The Gaia-ESO Survey: CNO abundances in the open clusters Trumpler 20, NGC 4815, and NGC 6705 ^⋆

¹ Institute of Theoretical Physics and Astronomy, Vilnius University, A. Gostauto 12, 01108 Vilnius, Lithuania
e-mail: grazina.tautvaisiene@tfai.vu.lt
² Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Florence, Italy
⁴ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
⁶ ESO, Alonso de Cordova 3107, 19001 Santiago de Chile, Chile
⁷ Department of Astronomy, Indiana University, Bloomington, IN 47405, USA
⁸ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, Bât. B5c, 4000 Liège, Belgium
⁹ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
¹⁰ Spanish Virtual Observatory, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Caada, Madrid, Spain
¹¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹² Astrophysics Group, Research Institute for the Environment, Physical Sciences and Applied Mathematics, Keele University, Keele, Staffordshire ST5 5BG, UK
¹³ INAF–Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁴ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
¹⁵ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁶ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
¹⁷ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁸ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Received: 15 September 2014
Accepted: 20 October 2014

Abstract

Context. The Gaia-ESO Public Spectroscopic Survey will observe a large sample of clusters and cluster stars, covering a wide age-distance-metallicity-position-density parameter space.

Aims. We aim to determine C, N, and O abundances in stars of Galactic open clusters of the Gaia-ESO survey and to compare the observed abundances with those predicted by current stellar and Galactic evolution models. In this pilot paper, we investigate the first three intermediate-age open clusters.

Methods. High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO VLT, were analysed using a differential model atmosphere method. Abundances of carbon were derived using the C₂ band heads at 5135 and 5635.5 Å. The wavelength interval 6470–6490 Å, with CN features, was analysed to determine nitrogen abundances. Oxygen abundances were determined from the [O i] line at 6300 Å.

Results. The mean values of the elemental abundances in Trumpler 20 as determined from 42 stars are: [ Fe/H ] = 0.10 ± 0.08 (s.d.), [ C/H ] = −0.10 ± 0.07, [ N/H ] = 0.50 ± 0.07, and consequently C/N = 0.98 ± 0.12. We measure from five giants in NGC 4815: [ Fe/H ] = −0.01 ± 0.04, [ C/H ] = −0.17 ± 0.08, [ N/H ] = 0.53 ± 0.07, [ O/H ] = 0.12 ± 0.09, and C/N = 0.79 ± 0.08. We obtain from 27 giants in NGC 6705: [ Fe/H ] = 0.0 ± 0.05, [ C/H ] = −0.08 ± 0.06, [ N/H ] = 0.61 ± 0.07, [ O/H ] = 0.13 ± 0.05, and C/N = 0.83 ± 0.19. The C/N ratios of stars in the investigated open clusters were compared with the ratios predicted by stellar evolutionary models. For the corresponding stellar turn-off masses from 1.9 to 3.3 M_⊙, the observed C/N ratio values are very close to the predictions of standard first dredge-up models as well as to models of thermohaline extra-mixing. They are not decreased as much as predicted by the recent model in which the thermohaline- and rotation-induced extra-mixing act together. The average [O/H] abundance ratios of NGC 4815 and NGC 6705 are compared with the predictions of two Galactic chemical evolution models. The data are consistent with the evolution at the solar radius within the errors.

Conclusions. The first results of CNO determinations in open clusters show the potential of the Gaia-ESO Survey to judge stellar and Galactic chemical evolution models and the validity of their physical assumptions through a homogeneous and detailed spectral analysis.