TOI-4438 b: a transiting mini-Neptune amenable to atmospheric characterization

Goffo, E.; Chaturvedi, P.; Murgas, F.; Morello, G.; Orell-Miquel, J.; Acuña, L.; Peña-Moñino, L.; Pallé, E.; Hatzes, A. P.; Geraldía-González, S.; Pozuelos, F. J.; Lanza, A. F.; Gandolfi, D.; Caballero, J. A.; Schlecker, M.; Pérez-Torres, M.; Lodieu, N.; Schweitzer, A.; Hellier, C.; Jeffers, S. V.; Duque-Arribas, C.; Cifuentes, C.; Béjar, V. J. S.; Daspute, M.; Dubois, F.; Dufoer, S.; Esparza-Borges, E.; Fukui, A.; Hayashi, Y.; Herrero, E.; Mori, M.; Narita, N.; Parviainen, H.; Tal-Or, L.; Vanaverbeke, S.; Hermelo, I.; Amado, P. J.; Dreizler, S.; Henning, Th.; Lillo-Box, J.; Luque, R.; Mallorquín, M.; Nagel, E.; Quirrenbach, A.; Reffert, S.; Reiners, A.; Ribas, I.; Schöfer, P.; Tabernero, H. M.; Zechmeister, M.
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Astronomy and Astrophysics

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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.