HADES RV Programme with HARPS-N at TNG

XIII. A sub-Neptune around the M dwarf GJ 720 A^★

¹ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: egonzalez@cab.inta-csic.es
² INAF-Osservatorio Astronomico di Palermo, Piazza Parlamento 1, 90134 Palermo, Italy
³ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁴ INAF-Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁶ Institut de Ciències de l’Espai (IEEC-CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193, Bellaterra, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
⁸ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁹ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
¹⁰ INAF-Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹² Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain

Received: 3 February 2021
Accepted: 16 March 2021

Abstract

Context. The high number of super-Earth and Earth-like planets in the habitable zone detected around M-dwarf stars in recent years has revealed these stellar objects to be the key to planetary radial velocity (RV) searches.

Aims. Using the HARPS-N spectrograph within The HArps-n red Dwarf Exoplanet Survey (HADES) we have reached the precision needed to detect small planets with a few Earth masses using the spectroscopic radial velocity technique. HADES is mainly focused on the M-dwarf population of the northern hemisphere.

Methods. We obtained 138 HARPS-N RV measurements between 2013 May and 2020 September of GJ 720 A, classified as an M0.5 V star located at a distance of 15.56 pc. To characterize the stellar variability and to distinguish the periodic variation due to the Keplerian signals from those related to stellar activity, the HARPS-N spectroscopic activity indicators and the simultaneous photometric observations with the APACHE and EXORAP transit surveys were analyzed. We also took advantage of TESS, MEarth, and SuperWASP photometric surveys. The combined analysis of HARPS-N RVs and activity indicators let us address the nature of the periodic signals. The final model and the orbital planetary parameters were obtained by simultaneously fitting the stellar variability and the Keplerian signal using a Gaussian process regression and following a Bayesian criterion.

Results. The HARPS-N RV periodic signals around 40 days and 100 days have counterparts at the same frequencies in HARPS-N activity indicators and photometric light curves. We thus attribute these periodicities to stellar activity; the first period is likely associated with the stellar rotation. GJ 720 A shows the most significant signal at 19.466 ± 0.005 days with no counterparts in any stellar activity indices. We hence ascribe this RV signal, having a semi-amplitude of 4.72 ± 0.27 m s⁻¹, to the presence of a sub-Neptune mass planet. The planet GJ 720 Ab has a minimum mass of 13.64 ± 0.79 M_⊕, it is in circular orbit at 0.119 ± 0.002 AU from its parent star, and lies inside the inner boundary of the habitable zone around its parent star.