Astron. Astrophys. 343, 813-824 (1999)

The outer-halo globular cluster NGC 6229

III. Deep CCD photometry

J. Borissova ¹, M. Catelan  ^{**  2}, F.R. Ferraro ³, N. Spassova ¹, R. Buonanno ⁴, G. Iannicola ⁴, T. Richtler  ^{***  5} and A.V. Sweigart <sup>2

1</sup> Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarigradsko chaussèe, BG-1784 Sofia, Bulgaria (jura@haemimont.bg)
² NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA (catelan@stars.gsfc.nasa.gov; sweigart@bach.gsfc.nasa.gov)
³ Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy (ferraro@astbo3.bo.astro.it)
⁴ Osservatorio Astronomico di Roma, I-00040 Monte Porzio Catone, Italy (buonanno@astrmp.mporzio.astro.it; iannicola@coma.mporzio.astro.it)
⁵ Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany (richtler@astro.uni-bonn.de)

Received 22 October 1998 / Accepted 22 December 1998

Abstract

Deep BV photometry for a large field covering the outer-halo globular cluster NGC 6229 is presented. For the first time, a colour-magnitude diagram (CMD) reaching below the main sequence turnoff has been obtained for this cluster. Previous results regarding the overall morphology of the horizontal and giant branches are confirmed. In addition, nine possible extreme horizontal-branch stars have been identified in our deep images, as well as thirty-three candidate blue stragglers. We also find the latter to be more centrally concentrated than subgiant branch stars covering the same range in V magnitude.

A comparison of the cluster CMD with the M 5 (NGC 5904) ridgeline from Sandquist et al. (1996) reveals that:

i) NGC 6229 and M 5 have essentially identical metallicities;

ii) NGC 6229 and M 5 have the same ages within the errors in spite of their different horizontal-branch morphologies.

Key words: stars: blue stragglers stars: Hertzsprung Russel (HR) and C-M diagrams stars: Population II Galaxy: globular clusters: individual: Galaxy: globular clusters: individual:

* Hubble Fellow
** Visiting Scientist, Universities Space Research Association
*** Visiting Astronomer, German-Spanish Astronomical Center, Calar Alto, operated by the Max Planck-Institut für Astronomie jointly with the Spanish National Commission for Astronomy

Send offprint requests to: F.R. Ferraro

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations
  • 2.1. CCD photometry
  • 2.2. Data reduction
  • 2.3. Photometric errors
  • 2.4. Completeness corrections
• 3. The Colour-Magnitude Diagram
  • 3.1. Overall CMD morphology
  • 3.2. Number ratios
  • 3.3. Blue straggler stars
  • 3.4. Extreme HB stars
• 4. Metallicity
• 5. Age determination
• 6. Summary and concluding remarks
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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