[Paper Review] Global survey of star clusters in the Milky Way II. The catalogue of basic parameters

N. V. Kharchenko, А. Э. Пискунов+3 more|arXiv (Cornell University)|Aug 27, 2013

Stellar, planetary, and galactic studies21 references310 citations

TL;DR

This paper presents the Milky Way Star Clusters (MWSC) catalogue, a comprehensive, homogeneous survey of 3,006 star clusters in the Milky Way using uniform astrometric and near-infrared photometric data from PPMXL and 2MASS. It delivers precise, consistent parameters—position, proper motion, distance, age, reddening, and tidal radius—for 1,500 clusters with parameters determined for the first time, achieving near-complete coverage up to 1.8 kpc from the Sun, except for the oldest open clusters within 1 kpc due to input list incompleteness.

ABSTRACT

Although they are the main constituents of the Galactic disk population, for half of the open clusters in the Milky Way reported in the literature nothing is known except the raw position and an approximate size. The main goal of this study is to determine a full set of uniform spatial, structural, kinematic, and astrophysical parameters for as many known open clusters as possible. On the basis of stellar data from PPMXL and 2MASS, we used a dedicated data-processing pipeline to determine kinematic and photometric membership probabilities for stars in a cluster region. For an input list of 3784 targets from the literature, we confirm that 3006 are real objects, the vast majority of them are open clusters, but associations and globular clusters are also present. For each confirmed object we determined the exact position of the cluster centre, the apparent size, proper motion, distance, colour excess, and age. For about 1500 clusters, these basic astrophysical parameters have been determined for the first time. For the bulk of the clusters we also derived the tidal radius. We estimated additionally average radial velocities for more than 30% of the confirmed clusters. The present sample (called MWSC) reaches both the central parts of the Milky Way and its outer regions. It is almost complete up to 1.8 kpc from the Sun and also covers neighbouring spiral arms. However, for a small subset of the oldest open clusters ($\log t \gtrsim 9$) we found some evidence of incompleteness within about 1 kpc from the Sun.

Motivation & Objective

• To create a complete, homogeneous, and unbiased sample of Galactic star clusters with well-determined astrophysical parameters.
• To address the lack of reliable, uniform data for half of the known open clusters, which were previously known only by approximate position and size.
• To improve the completeness and accuracy of cluster parameters across the Galactic disk, including the central regions and outer spiral arms.
• To identify and quantify incompleteness in the literature-based input list, particularly for old open clusters near the Sun.
• To provide a foundation for future studies of cluster formation, evolution, and Galactic structure by delivering a consistent, all-sky catalogue of cluster parameters.

Proposed method

• The study uses a uniform data-processing pipeline applied to stellar data from the PPMXL and 2MASS all-sky catalogues, combining astrometry and near-infrared photometry.
• Kinematic and photometric membership probabilities are computed for stars in cluster regions to distinguish cluster members from field stars.
• Cluster centre positions, apparent radii, proper motions, distances, colour excesses, and ages are derived using a consistent method across all objects.
• Tidal radii are estimated for 98% of the confirmed clusters using the derived cluster parameters and stellar density profiles.
• Radial velocities are estimated for 30% of confirmed clusters by combining literature measurements with cluster membership data.
• The input list of 3,784 named clusters from the literature was validated against the data, resulting in 3,006 confirmed real objects.

Experimental results

Research questions

• RQ1 What is the true number of real star clusters in the Milky Way, and how many are missing from the literature?
• RQ2 How complete is the current sample of open clusters in the solar neighbourhood, especially for the oldest clusters?
• RQ3 To what extent does the input list of named clusters introduce systematic incompleteness in the survey, particularly for old open clusters?
• RQ4 Can a uniform, all-sky pipeline reliably derive astrophysical parameters for star clusters using PPMXL and 2MASS data?
• RQ5 How do the spatial and kinematic distributions of open and globular clusters relate across different ages?

Key findings

• The MWSC catalogue contains 3,006 confirmed star clusters, with 79% of the input list of 3,784 named clusters being real objects.
• For approximately 1,500 clusters—about 50% of the confirmed sample—basic astrophysical parameters such as distance, age, and reddening are derived for the first time.
• The survey achieves near-complete coverage up to 1.8 kpc from the Sun, with completeness limited to 1.8 kpc in the Galactic disk.
• Evidence of incompleteness is found for the oldest open clusters (log t ≥ 9) within 1 kpc of the Sun, with an estimated 40 clusters missing from the input list.
• Tidal radii were determined for 98% of the confirmed clusters, and mean radial velocities were estimated for 30% of the sample using literature data.
• The distribution of clusters along the Galactic Z-axis shows a smooth transition from old open clusters to young globular clusters at around 6 Gyr, with increasing vertical dispersion for older globular clusters.