The Gaia DR2 view of the Gamma Velorum cluster: resolving the 6D structure

Gaia-ESO Survey observations of the young Gamma Velorum cluster led to the discovery of two kinematically distinct populations, Gamma Vel A and B, respectively, with population B extended over several square degrees in the Vela OB2 association. Using the Gaia DR2 data for a sample of high-probability cluster members, we find that the two populations differ not only kinematically, but are also located at different distances along the line of sight, with the main cluster Gamma Vel A being closer. A combined fit of the two populations yields ϖ_A = 2.895 ± 0.008 mas and ϖ_B = 2.608 ± 0.017 mas, with intrinsic dispersions of 0.038 ± 0.011 mas and 0.091 ± 0.016 mas, respectively. This translates into distances of 345.4+1.0+12.-1.0-11.5 pc and 383.4+2.5+15.3-2.5-14.2 pc, respectively, showing that Gamma Vel A is closer than Gamma Vel B by 38 pc. We find that the two clusters are nearly coeval, and that Gamma Vel B is expanding. We suggest that Gamma Vel A and B are two independent clusters located along the same line of sight.