The Gaia-ESO Survey: impact of extra mixing on C and N abundances of giant stars

¹ Institut UTINAM, CNRS UMR6213, Univ. Bourgogne Franche-Comté, OSU THETA Franche-Comté-Bourgogne, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
e-mail: nadege.lagarde@utinam.cnrs.fr
² Institute of Theoretical Physics and Astronomy, Vilnius University, Saulėtekio av. 3, 10257 Vilnius, Lithuania
³ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁶ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
⁷ INAF – Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹⁰ Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
¹¹ Space Science Data Center - Agenzia Spaziale Italiana, via del Politecnico, s.n.c., 00133 Roma, Italy
¹² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
¹³ Instituto de Física y Astronomía, Universidad de Valparaíso, Chile
¹⁴ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁵ Departamento de Didáctica, Universidad de Cádiz, 11519 Puerto Real, Cádiz, Spain
¹⁶ Departmento de Astrofísica, Centro de Astrobiología (INTA-CSIC), ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁷ Suffolk University, Madrid Campus, C/ de la Viña 3, 28003 Madrid, Spain
¹⁸ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹⁹ School of Physics, University of New South Wales, 2052 Sydney, Australia
²⁰ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
²¹ Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
²² Departamento de Ciencias Físicas, Universidad Andres Bello, Fernández Concha 700, Las Condes, Santiago, Chile
²³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
²⁴ European Southern Observatory, Alonso de Córdova 3107 Vitacura, Santiago de Chile, Chile
²⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²⁶ INAF – Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 7 December 2017
Accepted: 5 June 2018

Abstract

Context. The Gaia-ESO Public Spectroscopic Survey using FLAMES at the VLT has obtained high-resolution UVES spectra for a large number of giant stars, allowing a determination of the abundances of the key chemical elements carbon and nitrogen at their surface. The surface abundances of these chemical species are known to change in stars during their evolution on the red giant branch (RGB) after the first dredge-up episode, as a result of the extra mixing phenomena.

Aims. We investigate the effects of thermohaline mixing on C and N abundances using the first comparison between the Gaia-ESO survey [C/N] determinations with simulations of the observed fields using a model of stellar population synthesis.

Methods. We explore the effects of thermohaline mixing on the chemical properties of giants through stellar evolutionary models computed with the stellar evolution code STAREVOL. We include these stellar evolution models in the Besançon Galaxy model to simulate the [C/N] distributions determined from the UVES spectra of the Gaia-ESO survey and to compare them with the observations.

Results. Theoretical predictions including the effect of thermohaline mixing are in good agreement with the observations. However, the field stars in the Gaia-ESO survey with C and N abundance measurements have a metallicity close to solar, where the efficiency of thermohaline mixing is not very large. The C and N abundances derived by the Gaia-ESO survey in open and globular clusters clearly show the impact of thermohaline mixing at low metallicity, which explains the [C/N] value observed in lower mass and older giant stars. Using independent observations of carbon isotopic ratio in clump field stars and open clusters, we also confirm that thermohaline mixing should be taken into account to explain the behaviour of ¹²C/¹³C as a function of stellar age.

Conclusions. Overall, the current model including thermohaline mixing is able to reproduce very well the C and N abundances over the whole metallicity range investigated by the Gaia-ESO survey data.