orbiting the thick-disk, mid-M dwarf star TOI-2406. 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. We determine the host to be a metal-poor M4V star, located at a distance of 56 pc, with a sub-solar metallicity $(\mathrm{[Fe/H] = -0.38 \pm 0.07})$, and a member of the thick disk. The planet is a relatively large sub-Neptune for the M-dwarf planet population, with $\mathrm{R_p = 2.94 \pm 0.17} \mathrm{R_\oplus}$ and $\mathrm{P = 3.077}$ d, producing transits of 2% depth. We note the orbit has a non-zero eccentricity to 3$\mathrmσ$, prompting questions about the dynamical history of the system. 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, particularly in the category of warm sub-Neptunes.
description: Accepted for publication in Astronomy and Astrophysics