The Gaia-ESO Survey: a kinematical and dynamical study of four young open clusters

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
e-mail: bravi@arcetri.astro.it
² INAF – Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
³ Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁴ Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK
⁵ Université Grenoble Alpes, IPAG, CS 40700, 38085 Grenoble Cedex 9, France
⁶ CNRS, IPAG, 38000 Grenoble, France
⁷ Department of Signal Theory and Communications, Universidad Carlos III de Madrid 28911 Leganés, Madrid, Spain
⁸ ASI Space Science Data Center, via del Politecnico snc, 00133 Roma
⁹ Universidade de São Paulo, IAG, Departamento de Astronomia, Rua do Matão 1226, São Paulo, 05509-900 SP, Brasil
¹⁰ Departmento de Astrofísica, Centro de Astrobiología (INTA-CSIC), ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹¹ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, via Gobetti 93/3, 40129 Bologna, Italy
¹⁴ INAF – Osservatorio Astronomico di Palermo G. S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁵ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹⁶ McWilliams Center for Cosmology, Department of Physics, Carnegie Mellon University, 5000 Forbes avenue, Pittsburgh, PA 15213, USA
¹⁷ Instituto de Física y Astronomía, Universidad de Valparaíso, Chile
¹⁸ Millennium Nucleus “Núcleo Planet Formation”, Universidad de Valparaíso, Chile
¹⁹ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²⁰ Departamento de Didáctica, Universidad de Cádiz, 11519 Puerto Real, Cádiz, Spain
²¹ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
²² Laboratoire d’astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
²³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²⁴ INAF – Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 15 January 2018
Accepted: 3 March 2018

Abstract

Context. The origin and dynamical evolution of star clusters is an important topic in stellar astrophysics. Several models have been proposed in order to understand the formation of bound and unbound clusters and their evolution, and they can be tested by examining the kinematical and dynamical properties of clusters over a wide range of ages and masses.

Aims. We use the Gaia-ESO Survey products to study four open clusters (IC 2602, IC 2391, IC 4665, and NGC 2547) that lie in the age range between 20 and 50 Myr.

Methods. We employ the gravity index γ and the equivalent width of the lithium line at 6708 Å together with effective temperature T_eff and the metallicity of the stars in order to discard observed contaminant stars. Then we derive the cluster radial velocity dispersions σ_c, the total cluster mass M_tot, and the half mass radius r_hm. Using the Gaia-DR1 TGAS catalogue, we independently derive the intrinsic velocity dispersion of the clusters from the astrometric parameters of cluster members.

Results. The intrinsic radial velocity dispersions derived by the spectroscopic data are higher than those derived from the TGAS data, possibly due to the different masses of the considered stars. Using M_tot and r_hm we derive the virial velocity dispersion σ_vir and we find that three out of four clusters are supervirial. This result is in agreement with the hypothesis that these clusters are dispersing, as predicted by the “residual gas expulsion” scenario. However, recent simulations show that the virial ratio of young star clustersmay be overestimated if it is determined using the global velocity dispersion, since the clusters are not fully relaxed.