2014A&A...563A..94J

A key science goal of the Gaia-ESO survey (GES) at the VLT is to use the kinematics of low-mass stars in young clusters and star forming regions to probe their dynamical histories and how they populate the field as they become unbound. The clustering of low-mass stars around the massive Wolf-Rayet binary system γ² Velorum was one of the first GES targets. We empirically determine the radial velocity precision of GES data, construct a kinematically unbiased sample of cluster members and characterise their dynamical state. Targets were selected from colour-magnitude diagrams and intermediate resolution spectroscopy was used to derive radial velocities and assess membership from the strength of the Li i 6708 Å line. The radial velocity distribution was analysed using a maximum likelihood technique that accounts for unresolved binaries. The GES radial velocity precision is about 0.25km/s and sufficient to resolve velocity structure in the low-mass population around γ² 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-type stars and is probably more centrally concentrated around γ² Vel. We consider several formation scenarios, concluding that the two kinematic components are a bound remnant of the original, denser cluster that formed γ² Vel, and a dispersed population from the wider Vela OB2 association, of which γ² Vel is the most massive member. The apparent youth of γ² Vel compared to the older (≥10Myr) low-mass population surrounding it suggests a scenario in which the massive binary formed in a clustered environment after the formation of the bulk of the low-mass stars.