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

Investigating the physical mechanisms driving the dynamical evolution ofyoung star clusters is fundamental to our understanding of the starformation process and the properties of the Galactic field stars. Theyoung (similar to 2 Myr) and partially embedded cluster Chamaeleon I isone of the closest laboratories for the study of the early stages ofstar cluster dynamics in a low-density environment. The aim of this workis to study the structural and kinematical properties of this clustercombining parameters from the high-resolution spectroscopic observationsof the Gaia-ESO Survey with data from the literature. Our main result isthe evidence of a large discrepancy between the velocity dispersion(sigma(stars) = 1.14 +/- 0.35 km s(-1)) of the stellar population andthe dispersion of the pre-stellar cores (similar to 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. Ithas been suggested that it may be due to either the e ff ect of themagnetic field on the protostars and the filaments or to the dynamicalevolution of stars driven by two-body interactions. Furthermore, theanalysis of the kinematic properties of the stellar population puts inevidence a significant velocity shift (similar to 1 km s(-1)) betweenthe two subclusters located around the north and south main clouds ofthe cluster. This result further supports a scenario where clusters formfrom the evolution of multiple substructures rather than from amonolithic collapse. Using three independent spectroscopic indicators(the gravity indicator gamma, the equivalent width of the Li line at6708 angstrom, and the H alpha 10\% width), we performed a newmembership selection. We found six new cluster members all located inthe outer region of the cluster, proving that Chamaeleon I is probablymore extended than previously thought. Starting from the positions andmasses of the cluster members, we derived the level of substructure Q,the surface density Sigma, and the level of mass segregation Lambda(MSR)of the cluster. The comparison between these structural properties andthe results of N-body simulations suggests that the cluster formed in alow-density environment, in virial equilibrium or a supervirial state,and highly substructured.