The CARMENES search for exoplanets around M dwarfs: Two planets on opposite sides of the radius gap transiting the nearby M dwarf LTT 3780
Creators: name: Parviainen, H.
name: Molaverdikhani, K.
name: Jeffers, S.V.
name: Diéz Alonso, E.
name: Trifonov, T.
name: Rodríguez-López, C.
name: Caballero, J.A.
name: Schlecker, M.
name: Esparza-Borges, E.
name: Montes, D.
name: Lodieu, N.
name: Nagel, E.
name: Massey, B.
name: Vanderspek, R.
name: Stangret, M.
name: Tanner, A.
name: Guerra, P.
name: Cale, B.
name: Zapatero Osorio, M.R.
name: Reiners, A.
name: Rose, M.E.
name: Kotani, T.
name: Wittrock, J.
name: Mann, A.W.
name: Matthews, E.
name: Latham, D.W.
name: Barkaoui, K.
name: Ribas, I.
name: Gillon, M.
name: Morales, J.C.
name: Nowak, G.
name: Pedraz, S.
name: Reffert, S.
name: Quirrenbach, A.
name: Herrero, E.
name: Collins, K.I.
name: Ricker, G.R.
name: Tamura, M.
name: Béjar, V.J.S.
name: Lillo-Box, J.
name: Jenkins, J.M.
name: Teske, J.
name: Kemmer, J.
name: Hirano, T.
name: Lissauer, J.J.
name: Plavchan, P.
name: Schlieder, J.E.
name: Kossakowski, D.
name: Stock, S.
name: El Mufti, M.
name: Montañés-Rodríguez, P.
name: Mori, M.
name: Gaidos, E.
name: De León, J.P.
name: Benkhaldoun, Z.
name: Oshagh, M.
name: Schweitzer, A.
name: Kürster, M.
name: Narita, N.
name: Casasayas Barris, N.
name: Seager, S.
name: Howell, S.B.
name: Gonzales, E.J.
name: Jehin, E.
name: Kielkopf, J.F.
name: Dreizler, S.
name: Zechmeister, M.
name: Hidalgo, D.
name: Chaturvedi, P.
name: Madrigal-Aguado, A.
name: Matson, R.A.
name: Lafarga, M.
name: Amado, P.J.
name: Essack, Z.
name: Bauer, F.F.
name: Kaminski, A.
name: Law, N.
name: Henning, T.
name: Pollacco, D.
name: Collins, K.A.
name: Zohrabi, F.
name: Cortés-Contreras, M.
name: Pallé, E.
name: Espinoza, N.
name: Hesse, K.
name: Murgas, F.
name: Aceituno, F.J.
name: Pozuelos, F.J.
name: Crossfield, I.J.M.
name: Barrado, D.
name: Watanabe, D.
name: Ziegler, C.
name: Fukui, A.
name: Hatzes, A.
name: Twicken, J.D.
name: Passegger, V.M.
name: Cifuentes, C.
name: Caldwell, D.A.
name: Chen, G.
name: Luque, R.
Publisher: name: EDP Sciences
Year: 2020

description: We present the discovery and characterisation of two transiting planets observed by the Transiting Exoplanet Survey Satellite (TESS) orbiting the nearby (d∗ ≈ 22 pc), bright (J ≈ 9 mag) M3.5 dwarf LTT 3780 (TOI-732). We confirm both planets and their association with LTT 3780 via ground-based photometry and determine their masses using precise radial velocities measured with the CARMENES spectrograph. Precise stellar parameters determined from CARMENES high-resolution spectra confirm that LTT 3780 is a mid-M dwarf with an effective temperature of Teff = 3360 ± 51 K, a surface gravity of log g∗ = 4.81 ± 0.04 (cgs), and an iron abundance of [Fe/H] = 0.09 ± 0.16 dex, with an inferred mass of M∗ = 0.379 ± 0.016M· and a radius of R∗ = 0.382 ± 0.012R·. The ultra-short-period planet LTT 3780 b (Pb = 0.77 d) with a radius of 1.35-0.06+0.06 R·, a mass of 2.34-0.23+0.24 M·, and a bulk density of 5.24-0.81+0.94 g cm-3 joins the population of Earth-size planets with rocky, terrestrial composition. The outer planet, LTT 3780 c, with an orbital period of 12.25 d, radius of 2.42-0.10+0.10 R·, mass of 6.29-0.61+0.63 M·, and mean density of 2.45-0.37+0.44 g cm-3 belongs to the population of dense sub-Neptunes. With the two planets located on opposite sides of the radius gap, this planetary system is anexcellent target for testing planetary formation, evolution, and atmospheric models. In particular, LTT 3780 c is an ideal object for atmospheric studies with the James Webb Space Telescope (JWST).