The Gaia-ESO Survey: Properties of the intermediate age open cluster NGC
  4815
  NGC 4815 is a populous ~500 Myr open cluster at a Galactocentric radius of 7
kpc observed in the first six months of the Gaia-ESO Survey. Located in the
inner Galactic disk, NGC 4815 is an important tracer of the abundance gradient,
where few intermediate age open clusters are found. We use the survey derived
radial velocities, stellar atmospheric parameters, metallicity, and elemental
abundances for stars targeted as potential members of this cluster to carry out
an analysis of cluster properties. The radial velocity distribution of stars in
the cluster field is used to define the cluster systemic velocity and derive
likely cluster membership for stars observed by the Gaia-ESO Survey. We
investigate the distributions of Fe and Fe-peak elements, alpha-elements, and
the light elements Na and Al and characterize the cluster's internal chemical
homogeneity comparing it to the properties of radial velocity non-member stars.
Utilizing these cluster properties, the cluster color-magnitude diagram is
analyzed and theoretical isochrones are fit to derive cluster reddening,
distance, and age. NGC 4815 is found to have a mean [Fe/H]=+0.03. Elemental
abundances of cluster members show typically small internal variation, with
internal 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. As
with many open clusters, [Na/Fe] and [Al/Fe] are enhanced, [Na/Fe]
significantly so, although the role of internal mixing and the assumption of
LTE in the analysis remain to be investigated. From isochrone fits to
color-magnitude diagrams, we find a cluster age of 0.5 to 0.63 Gyr, a reddening
of 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 Galactocentric
distance of 6.9 kpc.
