The Gaia-ESO Survey: Asymmetric expansion of the Lagoon Nebula cluster
  NGC 6530 from GES and Gaia DR2
  The combination of precise radial velocities from multi-object spectroscopy
and highly accurate proper motions from Gaia DR2 opens up the possibility for
detailed 3D kinematic studies of young star forming regions and clusters. Here,
we perform such an analysis by combining Gaia-ESO Survey spectroscopy with Gaia
astrometry for ~900 members of the Lagoon Nebula cluster, NGC 6530. We measure
the 3D velocity dispersion of the region to be
$5.35^{+0.39}_{-0.34}$~km~s$^{-1}$, which is large enough to suggest the region
is gravitationally unbound. The velocity ellipsoid is anisotropic, implying
that the region is not sufficiently dynamically evolved to achieve isotropy,
though the central part of NGC 6530 does exhibit velocity isotropy that
suggests sufficient mixing has occurred in this denser part. We find strong
evidence that the stellar population is expanding, though this is
preferentially occurring in the declination direction and there is very little
evidence for expansion in the right ascension direction. This argues against a
simple radial expansion pattern, as predicted by models of residual gas
expulsion. We discuss these findings in the context of cluster formation,
evolution and disruption theories.
