The Gaia-ESO Survey: Structural and dynamical properties of the young cluster Chamaeleon i

Abstract

Investigating the physical mechanisms driving the dynamic al evolution of young star clusters is fundamental to our und erstanding of the star formation process and the properties of the Galac tic field stars. The young ( ∼ 2 Myr) and partially embedded cluster Chamaeleon I is one of the closest laboratories to study the e arly stages of star cluster dynamics in a low-density enviro nment. The aim of this work is to study the structural and kinematical pr operties of this cluster combining parameters from the high -resolution spectroscopic observations of the Gaia-ESO Survey with dat a from the literature. Our main result is the evidence of a lar ge discrepancy between the velocity dispersion ( σ stars = 1 . 14 ± 0 . 35 km s − 1 ) of the stellar population and the dispersion of the pre-ste llar cores ( ∼ 0 . 3 km s − 1 ) derived from submillimeter observations. The origin of th is 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 e ff ect of the magnetic field on the protostars and the filaments, or to the dynamical evolution of stars driven by two-body int eractions. Furthermore, the analysis of the kinematic prop erties of the stellar population put in evidence a significant velocity sh ift ( ∼ 1 km s − 1 ) between the two sub-clusters located around the North and South main clouds of the cluster. This result further sup ports a scenario, where clusters form from the evolution of m ultiple substructures rather than from a monolithic collapse. Using three independent spectroscopic indicators (the gra vity indicator γ , the equivalent width of the Li line at 6708 Å, and the H α 10% width), we performed a new membership selection. We foun d six new cluster members all located in the outer region of th e cluster, proving that Chamaeleon I is probably more extende d than previously thought. Starting from the positions and m asses of the cluster members, we derived the level of substructure Q , the surface density Σ and the level of mass segregation Λ MS R of the cluster. The comparison between these structural properties and the results of N-body simulations suggests that the cluster for med in a low density environment, in virial equilibrium or supervirial , and highly substructured.