[Paper Review] Absolute Proper Motions of Open Clusters: I. Observational data

Holger Baumgardt, C. Dettbarn+1 more|ArXiv.org|Oct 16, 2000

Stellar, planetary, and galactic studies3 references84 citations

TL;DR

This paper presents a comprehensive astrometric analysis of 205 open clusters using Hipparcos Catalogue data, combining ground-based photometric, radial velocity, and proper motion data with Hipparcos parallaxes and proper motions to identify 630 certain and 100 possible cluster members. The study provides absolute proper motions and parallaxes for distant clusters (D > 200 pc), confirming good agreement with photometric distances and resolving membership of several Cepheid variables.

ABSTRACT

Mean proper motions and parallaxes of 205 open clusters were determined from their member stars found in the Hipparcos Catalogue. 360 clusters were searched for possible members, excluding nearby clusters with distances D < 200 pc. Members were selected using ground based information (photometry, radial velocity, proper motion, distance from the cluster centre) and information provided by Hipparcos (proper motion, parallax). Altogether 630 certain and 100 possible members were found. A comparison of the Hipparcos parallaxes with photometric distances of open clusters shows good agreement. The Hipparcos data confirm or reject the membership of several Cepheids in the studied clusters.

Motivation & Objective

• To determine absolute proper motions and parallaxes for open clusters beyond 200 pc to improve kinematic modeling of the Galactic rotation curve and distance to the Galactic center.
• To identify cluster members among Hipparcos stars by combining Hipparcos astrometry with ground-based photometric, radial velocity, and proper motion data.
• To validate Hipparcos parallaxes against photometric distances for open clusters, assessing systematic errors in both methods.
• To test the membership of Cepheid variables in open clusters using high-precision astrometry from Hipparcos.
• To establish a reliable observational database of mean astrometric parameters for future studies on open cluster formation and evolution.

Proposed method

• Searched 360 open clusters from Lyngå (1987) for potential members, excluding those with D < 200 pc due to large angular diameters invalidating common proper motion assumption.
• Identified candidate stars within estimated tidal radii (12 pc) using cluster distance and angular size from Lyngå's catalogue.
• Applied multi-criteria membership selection: photometry, spectroscopy, radial velocities, proper motions, parallaxes, and angular distance from cluster center.
• Used weighted least-squares fitting to compute mean proper motions and parallaxes, with error estimates and correlation coefficients.
• Calculated photometric distances from B-V and V magnitude data and compared them with Hipparcos parallaxes to assess consistency.
• Evaluated Cepheid membership by comparing their Hipparcos astrometry with cluster mean parameters and internal consistency.

Experimental results

Research questions

• RQ1 Do Hipparcos parallaxes agree with photometric distances for open clusters, and what does this imply about systematic errors in either method?
• RQ2 Can high-precision Hipparcos astrometry resolve the membership status of Cepheid variables in open clusters?
• RQ3 What is the reliability of combining Hipparcos data with ground-based data to identify cluster members when only a few stars are available per cluster?
• RQ4 How do the mean proper motions and parallaxes of distant open clusters (D > 200 pc) contribute to kinematic modeling of the Milky Way?
• RQ5 What is the impact of excluding nearby clusters (D < 200 pc) on the accuracy of mean astrometric parameter estimation?

Key findings

• The Hipparcos parallaxes show good agreement with photometric distances for open clusters, indicating consistency between the two methods and minimal systematic errors.
• The study confirmed or rejected the membership of several Cepheid variables in open clusters based on their astrometric consistency with cluster mean parameters.
• A total of 630 certain and 100 possible members were identified across 205 open clusters, with mean proper motions and parallaxes derived for each cluster.
• The method successfully identified cluster members even with low numbers of stars per cluster (sometimes only one), demonstrating robustness under sparse data conditions.
• The correlation coefficients between parallax and proper motion parameters were computed and reported, enabling error propagation and uncertainty assessment in subsequent kinematic studies.
• The dataset provides a reliable observational foundation for future studies on Galactic structure, cluster kinematics, and the distance to the Galactic center.