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NGC 4815 is a populous ~500 Myr open cluster at a Galactocentric radius of 7kpc observed in the first six months of the Gaia-ESO Survey. Located in theinner Galactic disk, NGC 4815 is an important tracer of the abundance gradient,where few intermediate age open clusters are found. We use the survey derivedradial velocities, stellar atmospheric parameters, metallicity, and elementalabundances for stars targeted as potential members of this cluster to carry outan analysis of cluster properties. The radial velocity distribution of stars inthe cluster field is used to define the cluster systemic velocity and derivelikely cluster membership for stars observed by the Gaia-ESO Survey. Weinvestigate the distributions of Fe and Fe-peak elements, alpha-elements, andthe light elements Na and Al and characterize the cluster's internal chemicalhomogeneity comparing it to the properties of radial velocity non-member stars.Utilizing these cluster properties, the cluster color-magnitude diagram isanalyzed and theoretical isochrones are fit to derive cluster reddening,distance, and age. NGC 4815 is found to have a mean [Fe/H]=+0.03. Elementalabundances of cluster members show typically small internal variation, withinternal dispersions of ~0.05 dex. [Ca/Fe] and [Si/Fe] show solar ratios, but[Mg/Fe] is moderately enhanced, while [Ti/Fe] appears slightly deficient. Aswith many open clusters, [Na/Fe] and [Al/Fe] are enhanced, [Na/Fe]significantly so, although the role of internal mixing and the assumption ofLTE in the analysis remain to be investigated. From isochrone fits tocolor-magnitude diagrams, we find a cluster age of 0.5 to 0.63 Gyr, a reddeningof E(B-V) = 0.59 to 0.65, and a true distance modulus (m-M) = 11.95 to 12.20,depending on the choice of theoretical models, leading to a Galactocentricdistance of 6.9 kpc.

The Gaia-ESO Survey: Properties of the intermediate age open cluster NGC 4815