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

Friel, E. D.; Donati, P.; Bragaglia, A.; Jacobson, H. R.; Magrini, L.; Prisinzano, L.; Randich, S.; Tosi, M.; Cantat-Gaudin, T.; Vallenari, A.; Smiljanic, R.; Carraro, G.; Sordo, R.; Maiorca, E.; Tautvaišienė, G.; Sestito, P.; Zaggia, S.; Jiménez-Esteban, F. M.; Gilmore, G.; Jeffries, R. D.; Alfaro, E.; Bensby, T.; Koposov, S. E.; Korn, A. J.; Pancino, E.; Recio-Blanco, A.; Franciosini, E.; Hill, V.; Jackson, R. J.; de Laverny, P.; Morbidelli, L.; Sacco, G. G.; Worley, C. C.; Hourihane, A.; Costado, M. T.; Jofré, P.; Lind, K.
Astronomy & Astrophysics, Volume 563, id.A117, 10 pp. (2014).


Context. NGC 4815 is a populous ~500 Myr open cluster at Rgc ~ 7 kpc observed in the first six months of the Gaia-ESO Survey. Located in the inner Galactic disk, NGC 4815 is an important potential tracer of the abundance gradient, where relatively few intermediate age open clusters are found. 
Aims: The Gaia-ESO Survey data can provide an improved characterization of the cluster properties, such as age, distance, reddening, and abundance profile. 
Methods: 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. 
Results: NGC 4815 is found to have a mean metallicity of [Fe/H] = +0.03 ± 0.05 dex (s.d.). Elemental abundances of cluster members show typically very small internal variation, with internal dispersions of ~0.05 dex. The alpha-elements [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, the light elements [Na/Fe] and [Al/Fe] are enhanced, [Na/Fe] significantly so, although the role of internal mixing and the assumption of local thermodynamical equilibrium 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 distance modulus (m - M)0 = 11.95 to 12.20, depending on the choice of theoretical models, leading to a Galactocentric distance of 6.9 kpc.

Based on observations made with the ESO/VLT, at Paranal Observatory, under program 188.B-3002 (The Gaia-ESO Public Spectroscopic Survey).