TOI-4438 b: a transiting mini-Neptune amenable to atmospheric characterization
Nagel, E.; Amado, P. J.; Caballero, J. A.; Pallé, E.; Reiners, A.; Ribas, I.; Quirrenbach, A.; Béjar, V. J. S.; Tal-Or, L.; Zechmeister, M.; Schweitzer, A.; Dreizler, S.; Jeffers, S. V.; Gandolfi, D.; Hatzes, A. P.; Narita, N.; Henning, Th.; Lanza, A. F.; Goffo, E.; Luque, R.; Lillo-Box, J.; Morello, G.; Esparza-Borges, E.; Murgas, F.; Hellier, C.; Fukui, A.; Parviainen, H.; Mori, M.; Mallorquín, M.; Lodieu, N.; Herrero, E.; Vanaverbeke, S.; Pérez-Torres, M.; Schlecker, M.; Reffert, S.; Orell-Miquel, J.; Duque-Arribas, C.; Cifuentes, C.; Dufoer, S.; Tabernero, H. M.; Dubois, F.; Chaturvedi, P.; Schöfer, P.; Acuña, L.; Hermelo, I.; Pozuelos, F. J.; Hayashi, Y.; Peña-Moñino, L.; Geraldía-González, S.; Daspute, M.
Italy, Germany, Spain, Sweden, United States, Cyprus, United Kingdom, Israel, Belgium, Japan
Abstract
We report the confirmation and mass determination of a mini-Neptune transiting the M3.5 V star TOI-4438 (G 182-34) every 7.44 days. A transit signal was detected with NASA's TESS space mission in the sectors 40, 52, and 53. In order to validate the planet TOI-4438 b and to determine the system properties, we combined TESS data with high-precision radial velocity measurements from the CARMENES spectrograph, spanning almost one year, and ground-based transit photometry. We found that TOI-4438 b has a radius of Rb = 2.52 ± 0.13 R⊕ (5% precision), which together with a mass of Mb = 5.4 ± 1.1 M⊕ (20% precision), results in a bulk density of ρb = 1.85−0.44+0.51 g cm−3 (~28% precision), aligning the discovery with a volatile-rich planet. Our interior structure retrieval with a pure water envelope yields aminimum water mass fraction of 46% (1σ). TOI-4438 b is a volatile-rich mini-Neptune with likely H/He mixed with molecules, such as water, CO2, and CH4. The primary star has a J-band magnitude of 9.7, and the planet has a high transmission spectroscopy metric (TSM) of 136 ± 13. Taking into account the relatively warm equilibrium temperature of Teq = 435 ± 15 K, and the low activity level of its host star, TOI-4438 b is one of the most promising mini-Neptunes around an M dwarf for transmission spectroscopy studies.