The GALAH Survey and Gaia DR2: (Non)existence of five sparse high-latitude open clusters

TL;DR

This study combines GALAH survey data with Gaia DR2 to evaluate the existence of five high-latitude open clusters, revealing four are not real clusters and confirming NGC 1901 as a genuine sparse cluster.

Contribution

It demonstrates the importance of multi-dimensional analysis in confirming the reality of sparse open clusters using combined spectral and astrometric data.

Findings

Four purported clusters are chance projections, not real clusters.

A 6D phase space analysis is essential for reliable cluster membership identification.

NGC 1901 is confirmed as a genuine sparse high-latitude cluster.

Abstract

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.