The Gaia-ESO Survey: open clusters in Gaia-DR1

A way forward to stellar age calibration^★,★★

¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: randich@arcetri.astro.it
² Department of Physics “E. Fermi”, University of Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
³ INFN, Section of Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
⁴ Astrophysics Group, University of Keele, Keele, Staffordshire ST5 5BG, UK
⁵ INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁶ INAF–Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁷ Instituto de Astrofìsica de Andalucìa (IAA-CSIC), Glorieta de la Astronomìa, 18008 Granada, Spain
⁸ Laboratoire d’Astrophysique de Bordeaux, Université Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
¹⁰ Centro de Astrobiología (INTA-CSIC), Departamento de Astrofísica, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹¹ Departament de Fìsìca Quantìca i Astrofìsica, Institut Cìencies Cosmos (ICCUB), Universitat de Barcelona, 08007 Barcelona, Spain
¹² Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
¹³ Institute of Theoretical Physics and Astronomy, Vilnius University, Saulėtekio al. 3, 10222 Vilnius, Lithuania
¹⁴ Institute of Astronomy, Madingley Road, University of Cambridge, CB3 0HA, UK
¹⁵ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00, Lund Sweden
¹⁶ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁷ Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 city, Sweden
¹⁸ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy
¹⁹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
²⁰ Instituto de Física y Astronomiía, Universidad de Valparíso, Chile
²¹ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²² Núcleo de Astronomá, Universidad Diego Portales, Av. Ejéxrcito 441, Santiago, Chile
²³ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
²⁴ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
²⁵ European Southern Observatory, Alonso de Cordova, 3107 Vitacura, Santiago de Chile, Chile
²⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal

Received: 8 August 2017
Accepted: 4 November 2017

Abstract

Context. Determination and calibration of the ages of stars, which heavily rely on stellar evolutionary models, are very challenging, while representing a crucial aspect in many astrophysical areas.

Aims. We describe the methodologies that, taking advantage of Gaia-DR1 and the Gaia-ESO Survey data, enable the comparison of observed open star cluster sequences with stellar evolutionary models. The final, long-term goal is the exploitation of open clusters as age calibrators.

Methods. We perform a homogeneous analysis of eight open clusters using the Gaia-DR1 TGAS catalogue for bright members and information from the Gaia-ESO Survey for fainter stars. Cluster membership probabilities for the Gaia-ESO Survey targets are derived based on several spectroscopic tracers. The Gaia-ESO Survey also provides the cluster chemical composition. We obtain cluster parallaxes using two methods. The first one relies on the astrometric selection of a sample of bona fide members, while the other one fits the parallax distribution of a larger sample of TGAS sources. Ages and reddening values are recovered through a Bayesian analysis using the 2MASS magnitudes and three sets of standard models. Lithium depletion boundary (LDB) ages are also determined using literature observations and the same models employed for the Bayesian analysis.

Results. For all but one cluster, parallaxes derived by us agree with those presented in Gaia Collaboration (2017, A&A, 601, A19), while a discrepancy is found for NGC 2516; we provide evidence supporting our own determination. Inferred cluster ages are robust against models and are generally consistent with literature values.

Conclusions. The systematic parallax errors inherent in the Gaia DR1 data presently limit the precision of our results. Nevertheless, we have been able to place these eight clusters onto the same age scale for the first time, with good agreement between isochronal and LDB ages where there is overlap. Our approach appears promising and demonstrates the potential of combining Gaia and ground-based spectroscopic datasets.