Clusters and mirages: cataloguing stellar aggregates in the Milky Way

TL;DR

This study updates the Gaia DR2 cluster census by confirming the existence of many stellar groups, identifying new members, and critically assessing the reality of previously catalogued clusters to improve our understanding of the Milky Way's structure.

Contribution

The paper provides new membership lists for 150 clusters, including 10 previously known, and evaluates the physical reality of numerous catalogued stellar groups using Gaia data.

Findings

Confirmed 150 clusters with Gaia DR2 data.

Compiled a final list of 1481 cluster members.

Many catalogued clusters are likely non-physical groupings.

Abstract

Many open clusters listed in modern catalogues were initially reported by visual astronomers as apparent overdensities of bright stars. As observational techniques and analysis methods continue to improve, some of them have been shown to be chance alignments of stars and not true clusters. Recent publications have provided membership lists for over a thousand clusters, however, many nearby objects listed in the literature have so far evaded detection. We aim to update the Gaia DR2 cluster census by performing membership determinations for known clusters that had been missed by previous studies and for recently-discovered clusters. We investigate a sub-set of non-detected clusters that, according to their literature parameters, should be easily visible in Gaia. Confirming or disproving the existence of old, inner-disc, high-altitude clusters is especially important as their survival or disruption is linked to the dynamical processes that drive the evolution of the Milky Way. We employed the Gaia DR2 catalogue and a membership assignment procedure, as well as visual inspections of spatial, proper motion, and parallax distributions. We used membership lists provided by other authors when available. We derived membership lists for 150 objects, including ten that were already known prior to Gaia. We compiled a final list of members for 1481 clusters. Among the objects that we are still unable to identify with the Gaia data, we argue that many (mostly putatively old, relatively nearby, high-altitude objects) are not true clusters. At present, the only confirmed cluster located further than 500pc away from the Galactic plane within the Solar circle is NGC 6791. It is likely that the objects discussed in this study only represent a fraction of the non-physical groupings erroneously listed in the catalogues as genuine open clusters and that those lists need further cleaning.