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Context. The Gaia-ESO Public Spectroscopic Survey will observe a large sample of clusters and cluster stars, covering a wide age-distancemetallicity-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 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⊙, 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. [ABSTRACT FROM AUTHOR]

In this work we present the main atmospheric parameters, carbon, nitrogen and oxygen abundances, and 12C/13C ratios determined in a sample of 28 Galactic clump stars. Abundances of carbon were studied using the C2 band at 5086.2 Å. The wavelength interval 7980–8130 Å with strong CN features was analysed in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O i] line at 6300 Å. The mean abundances of C, N and O abundances in the investigated clump stars support our previous estimations that, compared to the Sun and dwarf stars of the Galactic disc, carbon is depleted by about 0.2 dex, nitrogen is enhanced by 0.2 dex and oxygen is close to abundances in dwarfs. The 12C/13C and C/N ratios for galactic red clump stars analysed were compared to the evolutionary models of extra-mixing. The steeper drop of 12C/13C ratio in the model of thermohaline mixing by Charbonnel & Lagarde better reflects the observational data at low stellar masses than the more shallow model of cool bottom processing by Boothroyd & Sackmann. For stars of about 2 M⊙ masses a modelling of rotationally induced mixing should be considered with rotation of about 250 km s−1 at the time when a star was at the hydrogen-core-burning stage. [ABSTRACT FROM AUTHOR]

The Hipparcos orbiting observatory has revealed a large number of helium-core-burning 'clump' stars in the Galactic field. These low-mass stars exhibit signatures of extra mixing processes that require modelling beyond the first dredge-up of standard models. The ratio is the most robust diagnostic of deep mixing because it is insensitive to the adopted stellar parameters. In this paper we present determinations in a sample of 34 Galactic clump stars as well as abundances of nitrogen, carbon and oxygen. Abundances of carbon were studied using the Swan (0,1) band head at 5635.5 Å. The wavelength interval 7980-8130 Å with strong CN features was analysed in order to determine nitrogen abundances and isotope ratios. The oxygen abundances were determined from the [O ] line at 6300 Å. Compared with the Sun and dwarf stars of the Galactic disc, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by 0.2 dex and oxygen is close to abundances in dwarfs. Comparisons to evolutionary models show that the stars fall into two groups: one is of first ascent giants with carbon isotope ratios altered according to the first dredge-up prediction and the other is of helium-core-burning stars with carbon isotope ratios altered by extra mixing. The stars investigated fall to these groups in approximately equal numbers. [ABSTRACT FROM AUTHOR]

The main atmospheric parameters and abundances of the iron-group elements (vanadium, chromium, iron, cobalt and nickel) are determined for 62 red giant 'clump' stars revealed in the Galactic field by the Hipparcos orbiting observatory. The stars form a homogeneous sample with the mean value of temperature K, of surface gravity log and the mean value of metallicity dex. A Gaussian fit to the [Fe/H] distribution produces the mean and dispersion . The near-solar metallicity and small dispersion of of clump stars of the Galaxy obtained in this paper confirm the theoretical model of the Hipparcos clump by Girardi & Salaris. This suggests that nearby clump stars are (in the mean) relatively young objects, reflecting mainly the near-solar metallicities developed in the local disc during the last few Gyr of its history. We find iron-group element to iron-abundance ratios in clump giants to be close to solar. [ABSTRACT FROM AUTHOR]

A large population of helium-core-burning clump stars of the Galactic field, revealed by the Hipparcos orbiting observatory, is an excellent target for the analysis of mixing processes in evolved low-mass metal-abundant stars. In this contribution we overview first result of 12C, 13C, N and O abundance determinations in our extensive project which we started with the aim at deriving the fundamental parameters and abundances of more than 20 chemical elements in a large sample of Galactic clump stars. [ABSTRACT FROM PUBLISHER]

Hipparcos orbiting observatory has revealed a large number of helium-core- burning “clump” stars of the Galactic field. These low-mass stars exhibit signatures of extra-mixing processes that require modeling beyond the standard stellar theory. In this contribution we overview available results of 12C, 13C, N and O abundances obtained by high-resolution spectra for clump stars and discuss them in the light of current predictions of stellar evolution models. [ABSTRACT FROM PUBLISHER]