The Gaia-ESO Survey: Abundance ratios in the inner-disk open clusters Trumpler 20, NGC 4815, NGC 6705^⋆

¹ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: laura@arcetri.astro.it
² INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Department of Astronomy, Indiana University, Bloomington, USA
⁴ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁶ MIT Kavli Institute, Boston, USA
⁷ Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Dipartimento di Fisica, sezione di Astronomia, Largo E. Fermi 2, 50125 Firenze, Italy
⁹ Dipartimento di Fisica e Astronomia, via Ranzani 1, 40127 Bologna, Italy
¹⁰ Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹² Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹³ Institute of Theoretical Physics and Astronomy, Vilnius University, A. Gostauto 12, 01108 Vilnius, Lithuania
¹⁴ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
¹⁵ CNRS, LAB, UMR 5804, 33270 Floirac, France
¹⁶ Centro de Astrobiología (INTA-CSIC) PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
¹⁷ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
¹⁸ Astronomy Department, University of Geneva, Ch. des Maillettes 51, 1290 Versoix, Switzerland
¹⁹ Research School of Astronomy and Astrophysics, Australian National University, ACT, 2611 Canberra, Australia
²⁰ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
²¹ Astrophysics Group, Keele University, Keele ST5 5BG, UK
²² Moscow M.V. Lomonosov State University, Sternberg Astronomical Institute, Universitetskij pr., 13, 119992 Moscow, Russia
²³ ASI Science Data Center, 00044 Frascati, Italy
²⁴ Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
²⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, 18008 Granada, Spain
²⁶ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

Received: 4 November 2013
Accepted: 20 December 2013

Abstract

Context. Open clusters are key tools to study the spatialdistribution 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 ~ 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: α-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 α-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 3σ in all analysed elements; ii) the three clusters have comparable [El/Fe] patterns within ~1σ, 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.