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
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
Accepted: 5 June 2018
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.
Key words: stars: abundances / stars: evolution / Galaxy: stellar content / Galaxy: abundances
© ESO 2018
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.