The CARMENES search for exoplanets around M dwarfs

Three temperate-to-warm super-Earths^★

¹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
e-mail: [email protected]
² Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK 73019, USA
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁷ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁸ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, C/Can Magrans s/n, 08193 Bellaterra, Spain
⁹ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206, La Laguna, Tenerife, Spain
¹³ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁴ Department of Exploitation and Exploration of Mines, University of Oviedo, Oviedo, Spain
¹⁵ Department of Physics, Ariel University, Ariel 40700, Israel
¹⁶ Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain

Received: 2 July 2020
Accepted: 11 September 2020

Abstract

We announce the discovery of two planets orbiting the M dwarfs GJ 251 (0.360 ± 0.015M_⊙) and HD 238090 (0.578 ± 0.021M_⊙) based on CARMENES radial velocity (RV) data. In addition, we independently confirm with CARMENES data the existence of Lalande 21185 b, a planet that has recently been discovered with the SOPHIE spectrograph. All three planets belong to the class of warm or temperate super-Earths and share similar properties. The orbital periods are 14.24 d, 13.67 d, and 12.95 d and the minimum masses are 4.0 ± 0.4 M_⊕, 6.9 ± 0.9 M_⊕, and 2.7 ± 0.3 M_⊕ for GJ 251 b, HD 238090 b, and Lalande 21185 b, respectively. Based on the orbital and stellar properties, we estimate equilibrium temperatures of 351.0 ± 1.4 K for GJ 251 b, 469.6 ± 2.6 K for HD 238090 b, and 370.1 ± 6.8 K for Lalande 21185 b. For the latter we resolve the daily aliases that were present in the SOPHIE data and that hindered an unambiguous determination of the orbital period. We find no significant signals in any of our spectral activity indicators at the planetary periods. The RV observations were accompanied by contemporaneous photometric observations. We derive stellar rotation periods of 122.1 ± 2.2 d and 96.7 ± 3.7 d for GJ 251 and HD 238090, respectively. The RV data of all three stars exhibit significant signals at the rotational period or its first harmonic. For GJ 251 and Lalande 21185, we also find long-period signals around 600 d, and 2900 d, respectively, which we tentatively attribute to long-term magnetic cycles. We apply a Bayesian approach to carefully model the Keplerian signals simultaneously with the stellar activity using Gaussian process regression models and extensively search for additional significant planetary signals hidden behind the stellar activity. Current planet formation theories suggest that the three systems represent a common architecture, consistent with formation following the core accretion paradigm.