The Gaia-ESO survey: Mixing processes in low-mass stars traced by lithium abundance in cluster and field stars^⋆,⋆⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E.Fermi, 5. 50125 Firenze, Italy
e-mail: laura.magrini@inaf.it
² Institut UTINAM, CNRS UMR 6213, Univ. Bourgogne Franche-Comté, OSU THETA Franche-Comté-Bourgogne, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
³ Department of Astronomy, University of Geneva, Chemin de Pégase 51, 1290 Versoix, Switzerland
⁴ IRAP, UMR 5277 CNRS and Université de Toulouse, 14 Av. E.Belin, 31400 Toulouse, France
⁵ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
⁷ Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio Av. 3, 10257 Vilnius, Lithuania
⁸ INAF – Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ INAF – Rome Observatory, Via Frascati, 33, Monte Porzio Catone, Rome, Italy
¹⁰ ESO, Karl Schwarzschild Strasse 2, 85748 Garching, Germany
¹¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹² Science and Technology Department, Parthenope University of Naples, Centro Direzionale, Isola C4, 80143 Naples, Italy
¹³ INAF – Osservatorio Astronomico di Capodimonte, Salita Moraliello, Napoli, Italy
¹⁴ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁵ Dipartimento di Fisica “E.Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
¹⁶ INFN, Sezione di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
¹⁷ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
¹⁸ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
¹⁹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²⁰ Dipartimento di Fisica e Astronomia Galileo Galilei, Vicolo Osservatorio 3, 35122 Padova, Italy
²¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
²² Departamento de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
²³ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
²⁴ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43 221 00 Lund, Sweden
²⁵ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
²⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
²⁷ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
²⁸ Núcleo Milenio Formación Planetaria – NPF, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
²⁹ Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK
³⁰ Space Science Data Center – Agenzia Spaziale Italiana, Via del Politecnico, s.n.c., 00133 Rome, Italy

Received: 30 March 2021
Accepted: 5 May 2021

Abstract

Aims. We aim to constrain the mixing processes in low-mass stars by investigating the behaviour of the Li surface abundance after the main sequence. We take advantage of the data from the sixth internal data release of Gaia-ESO, IDR6, and from the Gaia Early Data Release 3, EDR3s.

Methods. We selected a sample of main-sequence, sub-giant, and giant stars in which the Li abundance is measured by the Gaia-ESO survey. These stars belong to 57 open clusters with ages from 130 Myr to about 7 Gyr and to Milky Way fields, covering a range in [Fe/H] between ∼​ − 1.0 and ∼​ + 0.5 dex, with few stars between ∼​ − 1.0 and ∼​ − 2.5 dex. We studied the behaviour of the Li abundances as a function of stellar parameters. We inferred the masses of giant stars in clusters from the main-sequence turn-off masses, and for field stars through comparison with stellar evolution models using a maximum likelihood technique. We compared the observed Li behaviour in field giant stars and in giant stars belonging to individual clusters with the predictions of a set of classical models and of models with mixing induced by rotation and thermohaline instability.

Results. The comparison with stellar evolution models confirms that classical models cannot reproduce the observed lithium abundances in the metallicity and mass regimes covered by the data. The models that include the effects of both rotation-induced mixing and thermohaline instability account for the Li abundance trends observed in our sample in all metallicity and mass ranges. The differences between the results of the classical models and of the rotation models largely differ (up to 2 dex), making lithium the best element with which to constrain stellar mixing processes in low-mass stars. We discuss the nature of a sample of Li-rich stars.

Conclusions. We demonstrate that the evolution of the surface abundance of Li in giant stars is a powerful tool for constraining theoretical stellar evolution models, allowing us to distinguish the effect of different mixing processes. For stars with well-determined masses, we find a better agreement of observed surface abundances and models with rotation-induced and thermohaline mixing. Rotation effects dominate during the main sequence and the first phases of the post-main-sequence evolution, and the thermohaline induced mixing after the bump in the luminosity function.