Aggregates of clusters in the Gaia data

TL;DR

This paper investigates the phenomenon of cluster aggregates in Gaia data, comparing their parallax distributions with literature values, and highlights the need for refined analysis considering multiple star membership probabilities.

Contribution

It introduces a method to analyze cluster aggregates in Gaia data and emphasizes the importance of accounting for multiple star memberships in parallax analysis.

Findings

Many clusters' parallaxes match literature values

Clusters sharing many members show discrepancies in parallax distributions

Ignoring multiple membership probabilities affects cluster parallax accuracy

Abstract

The precision of the parallax measurements by Gaia is unprecedented. As of Gaia Data Release 2, the number of known nearby open clusters has increased. Some of the clusters appear to be relatively close to each other and form aggregates, which makes them interesting objects to study. We study the aggregates of clusters which share several of the assigned member stars in relatively narrow volumes of the phase space. Using the most recent list of open clusters, we compare the cited central parallaxes with the histograms of parallax distributions of cluster aggregates. The aggregates were chosen based on the member stars which are shared by multiple clusters. Many of the clusters in the aggregates have been assigned parallaxes which coincide with the histograms. However, clusters that share a large number of members in a small volume of the phase space display parallax distributions which do not coincide with the values from the literature. This is the result of ignoring a possibility of assigning multiple probabilities to a single star. We propose that this small number of clusters should be analysed anew.