TheGaia-ESO Survey: Kinematic structure in the Gamma Velorum cluster

Abstract

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 γ 2 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 γ 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 ± 0.16 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 γ 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 γ 2 Vel, and a dispersed population from the wider Vela OB2 association, of which γ 2 Vel is the most massive member. The apparent youth of γ 2 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.

Gaia-ESO Survey: Empirical classification of VLT/Giraffe stellar spectra in the wavelength range 6440–6810 Å in theγVelorum cluster, and calibration of spectral indices

“…Our redetermined RV values have a likely larger error (∼1-2 km s −1 ) compared to that typical of GESiDR1Final released RVs (Jeffries et al 2014), but this is not critical for the present work. In most cases, the incorrect RV values were due to SB2 nature, and are being revised in forthcoming data releases.…”

“…4.1 and 4.2 for details. Blue crosses indicate bona-fide cluster members, selected by the presence of at least two indicators among: X-ray emission (from Jeffries et al 2009), lithium EW > 150 mÅ, radial velocity (RV) in the range 10-22 km s −1 (the latter two membership indicators for γ Vel stars are studied by Jeffries et al 2014;and Prisinzano et al, in prep.). To these membership criteria we have added the presence of wide Hα emission (green circles, see Sect.…”

TheGaia-ESO survey: Discovery of a spatially extended low-mass population in the Vela OB2 association

“…Recently, the young cluster (hereafter, the Gamma Vel cluster) around γ 2 Vel has been observed by the Gaia-ESO Survey (Gilmore et al 2012;Randich & Gilmore 2013). The results of these observations, covering about one square degree around the massive binary, are discussed in Jeffries et al (2014). They found that the cluster is composed of two kinematically distinct populations, which are older than previously thought (≥10 Myr) and have an age difference of ∼1-2 Myr.…”

“…The strategy for the selection of targets for young clusters in the Gaia-ESO Survey is tailored to observe an unbiased and nearly complete sample, but can lead to significant contamination by field stars (e.g., Jeffries et al 2014). To cleanly study the kinematics of the young NGC 2547 population and search for additional young populations, we need to remove these contaminants.…”

“…Alternatively, stars may form in a wide range of environments, from the dense cores of massive clusters to sparse associations (Bressert et al 2010;Wright et al 2014). High-resolution spectroscopic surveys of SFRs and young clusters are powerful tools for investigating this topic, since stellar radial velocities (RVs) allow us to A&A 574, L7 (2015) characterize their current dynamical state, establishing whether a SFR is bound or expanding into the field (e.g., Cottaar et al 2012a;Jeffries et al 2014), and for identifying and disentangling multiple populations seen in the same line of sight (e.g., Jeffries et al 2006b;Alves & Bouy 2012).…”