The GAPS programme with HARPS-N at TNG

X. Differential abundances in the XO-2 planet-hosting binary^⋆,⋆⋆

¹ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
e-mail: katia.biazzo@oact.inaf.it
² INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Dipartimento di Fisica, Università di Torino, via P. Giuria 1, 10125 Torino, Italy
⁵ Landessternwarte Königstuhl, Zentrum für Astronomie der Universitat Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁶ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁸ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁹ INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁰ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹¹ Dipartimento di Fisica e Astronomia Galileo Galilei – Università di Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
¹² Instituto de Astrofísica e Ciências do Espaço, Universitade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹³ Centro de Astrofísica, Universitade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁴ Departamento de Física e Astronomia, Facultade de Ciências, Univ. do Porto, Rua do Campo Alegre, s/n, 4169-007 Porto, Portugal

Received: 21 April 2015
Accepted: 24 August 2015

Abstract

Binary stars hosting exoplanets are a unique laboratory where chemical tagging can be performed to measure the elemental abundances of both stellar components with high accuracy, with the aim to investigate the formation of planets and their subsequent evolution. Here, we present a high-precision differential abundance analysis of the XO-2 wide stellar binary based on high-resolution HARPS-N at TNG spectra. Both components are very similar K-dwarfs and host planets. Since they formed presumably within the same molecular cloud, we expect that they possess the same initial elemental abundances. We investigated whether planets can cause some chemical imprints in the stellar atmospheric abundances. We measure abundances of 25 elements for both stars with a range of condensation temperature T_C = 40−1741 K, achieving typical precisions of ~0.07 dex. The northern component shows abundances in all elements higher by +0.067 ± 0.032 dex on average, with a mean difference of +0.078 dex for elements with T_C > 800 K. The significance of the XO-2N abundance difference relative to XO-2S is at the 2σ level for almost all elements. We discuss that this result might be interpreted as the signature of the ingestion of material by XO-2N or depletion in XO-2S that is due to locking of heavy elements by the planetary companions. We estimate a mass of several tens of M_⊕ in heavy elements. The difference in abundances between XO-2N and XO-2S shows a positive correlation with the condensation temperatures of the elements, with a slope of (4.7 ± 0.9) × 10^-5 dex K^-1, which could mean that both components have not formed terrestrial planets, but first experienced the accretion of rocky core interior to the subsequent giant planets.