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

¹ Department of Astronomy, Indiana University, Bloomington, IN, USA
² Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
⁴ Massachusetts Institute of Technology, Kavli Institute of Astrophysics & Space Research, Cambridge MA, USA
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ Dipartimento di Fisica e Astronomia, Università di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
⁸ INAF – Osservatorio Astronomico di Padova, vicolo Osservatorio 5, 35122 Padova, Italy
⁹ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
¹⁰ European Southern Observatory, Karl-Schwarschild-Str 2, 85748 Garching bei München, Germany
¹¹ Institute of Theoretical Physics and Astronomy, Vilnius University, A. Gostauto 12, 01108 Vilnius, Lithuania
¹² Universidad Complutense de Madrid, Plaza de la Ciencia 3, 28040 Madrid, Spain
¹³ Centro de Astrobiología (INTA-CSIC), Departamento de Astrofísica, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁴ Suffolk University, Madrid Campus, C/ Valle de la Via 3, 28003 Madrid, Spain
¹⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁶ Astrophysics Group, Research Institute for the Environment, Physical Sciences and Applied Mathematics, Keele University, Keele, Staffordshire ST5 5BG, UK
¹⁷ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
¹⁸ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
¹⁹ Moscow M.V. Lomonosov State University, Sternberg Astronomical Institute, Universitetskij pr. 13, 119992 Moscow, Russia
²⁰ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
²¹ ASI Science Data Center, Via del Politecnico SNC, 00133 Roma, Italy
²² Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France

Received: 7 December 2013
Accepted: 9 February 2014

Abstract

Context. NGC 4815 is a populous ~500 Myr open cluster at R_gc ~ 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.