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

Jeffries, R. D.; Jackson, R. J.; Cottaar, M.; Koposov, S. E.; Lanzafame, A. C.; Meyer, M. R.; Prisinzano, L.; Randich, S.; Sacco, G. G.; Brugaletta, E.; Caramazza, M.; Damiani, F.; Franciosini, E.; Frasca, A.; Gilmore, G.; Feltzing, S.; Micela, G.; Alfaro, E.; Bensby, T.; Pancino, E.; Recio-Blanco, A.; de Laverny, P.; Lewis, J.; Magrini, L.; Morbidelli, L.; Costado, M. T.; Jofre, P.; Klutsch, A.; Lind, K.; Maiorca, E.
Astronomy & Astrophysics, Volume 563, id.A94, 15 pp. (2014).


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 γ2 Velorum was one of the first GES targets. 
Aims: We empirically determine the radial velocity 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 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. 
Results: The GES radial velocity precision is about 0.25 km s-1 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 ± 0.16 km s-1, consistent with virial equilibrium. The second has a broader dispersion of 1.60 ± 0.37 km s-1 and is offset from the first by ≃2 km s-1. 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 γ2 Vel. 
Conclusions: 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 (≥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 ( or via