HADES RV Programme with HARPS-N at TNG^⋆,⋆⋆

VI. GJ 3942 b behind dominant activity signals

¹ Institut de Ciéncies de l’Espai (IEEC-CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193, Cerdanyola del Vallés, Spain
e-mail: perger@ice.cat
² INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
³ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ Institute de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁵ Observatoire Astronomique de l’Université de Genève, 1205 Versoix, Switzerland
⁶ Universidad de La Laguna (ULL), Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
⁷ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
⁸ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁹ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹¹ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
¹² Osservatorio Astronomico della Regione Autonoma Valle d’Aosta, Fraz. Lignan 39, 11020 Nus ( Aosta), Italy
¹³ INAF–Osservatorio Astronomico di Capodimonte, via Moiariello, 16, 80131 Napoli, Italy
¹⁴ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
¹⁵ INAF–Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
¹⁶ Dipartimento di Fisica e Astronomia, G. Galilei, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁷ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
¹⁸ CNRS, IRAP, 14 avenue Edouard Belin, 31400 Toulouse, France

Received: 2 June 2017
Accepted: 12 September 2017

Abstract

Context. Short- to mid-term magnetic phenomena on the stellar surface of M-type stars can resemble the effects of planets in radial velocity data, and may also hide them.

Aims. We analyze 145 spectroscopic HARPS-N observations of GJ 3942 taken over the past five years and additional photometry in order to disentangle stellar activity effects from genuine Doppler signals as a result of the orbital motion of the star around the common barycenter with its planet.

Methods. To achieve this, we use the common methods of pre-whitening, and treat the correlated red noise by a first-order moving average term and by Gaussian-process regression following an MCMC analysis.

Results. We identify the rotational period of the star at 16.3 days and discover a new super-Earth, GJ 3942 b, with an orbital period of 6.9 days and a minimum mass of 7.1 M_⊕. An additional signal in the periodogram of the residuals is present, but at this point we cannot claim with sufficient significance that it is related to a second planet. If confirmed, this planet candidate would have a minimum mass of 6.3 M_⊕ and a period of 10.4 days, which might indicate a 3:2 mean-motion resonance with the inner planet.