Eyes on K2-3: A system of three likely sub-Neptunes characterized with HARPS-N and HARPS^★

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: damasso@oato.inaf.it
² Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland
³ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
⁷ Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Centre for Exoplanet Science, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, UK
¹⁰ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
¹¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹² INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹³ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
¹⁵ Centre for Star and Planet Formation, Niels Bohr Institute & Natural History Museum, University of Copenhagen, 1350 Copenhagen, Denmark
¹⁶ INAF – Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
¹⁷ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁸ Department of Astronomy, University of California, Berkeley, CA 94720, USA
¹⁹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
²⁰ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²¹ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
²² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²³ INAF – Osservatorio di Cagliari, via della Scienza 5, 09047 Selargius, CA, Italy
²⁴ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
²⁵ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
²⁶ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH93HJ, UK
²⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²⁸ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK

Received: 14 December 2017
Accepted: 20 February 2018

Abstract

Context. M-dwarf stars are promising targets for identifying and characterizing potentially habitable planets. K2-3 is a nearby (45 pc), early-type M dwarf hosting three small transiting planets, the outermost of which orbits close to the inner edge of the stellar (optimistic) habitable zone. The K2-3 system is well suited for follow-up characterization studies aimed at determining accurate masses and bulk densities of the three planets.

Aims. Using a total of 329 radial velocity measurements collected over 2.5 years with the HARPS-N and HARPS spectrographs and a proper treatment of the stellar activity signal, we aim to improve measurements of the masses and bulk densities of the K2-3 planets. We use our results to investigate the physical structure of the planets.

Methods. We analysed radial velocity time series extracted with two independent pipelines using Gaussian process regression. We adopted a quasi-periodic kernel to model the stellar magnetic activity jointly with the planetary signals. We used Monte Carlo simulations to investigate the robustness of our mass measurements of K2-3 c and K2-3 d, and to explore how additional high-cadence radial velocity observations might improve these values.

Results. Even though the stellar activity component is the strongest signal present in the radial velocity time series, we are able to derive masses for both planet b (M_b = 6.6 ± 1.1 M_⊕) and planet c (M_c = 3.1_−1.2^+1.3 M_⊕). The Doppler signal from K2-3 d remains undetected, likely because of its low amplitude compared to the radial velocity signal induced by the stellar activity. The closeness of the orbital period of K2-3 d to the stellar rotation period could also make the detection of the planetary signal complicated. Based on our ability to recover injected signals in simulated data, we tentatively estimate the mass of K2-3 d to be M_d = 2.7_−0.8^+1.2 M_⊕ M_⊕. These mass measurements imply that the bulk densities and therefore the interior structures of the three planets may be similar. In particular, the planets may either have small H/He envelopes (<1%) or massive water layers, with a water content ≥50% of their total mass, on top of rocky cores. Placing further constraints on the bulk densities of K2-3 c and d is difficult; in particular, we would not have been able to detect the Doppler signal of K2-3 d even by adopting a semester of intense, high-cadence radial velocity observations with HARPS-N and HARPS.