The GAPS Programme at TNG

LIII. New insights on the peculiar XO-2 system^★,★★

¹ Department of Physics and Astronomy, University of Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
e-mail: alessandro.ruggieri.1@phd.unipd.it
² INAF, Astrophysical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ INAF, Astronomical Observatory of Rome, Via Frascati 33, 00178 Monte Porzio Catone (RM), Italy
⁴ INAF, Astrophysical Observatory of Turin, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁵ Department of Physics and Astronomy, University of Padova, via Marzolo 8, 35131 Padova, Italy
⁶ INAF, Astrophysical Observatory of Catania, Via S. Sofia 78, 95123 Catania, Italy
⁷ INAF, Astronomical Observatory of Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁸ INAF, Astronomical Observatory of Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁹ Fundación Galileo Galilei – INAF, Rambla José Ana Fernández Pérez 7, 38712 Breña Baja, Tenerife, Spain
¹⁰ INAF, Astronomical Observatory of Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹¹ INAF, Astrophysical Observatory of Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

Received: 22 September 2023
Accepted: 12 January 2024

Abstract

Context. Planets in binary systems are a fascinating and yet poorly understood phenomenon. Since there are only a few known large-separation systems in which both components host planets, characterizing them is a key target for planetary science.

Aims. In this paper, we aim to carry out an exhaustive analysis of the interesting XO-2 system, where one component (XO-2N) appears to be a system with only one planet, while the other (XO-2S) has at least three planets.

Methods. Over the last 9 yr, we have collected 39 spectra of XO-2N and 106 spectra of XO-2S with the High Accuracy Radial velocity Planet Searcher for the Northern emisphere (HARPS-N) in the framework of the Global Architecture of Planetary Systems (GAPS) project, from which we derived precise radial velocity (RV) and activity indicator measurements. Additional spectroscopic data from the High Resolution Echelle Spectrometer (HIRES) and from the High Dispersion Spectrograph (HDS), and the older HARPS-N data presented in previous papers, have also been used to increase the total time span. We also used photometric data from TESS to search for potential transits that have not been detected yet. For our analysis, we mainly used PyORBIT, an advanced Python tool for the Bayesian analysis of RVs, activity indicators, and light curves.

Results. We found evidence for an additional long-period planet around XO-2S and characterized the activity cycle likely responsible for the long-term RV trend noticed for XO-2N. The new candidate is an example of a Jovian analog with m sin i ~ 3.7 M_J, a ~ 5.5 au, and e = 0.09. We also analyzed the stability and detection limits to get some hints about the possible presence of additional planets. Our results show that the planetary system of XO-2S is at least one order of magnitude more massive than that of XO-2N. The implications of these findings for the interpretation of the previously known abundance difference between components are also discussed.