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 $\varpi_A = 2.895 \pm 0.008$ mas and $\varpi_B = 2.608
\pm 0.017$ mas, with intrinsic dispersions of $0.038 \pm 0.011$ mas and $0.091
\pm 0.016$ mas, respectively. This translates into distances of
$345.4^{+1.0+12.4}_{-1.0-11.5}\,$ 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 $\sim$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.
