The Gaia-ESO Survey: Sodium and aluminium abundances in giants and dwarfs

Implications for stellar and Galactic chemical evolution^⋆,⋆⋆

¹ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
e-mail: rsmiljanic@ncac.torun.pl
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INAF–Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5 50125 Firenze, Italy
⁵ Department of Astronomy, Indiana University, Bloomington, IN 47405, USA
⁶ Kavli Institute for Astrophysics & Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁷ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone ( RM), Italy
⁸ Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
⁹ Max Planck Institute für Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, Bât. B5c, 4000 Liège, Belgium
¹¹ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
¹² Institute of Theoretical Physics and Astronomy, Vilnius University, Goštauto 12, 01108 Vilnius, Lithuania
¹³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁴ INAF–Osservatorio Astronomico di Padova, Vicolo Osservatorio 2, 35122 Padova, Italy
¹⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁶ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
¹⁷ GEPI, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, 61 avenue de l’Observatoire, 75014 Paris, France
¹⁸ Université de Picardie Jules Verne, Physics Dpt. 33 rue St. Leu, 80000 Amiens, France
¹⁹ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Universitá di Catania, via S. Sofia 78, 95123 Catania, Italy
²⁰ Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
²¹ Instituto de Física y Astronomía, Universidad de Valparaíso, Chile
²² European Southern Observatory, Alonso de Cordova 3107 Vitacura, Santiago de Chile, Chile
²³ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
²⁴ Department of Physics and Astronomy, UppsalaUniversity, Box 516, 751 20 Uppsala, Sweden
²⁵ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁶ Departamento de Ciencias Fisicas, Universidad Andres Bello, Republica 220, Santiago, Chile
²⁷ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
²⁸ Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile

Received: 20 December 2015
Accepted: 7 February 2016

Abstract

Context. Stellar evolution models predict that internal mixing should cause some sodium overabundance at the surface of red giants more massive than ~1.5–2.0 M_⊙. The surface aluminium abundance should not be affected. Nevertheless, observational results disagree about the presence and/or the degree of Na and Al overabundances. In addition, Galactic chemical evolution models adopting different stellar yields lead to very different predictions for the behavior of [Na/Fe] and [Al/Fe] versus [Fe/H]. Overall, the observed trends of these abundances with metallicity are not well reproduced.

Aims. We readdress both issues, using new Na and Al abundances determined within the Gaia-ESO Survey. Our aim is to obtain better observational constraints on the behavior of these elements using two samples: i) more than 600 dwarfs of the solar neighborhood and of open clusters and ii) low- and intermediate-mass clump giants in six open clusters.

Methods. Abundances were determined using high-resolution UVES spectra. The individual Na abundances were corrected for nonlocal thermodynamic equilibrium effects. For the Al abundances, the order of magnitude of the corrections was estimated for a few representative cases. For giants, the abundance trends with stellar mass are compared to stellar evolution models. For dwarfs, the abundance trends with metallicity and age are compared to detailed chemical evolution models.

Results. Abundances of Na in stars with mass below ~2.0 M_⊙, and of Al in stars below ~3.0 M_⊙, seem to be unaffected by internal mixing processes. For more massive stars, the Na overabundance increases with stellar mass. This trend agrees well with predictions of stellar evolutionary models. For Al, our only cluster with giants more massive than 3.0 M_⊙, NGC 6705, is Al enriched. However, this might be related to the environment where the cluster was formed. Chemical evolution models that well fit the observed [Na/Fe] vs. [Fe/H] trend in solar neighborhood dwarfs cannot simultaneously explain the run of [Al/Fe] with [Fe/H], and vice versa. The comparison with stellar ages is hampered by severe uncertainties. Indeed, reliable age estimates are available for only a half of the stars of the sample. We conclude that Al is underproduced by the models, except for stellar ages younger than about 7 Gyr. In addition, some significant source of late Na production seems to be missing in the models. Either current Na and Al yields are affected by large uncertainties, and/or some important Galactic source(s) of these elements has as yet not been taken into account.