[Paper Review] Analysing the database for stars in open clusters I. General methods and description of the data

J. C. Mermilliod, E. Paunzen|ArXiv.org|Oct 5, 2003

Stellar, planetary, and galactic studiesPhysics and Astronomy18 references77 citations

TL;DR

This paper presents a comprehensive statistical analysis of the WEBDA database, evaluating the internal and external accuracy of photometric data (UBV, VRI, uvbyβ) from photographic, photoelectric, and CCD sources for 573 open clusters and 469,820 stars. It establishes that over 97% of data meet sufficient accuracy standards, enabling robust astrophysical parameter derivation—such as age, distance, and reddening—via isochrone fitting in a follow-up study.

ABSTRACT

We present an overview and statistical analysis of the data included in WEBDA. This database includes valuable information such as coordinates, rectangular positions, proper motions, photometric as well as spectroscopic data, radial and rotational velocities for objects of open clusters in our Milky Way. It also contains miscellaneous types of data like membership probabilities, orbital elements of spectroscopic binaries and periods of variability for different kinds of variable stars. Our final goal is to derive astrophysical parameters (reddening, distance and age) of open clusters based on the major photometric system which will be presented in a follow-up paper. For this purpose we have chosen the Johnson UBV, Cousins VRI and Stroemgren uvbybeta photometric systems for a statistical analysis of published data sets included in WEBDA. Our final list contains photographic, photoelectric and CCD data for 469820 objects in 573 open clusters. We have checked the internal (data sets within one photometric system and the same detector technique) and external (different detector technique) accuracy and conclude that more than 97% of all investigated data exhibit a sufficient accuracy for our analysis. The way of weighting and averaging the data is described. In addition, we have compiled a list of deviating measurements which is available to the community through WEBDA.

Motivation & Objective

• To assess the internal and external accuracy of photometric data in the WEBDA database for open clusters in the Milky Way.
• To evaluate the reliability of measurements across different detector techniques (photographic, photoelectric, CCD) within the same photometric systems.
• To compile a list of deviating measurements for community use and to support future astrophysical analysis.
• To establish a foundation for deriving key astrophysical parameters—reddening, distance, and age—of open clusters using homogeneous data.
• To provide a statistically robust, weighted averaging framework for multi-source photometric data to improve parameter derivation accuracy.

Proposed method

• Conducted statistical analysis on 573 open clusters and 469,820 stellar objects using Johnson-Cousins UBVRI, Strömgren uvbyβ, and Geneva 7-color photometric systems.
• Performed internal accuracy checks by comparing independent measurements within the same photometric system and detector technique.
• Performed external accuracy checks by comparing measurements across different detector techniques (e.g., CCD vs. photographic) to assess inter-method consistency.
• Used Gaussian fitting to histograms of magnitude differences to quantify measurement scatter and identify outliers.
• Implemented a weighted mean and averaging scheme based on measurement precision, with emphasis on minimizing systematic errors.
• Generated a public list of 4,467 deviating measurements from 4,056 data sets for transparency and data validation.

Experimental results

Research questions

• RQ1 What is the internal accuracy of photometric data within the same photometric system and detector technique across open clusters?
• RQ2 How do measurement accuracies compare across different detector technologies (photographic, photoelectric, CCD) for the same photometric system?
• RQ3 What percentage of photometric measurements in WEBDA exhibit significant deviations from expected values, and can these be systematically identified?
• RQ4 To what extent can the data support reliable derivation of astrophysical parameters like age, distance, and reddening for open clusters?
• RQ5 How does the quantum efficiency difference between photographic and CCD detectors affect photometric consistency, and can this be quantified?

Key findings

• Over 97% of the 469,820 photometric measurements in WEBDA exhibit sufficient accuracy for astrophysical analysis, with only 1.7% (4,467 measurements) identified as deviating.
• The Strömgren uvbyβ system shows the highest internal accuracy, with Gaussian distribution widths between 0.01 and 0.03 mag.
• Photographic UBV data show the largest scatter (≈0.09 mag), likely due to UV sensitivity limitations in CCDs and lower detector efficiency.
• CCD vs. photographic comparisons show a 0.12 mag scatter, significantly higher than photoelectric vs. photographic (0.05 mag), reflecting differences in quantum efficiency.
• Photoelectric data serve as a critical bridge between photographic and CCD measurements due to their high consistency and intermediate precision.
• The external accuracy analysis of 1,960 data sets (292,770 measurements) revealed only 2.4% (7,061) deviating measurements, confirming the overall data quality and reliability.