Abstract

Context. Open clusters are key tools to study the spatial distribution of abundances in the disk and their evolution with time. Aims. Using the first release of stellar parameters and abundances of the Gaia-ESO Survey, we analyse the chemical properties of stars in three old/intermediate-age open clusters, namely NGC 6705, NGC 4815, and Trumpler 20, which are all located in the inner part of the Galactic disk at Galactocentric radius R-GC similar to 7 kpc. We aim to prove their homogeneity and to compare them with the field population. Methods. We study the abundance ratios of elements belonging to two different nucleosynthetic channels: alpha-elements and iron-peak elements. For each element, we analyse the internal chemical homogeneity of cluster members, and we compare the cumulative distributions of cluster abundance ratios with those of solar neighbourhood turn-off stars and of inner-disk/bulge giants. We compare the abundance ratios of field and cluster stars with two chemical evolution models that predict different alpha-enhancement dependences on the Galactocentric distance due to different assumptions on the infall and star-formation rates. Results. The main results can be summarised as follows: i) cluster members are chemically homogeneous within 3 sigma in all analysed elements; ii) the three clusters have comparable [El/Fe] patterns within similar to 1 sigma, but they differ in their global metal content [El/H] with NGC 4815 having the lowest metallicity; their [El/Fe] ratios show differences and analogies with those of the field population, in both the solar neighbourhood and the bulge/inner disk; iii) comparing the abundance ratios with the results of two chemical evolution models and with field star abundance distributions, we find that the abundance ratios of Mg, Ni, and Ca in NGC 6705 might require an inner birthplace, implying a subsequent variation in its R-GC during its lifetime, which is consistent with previous orbit determination. Conclusions. Using the results of the first internal data release, we show the potential of the Gaia-ESO Survey through a homogeneous and detailed analysis of the cluster versus field populations to reveal the chemical structure of our Galaxy using a completely uniform analysis of different populations. We verify that the Gaia-ESO Survey data are able to identify the unique chemical properties of each cluster by pinpointing the composition of the interstellar medium at the epoch and place of formation. The full dataset of the Gaia-ESO Survey is a superlative tool to constrain the chemical evolution of our Galaxy by disentangling different formation and evolution scenarios.