Gaia-ESO survey: Lithium abundances in open cluster Red Clump stars^⋆,⋆⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: laura.magrini@inaf.it
² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
³ ESO, Karl Schwarzschild Strasse 2, 85748 Garching, Germany
⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF – Astrophysics and Space Science Observatory Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio Av. 3, 10257 Vilnius, Lithuania
⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁸ INAF – Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ INAF – Rome Observatory, Via Frascati, 33, Monte Porzio Catone, RM, Italy
¹⁰ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹¹ Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹² Space Science Data Center – Agenzia Spaziale Italiana, Via del Politecnico, s.n.c., 00133 Roma, Italy
¹³ Departamento de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁵ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
¹⁶ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43 221 00 Lund, Sweden
¹⁷ Dipartimento di Fisica e Astronomia Galileo Galilei, Vicolo Osservatorio 3, 35122 Padova, Italy
¹⁸ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Ejército 441, Santiago, Chile
¹⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 10 May 2021
Accepted: 26 August 2021

Abstract

Context. It has recently been suggested that all giant stars with masses below 2 M_⊙ suffer an episode of surface lithium enrichment between the tip of the red giant branch (RGB) and the red clump (RC).

Aims. We test if the above result can be confirmed in a sample of RC and RGB stars that are members of open clusters.

Methods. We discuss Li abundances in six open clusters with ages between 1.5 and 4.9 Gyr (turn-off masses between 1.1 and 1.7 M_⊙). We compare these observations with the predictions of different models that include rotation-induced mixing, thermohaline instability, mixing induced by the first He flash, and energy losses by neutrino magnetic moment.

Results. In six clusters, we find close to 35% of RC stars have Li abundances that are similar or higher than those of upper RGB stars. This can be a sign of fresh Li production. Because of the extra-mixing episode connected to the luminosity bump, the expectation has been for RC stars to have systematically lower surface Li abundances. However, we cannot confirm that this possible Li production is ubiquitous. For about 65% of RC giants, we can only determine upper limits in abundances that could be hiding very low Li content.

Conclusions. Our results indicate the possibility that Li is being produced in the RC, at levels that would not typically permit the classification of these the stars as Li rich. The determination of their carbon isotopic ratio would help to confirm that the RC giants have suffered extra mixing followed by subsequent Li enrichment. The Li abundances of the RC stars can be qualitatively explained by the models including an additional mixing episode close to the He flash.