Gaia-ESO Survey: Tr 20, NGC4815, NGC6705

Oxygen abundances were determined from the [OI] line at 6300{AA}. 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 3.3M_{sun}_, 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.

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.