The HADES RV programme with HARPS-N at TNG

XI. GJ 685 b: a warm super-Earth around an active M dwarf^★

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: matteo.pinamonti@inaf.it
² INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
³ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, C/ de Can Magrans s/n, 08193 Cerdanyola del Vallès, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), C/ Gran Capità 2-4, 08034 Barcelona, Spain
⁵ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁶ Universidad de La Laguna, Departamento Astrofísica, 38206 La Laguna, Tenerife, Spain
⁷ INAF – Osservatorio Astronomico di Palermo, piazza del Parlamento 1, 90134 Palermo, Italy
⁸ Observatoire Astronomique de l’Université de Genéve, 1290 Versoix, Switzerland
⁹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
¹¹ Fundación Galileo Galilei – INAF, Ramble José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
¹² Dipartimento di Fisica e Astronomia, Università di Padova, Via Marzolo 8, 35131 Padova, Italy
¹³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 22 December 2018
Accepted: 26 March 2019

Abstract

Context. Small rocky planets seem to be very abundant around low-mass M-type stars. Their actual planetary population is however not yet precisely understood. Currently, several surveys aim to expand the statistics with intensive detection campaigns, both photometric and spectroscopic.

Aims. The HADES program aims to improve the current statistics through the in-depth analysis of accurate radial-velocity (RV) monitoring in a narrow range of spectral sub-types, with the precision needed to detect small planets with a few Earth masses.

Methods. We analyse 106 spectroscopic HARPS-N observations of the active M0-type star GJ 685 taken over the past five years. We combine these data with photometric measurements from different observatories to accurately model the stellar rotation and disentangle its signals from genuine Doppler planetary signals in the RV data. We run an MCMC analysis on the RV and activity index time series to model the planetary and stellar signals present in the data, applying Gaussian Process regression technique to deal with the stellar activity signals.

Results. We identify three periodic signals in the RV time series, with periods of 9, 24, and 18 d. Combining the analyses of the photometry of the star with the activity indexes derived from the HARPS-N spectra, we identify the 18 d and 9 d signals as activity-related, corresponding to the stellar rotation period and its first harmonic, respectively. The 24 d signal shows no relation to any activity proxy, and therefore we identify it as a genuine planetary signal. We find the best-fit model describing the Doppler signal of the newly found planet, GJ 685 b, corresponding to an orbital period P_b = 24.160_−0.047^+0.061 d and a minimum mass M_P sin i = 9.0_−1.8^+1.7 M_⊕. We also study a sample of 70 RV-detected M-dwarf planets, and present new statistical evidence of a difference in mass distribution between the populations of single- and multi-planet systems, which can shed new light on the formation mechanisms of low-mass planets around late-type stars.