[Paper Review] A Gaia DR2 view of the Open Cluster population in the Milky Way

T. Cantat-Gaudin, C. Jordi+11 more|arXiv (Cornell University)|May 22, 2018

Astronomy and Astrophysical Research4 citations

TL;DR

This study leverages Gaia DR2's sub-milliarcsecond astrometry and mmag photometry to identify 1,229 open clusters across the Milky Way, deriving precise homogeneous distances and membership lists using an unsupervised membership algorithm (UPMASK). It reports 60 new open cluster discoveries and confirms that young clusters trace the Galactic plane and spiral arms, while older clusters are more dispersed and located at higher Galactic latitudes and larger Galactocentric radii.

ABSTRACT

Open clusters are convenient probes of the structure and history of the Galactic disk. They are also fundamental to stellar evolution studies. The second Gaia data release contains precise astrometry at the sub-milliarcsecond level and homogeneous photometry at the mmag level, that can be used to characterise a large number of clusters over the entire sky. In this study we aim to a establish list of members and derive mean parameters, in particular distances, for as many clusters as possible, making use of Gaia data alone. We compile a list of thousands of known or putative clusters from the literature. We then apply an unsupervised membership assignment code, UPMASK, to the Gaia DR2 data contained within the fields of those clusters. We obtained a list of members and cluster parameters for 1229 clusters. As expected, the youngest clusters are seen to be tightly distributed near the Galactic plane and to trace the spiral arms of the Milky Way, while older objects are more uniformly distributed, deviate further from the plane, and tend to be located at larger Galactocentric distances. Thanks to the quality of GaiaDR2 astrometry, the fully homogeneous parameters derived in this study are the most precise to date. Furthermore, we report on the serendipitous discovery of 60 new open clusters in the fields analysed during this study.

Motivation & Objective

• To compile a comprehensive, homogeneous list of open cluster members and parameters using Gaia DR2 data alone.
• To derive precise distances and kinematic properties for open clusters across the Milky Way, independent of external photometric or spectroscopic data.
• To identify previously unknown open clusters through systematic analysis of Gaia DR2 astrometric and photometric data.
• To characterize the spatial distribution of open clusters as a function of age, revealing insights into Galactic disk structure and evolution.
• To provide a benchmark dataset of cluster parameters for future studies of stellar evolution, chemical evolution, and dynamical processes in the Milky Way.

Proposed method

• An unsupervised membership assignment algorithm, UPMASK, was applied to Gaia DR2 data within the spatial extent of 1,229 known or putative open clusters.
• Membership was determined based on the spatial, proper motion, and parallax distribution of stars relative to cluster centers, using a probabilistic clustering approach.
• Mean cluster parameters—especially parallaxes and thus distances—were derived from the median of member star astrometry, leveraging the high precision of Gaia DR2.
• Cluster age information from the literature was used to stratify the sample and analyze the spatial distribution of young versus old clusters.
• The method was validated by cross-checking with known clusters and identifying new candidates through excess spatial concentration and kinematic coherence.
• A Bayesian approach to distance estimation was implicitly assumed, though photometric data were not used in this study to avoid introducing external dependencies.

Experimental results

Research questions

• RQ1 What is the spatial distribution of open clusters in the Milky Way as revealed by Gaia DR2, and how does it vary with age?
• RQ2 Can an unsupervised algorithm like UPMASK reliably identify cluster members and derive homogeneous cluster parameters using Gaia DR2 astrometry alone?
• RQ3 How many new open clusters can be discovered in the Gaia DR2 data, and what are their basic characteristics?
• RQ4 To what extent do young open clusters trace the Galactic plane and spiral structure, compared to older clusters?
• RQ5 What are the limitations of deriving distances from Gaia DR2 parallaxes alone for distant clusters, and how can this be improved?

Key findings

• The study identified 1,229 open clusters, including 60 new discoveries, with homogeneous and precise distance estimates derived solely from Gaia DR2 parallaxes.
• Young open clusters are tightly concentrated near the Galactic plane and trace the spiral structure, while older clusters are more uniformly distributed and located at higher Galactic latitudes.
• Older clusters are rarer in the inner disk (R_GC < 7 kpc), suggesting possible disruption in the dense inner regions or dynamical evolution such as outward migration.
• The distribution of clusters confirms that young clusters follow the Galactic disk's symmetry plane, while older clusters show greater scale height and larger Galactocentric distances.
• The method successfully recovered known clusters and identified new candidates with high confidence, demonstrating the power of unsupervised clustering in large astrometric surveys.
• Despite the high precision of Gaia DR2, distance estimates for clusters beyond ~10 kpc remain uncertain, highlighting the need for photometric or spectroscopic priors in future work.