HADES RV program with HARPS-N at the TNG

IX. A super-Earth around the M dwarf Gl 686^★,★★

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: laura.affer@inaf.it
² INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
³ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ INAF – Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁵ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁶ INAF – Osservatorio Astronomico di Napoli, Salita Moiariello, 16, 80131 Napoli, Italy
⁷ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁸ Universidad de La Laguna, Departamento Astrofísica, 38206 La Laguna, Tenerife, Spain
⁹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy
¹⁰ Institut de Ciéncies de l’Espai (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra, Spain
¹¹ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
¹² Observatoire Astronomique de l’Université de Genève, 1290, Versoix, Switzerland
¹³ INAF – Osservatorio Astronomico di Cagliari, viale della Scienza, 5, 09047 Selargius, Cagliari, Italy

Received: 14 December 2018
Accepted: 13 January 2019

Abstract

Aims. The HArps-n red Dwarf Exoplanet Survey (HADES) is providing a major contribution to the widening of the current statistics of low-mass planets, through the in-depth analysis of precise radial-velocity (RV) measurements in a narrow range of spectral sub-types. Using the HARPS-N spectrograph we reach the precision needed to detect small planets with a few earth masses. Our survey is mainly focused on the M-dwarf population of the northern hemisphere.

Methods. As part of that program, we obtained RV measurements of Gl 686, an M1 dwarf at d = 8.2 pc. These measurements show a dispersion much in excess of their internal errors. The analysis of data obtained within an intensive observing campaign demonstrates that the excess dispersion is due to a coherent signal with a period of 15.53 d. Almost simultaneous photometric observations were carried out within the APACHE and EXORAP programs to characterize the stellar activity and to distinguish periodic variations related to activity from signals due to the presence of planetary companions, complemented also with ASAS photometric data. We used a Bayesian framework to estimate the orbital parameters and the planet minimum mass, and to properly treat the activity noise. We took advantage of the available RV measurements for this target from other observing campaigns. The analysis of the RV composite time series from the HIRES, HARPS, and HARPS-N spectrographs, consisting of 198 measurements taken over 20 yr, enabled us to address the nature of periodic signals and also to characterize stellar physical parameters (mass, temperature, and rotation).

Results. We report the discovery of a super-Earth orbiting at a distance of 0.092 AU from the host star Gl 686. The planet has a minimum mass of 7.1 ± 0.9 M_⊕ and an orbital period of 15.532 ± 0.002 d. The analysis of the activity indexes, of the correlated noise through a Gaussian process framework, and of the photometry provides an estimate of the stellar rotation period at 37 d, and highlights the variability of the spot configuration during the long timespan covering 20 yr. The observed periodicities around 2000 d likely point to the existence of an activity cycle.