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Thomas Bensby. Profile photo.

Thomas Bensby

Senior lecturer

Thomas Bensby. Profile photo.

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


  • G. Tautvaisiene
  • A. Drazdauskas
  • S. Mikolaitis
  • G. Barisevicius
  • E. Puzeras
  • E. Stonkute
  • Y. Chorniy
  • L. Magrini
  • D. Romano
  • R. Smiljanic
  • A. Bragaglia
  • G. Carraro
  • E. Friel
  • T. Morel
  • E. Pancino
  • P. Donati
  • F. Jimenez-Esteban
  • G. Gilmore
  • S. Randich
  • R. D. Jeffries
  • A. Vallenari
  • Thomas Bensby
  • E. Flaccomio
  • A. Recio-Blanco
  • M. T. Costado
  • V. Hill
  • P. Jofre
  • C. Lardo
  • P. de laverny
  • T. Masseron
  • L. Moribelli
  • S. G. Sousa
  • S. Zaggia

Summary, in English

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 C-2 band heads at 5135 and 5635.5 angstrom. The wavelength interval 6470-6490 angstrom, with CN features, was analysed to determine nitrogen abundances. Oxygen abundances were determined from the [O I] line at 6300 angstrom. 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 NGC4815: [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 NGC6705: [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-circle dot, 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 NGC4815 and NGC6705 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.


  • Lund Observatory - Has been reorganised

Publishing year





Astronomy & Astrophysics



Document type

Journal article


EDP Sciences


  • Astronomy, Astrophysics and Cosmology


  • stars: abundances
  • stars: evolution
  • Galaxy: evolution
  • open clusters
  • and associations: individual: Trumpler 20
  • open clusters and
  • associations: individual: NGC4815
  • open clusters and associations:
  • individual: NGC6705




  • ISSN: 0004-6361