Repository logo

The Gaia-ESO Survey: Kinematic structure in the Gamma Velorum cluster$^{⋆,⋆⋆}$

Published version



Change log


Jeffries, RD 
Jackson, RJ 
Cottaar, M 
Koposov, SE 
Lanzafame, AC 


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.



stars: pre-main sequence, stars: kinematics and dynamics, open clusters and associations: individual: gamma2 Velorum, stars: formation

Journal Title

Astronomy & Astrophysics

Conference Name

Journal ISSN


Volume Title



EDP Sciences
Science and Technology Facilities Council (ST/H00243X/1)
Science and Technology Facilities Council (ST/J00541X/1)
Science and Technology Facilities Council (ST/L001381/1)
European Research Council (320360)
European Commission (264895)
This work was partially supported by the Gaia Research for European Astronomy Training (GREAT-ITN) Marie Curie network, funded through the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement 264895 and supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Università e della Ricerca (MIUR) in the form of the grant “Premiale VLT 2012”. RJJ acknowledges financial support from the UK Science and Technology Facilities Council.