Structure of Open Clusters -- Gaia DR2 and its limitations
  Very precise observational data are needed for studying the stellar cluster
parameters (distance, reddening, age, metallicity) and cluster internal
kinematics. In turn, these give us an insight into the properties of our
Galaxy, for example, by giving us the ability to trace Galactic spiral
structure, star formation rates and metallicity gradients. We investigated the
available Gaia DR2 catalogue of 1229 open clusters and studied cluster
distances, sizes and membership distributions in the 3D space. An appropriate
analysis of the parallaxto-distance transformation problem is presented in the
context of getting distances toward open clusters and estimating their sizes.
Based on our investigation of the Gaia DR2 data we argue that, within 2 kpc,
the inverse-parallax method gives comparable results (distances and sizes) as
the Bayesian approach based on the exponentially decreasing volume density
prior. Both of these methods show very similar dependence of the line-of-sight
elongation of clusters (needle-like shapes resulting from the parallax
uncertainties) on the distance. We also looked at a measure of elongations of
the studied clusters and find the maximum distance of 665 pc at which a
spherical fit still contains about half of the stellar population of a cluster.
It follows from these results that the 3D structure of an open cluster cannot
be properly studied beyond about 500 pc when using any of mentioned standard
transformations of parallaxes to distances.
