The GALAH Survey and Gaia DR2: (Non)existence of five sparse
  high-latitude open clusters
  Sparse open clusters can be found at high galactic latitudes where loosely
populated clusters are more easily detected against the lower stellar
background. Because most star formation takes place in the thin disk, the
observed population of clusters far from the Galactic plane is hard to explain.
We combined spectral parameters from the GALAH survey with the Gaia DR2
catalogue to study the dynamics and chemistry of five old sparse high-latitude
clusters in more detail. We find that four of them (NGC 1252, NGC 6994, NGC
7772, NGC 7826) - originally classified in 1888 - are not clusters but are
instead chance projections on the sky. Member stars quoted in the literature
for these four clusters are unrelated in our multi-dimensional physical
parameter space; the published cluster properties are therefore irrelevant. We
confirm the existence of morphologically similar NGC 1901 for which we provide
a probabilistic membership analysis. An overdensity in three spatial dimensions
proves to be enough to reliably detect sparse clusters, but the whole
6-dimensional space must be used to identify members with high confidence, as
demonstrated in the case of NGC 1901.
