The Gaia-ESO Survey Kinematic structure in the Gamma Velorum cluster

Context. 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 gamma<SUP>2</SUP> Velorum was one of the first GES targets. <BR /> Aims We empirically determine the radial velocity precision of GES data, construct a kinematically unbiased sample of cluster members and characterise their dynamical state. <BR /> Methods 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 line. The radial velocity distribution was analysed using a maximum likelihood technique that accounts for unresolved binaries. <BR /> Results The GES radial velocity precision is about 0.25 km s<SUP>-1</SUP> and sufficient to resolve velocity structure in the low-mass population around gamma<SUP>2</SUP> 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 0.16 km s<SUP>-1</SUP>, consistent with virial equilibrium. The second has a broader dispersion of 1.60 0.37 km s<SUP>-1</SUP> and is offset from the first by ~=2 km s<SUP>-1</SUP>. The first population is older by 1-2 Myr based on a greater level of Li depletion seen among its M-type stars and is probably more centrally concentrated around gamma<SUP>2</SUP> Vel. <BR /> Conclusions We consider several formation scenarios, concluding that the two kinematic components are a bound remnant of the original, denser cluster that formed gamma<SUP>2</SUP> Vel, and a dispersed population from the wider Vela OB2 association, of which gamma<SUP>2</SUP> Vel is the most massive member. The apparent youth of gamma<SUP>2</SUP> Vel compared to the older (>=10 Myr) 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. Based on observations collected with the FLAMES spectrograph at VLT/UT2 telescope (Paranal Observatory, ESO, Chile), for the Gaia- ESO Large Public Survey (188.B-3002).Full Table 2 is only available at the CDS via anonymous ftp to <A href="http //cdsarc.u-strasbg.fr">http //cdsarc.u-strasbg.fr</A> (ftp //130.79.128.5) or via <A href="http //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/563/A94">http //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/563/A94</A>