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

September 2021 • 2021A&A...653A..97W

Authors • 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.

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 T_eff = 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 R_p = 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.

Links

• PREPRINT http://arxiv.org/abs/2107.14125
• POSTSCRIPT https://www.aanda.org/10.1051/0004-6361/202141277/postscript
• ELECTR https://doi.org/10.1051%2F0004-6361%2F202141277
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2021A%26A...653A..97W
• PDF https://www.aanda.org/10.1051/0004-6361/202141277/pdf
• DATA https://archive.gemini.edu/searchform/publication=2021A&A...653A..97W
• DATA https://irsa.ipac.caltech.edu/bibdata/2021/W/2021A&A...653A..97W.html