Astron. Astrophys. 343, 904-908 (1999)

On the gap in horizontal branches at B - V about zero

V. Caloi

Istituto di Astrofisica Spaziale CNR, Via Fosso del Cavaliere, I-00133 Roma, Italy (caloi@victoria.ias.rm.cnr.it)

Received 19 October 1998 / Accepted 3 December 1998

Abstract

It is proposed that the gap observed in the horizontal branch sequence of many globular clusters at about zero is due to a surface phenomenon. Stars on the hotter side of the gap - 10000 K - have an atmospheric chemical composition altered as in peculiar A and B stars. The appearance of a convective regime in the surface layers at the temperature and gravity of the gap cancel the peculiarities: it is proposed that the ensuing changes in surface physical conditions give origin to the observed gap.

Key words: Galaxy: globular clusters: general stars: horizontal-branch stars: chemically peculiar

Send offprint requests to: V. Caloi

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Contents

• 1. Introduction
• 2. Possible origins of the gap
  • 2.1. The gap and mass loss
  • 2.2. The gap and stellar evolution
• 3. The atmospheric connection
  • 3.1. Some properties of CP stars
  • 3.2. Theoretical interpretation of chemical peculiarities in HB stars
• 4. Above and below the gap
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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