The discovery of a Li/Na-rich giant star in Omega Centauri: formed from the pure ejecta of super-AGB stars?^★

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
e-mail: alessio.mucciarelli2@unibo.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
⁴ GEPI, Observatoire de Paris, Université PSL, CNRS, Place Jules Janssen, 92195 Meudon, France
⁵ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁶ Departamento de Astronomía, Casilla 160-C, Universidad de Concepción, Concepción, Chile

Received: 23 October 2018
Accepted: 27 December 2018

Abstract

We report the discovery of two Li-rich giant stars (fainter than the red giant branch (RGB) bump) in the stellar system Omega Centauri using GIRAFFE-FLAMES spectra. These two stars have A(Li) = 1.65 and 2.40 dex and they belong to the main population of the system ([Fe/H] = –1.70 and –1.82, respectively). The most Li-rich of them (#25664) has [Na/Fe] = +0.87 dex, which is ~0.5 dex higher than those measured in the most Na-rich stars of Omega Centauri of similar metallicity. The chemical abundances of Li and Na in #25664 can be qualitatively explained by deep extra mixing efficient within the star during its RGB evolution or by super-asymptotic giant branch (AGB) stars with masses between ~7 and 8 M_⊙. In the latter scenario, this Li/Na-rich star could be formed from the pure ejecta of super-AGB stars before the dilution with pristine material occurs, or, alternatively, be part of a binary system having experienced mass transfer from the companion when this latter evolved through the super-AGB phase. In both these cases, the chemical composition of this unique object could allow to look for the first time at the chemical composition of the gas processed in the interior of super-AGB stars.