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

• Published in: arXiv.org 2024-03
• Main Authors: 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
• Format: Article
• Language: English
• Subjects: Bulk density; Neptune; Planetary interiors; Radial velocity; Space missions; Spectrum analysis; Water masses
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2403.09833

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_Earth (5% precision), which together with a mass of Mb=5.4 +/- 1.1 M_Earth (20% precision), results in a bulk density of rho = 1.85+0.51-0.44 g cm-3 (28% precision), aligning the discovery with a volatile-rich planet. Our interior structure retrieval with a pure water envelope yields a minimum water mass fraction of 46% (1-sigma). TOI-4438 b is a volatile-rich mini-Neptune with likely H/He mixed with molecules, such as water, CO_2, and CH_4. 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 T_eq = 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.