The Gaia-ESO Survey: Kinematic structure in the Gamma Velorum Cluster

R. D. Jeffries, R. J. Jackson, M. Cottaar, S. E. Koposov, A. C. Lanzafame, M. R. Meyer, L. Prisinzano, S. Randich, G. G. Sacco, E. Brugaletta, M. Caramazza, F. Damiani, E. Franciosini, A. Frasca, G. Gilmore, S. Feltzing, G. Micela, E. Alfaro, T. Bensby, E. Pancino, A. Recio-Blanco, P. de Laverny, J. Lewis, L. Magrini, L. Morbidelli, M. T. Costado, P. Jofre, A. Klutsch, K. Lind, E. Maiorca

Published 2014-01-20Version 1

Context: A key science goal of the Gaia-ESO survey (GES) is to use the kinematics of low-mass stars in young clusters to probe their dynamical histories and how they populate the field as they become unbound. The clustering of low-mass stars around the massive W-R binary gamma^2 Velorum was one of the first GES targets. Aims: To empirically determine the radial velocity (RV) precision of GES data, construct a kinematically unbiased sample of cluster members and characterise their dynamical state. Methods: Targets were selected from colour-magnitude diagrams and intermediate resolution spectroscopy used to derive RVs and assess membership from the strength of the Li6708A line. The RV distribution was analysed using a maximum likelihood technique that accounts for unresolved binaries. Results: The GES RV precision is about 0.25km/s and sufficient to resolve velocity structure in the low-mass population around gamma^2 Vel. The structure is well fitted by two kinematic components with roughly equal numbers of stars; the first has an intrinsic dispersion of 0.34+/-0.16km/s, consistent with virial equilibrium. The second has a broader dispersion of 1.60+/-0.37km/s and is offset from the first by ~2km/s. The first population is older by 1-2Myr based on a greater level of Li depletion seen among its M-stars and is probably more centrally concentrated around gamma^2 Vel. Conclusions: We consider several formation scenarios, concluding that the two kinematic components are a bound remnant of the original, denser cluster that formed gamma^2 Vel, and a dispersed population from the wider Vela OB2 association, of which gamma^2 Vel is the most massive member. The apparent youth of gamma^2 Vel compared to the older (>=10Myr) low-mass population surrounding it suggests a scenario where the massive binary formed in a clustered environment after the formation of the bulk of the low-mass stars.[abridged]