Astron. Astrophys. 361, 929-936 (2000)

New color-magnitude diagram for the open cluster IC 4651

Deep Strömgren uvby CCD photometry in a large field ^* **

S. Meibom <sup>1,2

1</sup> Niels Bohr Institute for Astronomy, Physics, and Geophysics, Astronomical Observatory, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
² Astronomy Department, University of Wisconsin, Madison, WI-53706, USA (meibom@astro.wisc.edu)

Received 20 March 2000 / Accepted 26 July 2000

Abstract

New accurate CCD photometry in the u, v, b and y bands of the Strömgren system filters has been obtained for 17640 stars to in a field centered on the intermediate-age open cluster IC 4651. The resulting color-magnitude diagram (CMD) is substantially improved in completeness and accuracy compared to previous studies of IC 4651.

Specifically, this study has: 1) more than doubled the known size of the cluster area on the sky, 2) more than doubled the number of stars brighter than , 3) added 160 "new" candidate member stars to the turn-off region of the CMD, 4) identified fainter stars that trace a previously undetected and well-defined main-sequence down to , and 5) enabled a very accurate determination of true distance modulus ) by fitting the Hyades main-sequence over the long stretch of the cluster main-sequence. The CMD defines the main-sequence significantly better than the traditional or CMDs. These findings make IC 4651 a perfect target for testing stellar evolution models.

A correlation is found between the CMD and the radial distance of the stars to the cluster center. The fainter main-sequence stars are less centrally concentrated, as predicted by theory. Overall, IC 4651 appears to be a moderately dynamically evolved cluster ideally suited for testing of dynamical models.

Key words: Galaxy: open clusters and associations: individual: IC 4651 stars: Hertzsprung Russel (HR) and C-M diagrams

* Based on observations obtained with the Danish 1.5-m telescope at the European Southern Observatory, La Silla, Chile.
** Tables 1, 2 and 3 are only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/361/929 (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/361/929

SIMBAD Objects

Contents

• 1. Introduction
• 2. Recent studies of IC 4651
• 3. Observations and data reduction
  • 3.1. Basic reductions
  • 3.2. Point-Spread Function (PSF) photometry
• 4. Standard stars
  • 4.1. Transformation to standard system
  • 4.2. Photometric precision and accuracy
  • 4.3. Comparison with previous photometric studies
• 5. Star numbering conventions
• 6. The color-magnitude diagram
  • 6.1. The red giant region
  • 6.2. The turn-off region
  • 6.3. The lower main-sequence and the distance to IC 4651
  • 6.4. Why is the CMD so superior?
  • 6.5. Radial variations in the CMD
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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