The Gaia-ESO Survey: Structural and dynamical properties of the young cluster Chamaeleon I^{⋆,⋆⋆,⋆⋆⋆}

¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
e-mail: gsacco@arcetri.inaf.it
² Universidade de São Paulo, IAG, Departamento de Astronomia, Rua do Mãtao 1226, São Paulo, 05509-900 SP, Brazil
³ Department of Physics and Astronomy, University of Sheffield, The Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK
⁴ Astrophysics Group, Research Institute for the Environment, Physical Sciences and Applied Mathematics, Keele University, Keele, Staffordshire ST5 5BG, UK
⁵ Institute of Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁶ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Universitá di Catania, via S. Sofia 78, 95123 Catania, Italy
⁸ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁹ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁰ Dipartimento di Fisica Universitá di Pisa ed INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127, Pisa, Italy
¹¹ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
¹² ESA, ESTEC, Keplerlaan 1, PO Box 299 2200 AG Noordwijk, The Netherlands
¹³ Depto. de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC campus, 28691, Villanueva de la Cañada, Madrid, Spain
¹⁴ Universitá degli Studi di Firenze, Dipartimento di Fisica e Astrofisica, Sezione di Astronomia, Largo E. Fermi, 2, 50125 Firenze, Italy
¹⁵ Dpto. de Astrofísica y Cencias de la Atmósfera, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁶ Instituto de Física y Astronomiía, Universidad de Valparaiíso, Chile
¹⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁸ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁹ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
²⁰ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
²¹ Università di Bologna, Dipartimento di Fisica e Astronomia, viale Berti Pichat 6/2, 40127 Bologna, Italy
²² Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
²³ Departamento de Ciencias Fisicas, Universidad Andres Bello, Republica 220, Santiago, Chile
²⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal

Received: 12 September 2016
Accepted: 13 January 2017

Abstract

Investigating the physical mechanisms driving the dynamical evolution of young star clusters is fundamental to our understanding of the star formation process and the properties of the Galactic field stars. The young (~2 Myr) and partially embedded cluster Chamaeleon I is one of the closest laboratories for the study of the early stages of star cluster dynamics in a low-density environment. The aim of this work is to study the structural and kinematical properties of this cluster combining parameters from the high-resolution spectroscopic observations of the Gaia-ESO Survey with data from the literature. Our main result is the evidence of a large discrepancy between the velocity dispersion (σ_stars = 1.14 ± 0.35 km s^-1) of the stellar population and the dispersion of the pre-stellar cores (~0.3 km s^-1) derived from submillimeter observations. The origin of this discrepancy, which has been observed in other young star clusters, is not clear. It has been suggested that it may be due to either the effect of the magnetic field on the protostars and the filaments or to the dynamical evolution of stars driven by two-body interactions. Furthermore, the analysis of the kinematic properties of the stellar population puts in evidence a significant velocity shift (~1 km s^-1) between the two subclusters located around the north and south main clouds of the cluster. This result further supports a scenario where clusters form from the evolution of multiple substructures rather than from a monolithic collapse. Using three independent spectroscopic indicators (the gravity indicator γ, the equivalent width of the Li line at 6708 Å, and the Hα 10% width), we performed a new membership selection. We found six new cluster members all located in the outer region of the cluster, proving that Chamaeleon I is probably more extended than previously thought. Starting from the positions and masses of the cluster members, we derived the level of substructure Q, the surface density Σ, and the level of mass segregation Λ_MSR of the cluster. The comparison between these structural properties and the results of N-body simulations suggests that the cluster formed in a low-density environment, in virial equilibrium or a supervirial state, and highly substructured.