Astronomy & Astrophysics

A&A 451, 499-513 (2006)
DOI: 10.1051/0004-6361:20053940

The lack of close binaries among hot horizontal branch stars in NGC 6752

C. Moni Bidin^{1, 2}, S. Moehler³, G. Piotto¹, A. Recio-Blanco^{1, 4}, Y. Momany¹ and R. A. Méndez²

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'osservatorio 2, 35122 Padova, Italy
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
e-mail: mbidin@das.uchile.cl
³ Institut für Theoretische Physik und Astrophysik, Christan-Albrechts-Universität zu Kiel, 24098 Kiel, Germany
⁴ Observatoire de la Côte d'Azur, Dpt. Cassiopée, CNRS UMR 6202, BP 4229, 06304 Nice Cedex 04, France

(Received 29 July 2005 / Accepted 5 January 2006)

Abstract
Aims.We present the results of a spectroscopic search for close binaries among horizontal branch (HB) stars in NGC 6752 .
Methods.We used the ESO VLT-FORS2 instrument to obtain medium resolution (R=4100) spectra of 51 hot HB stars with $8 000~{\rm K} \leq T_{\rm eff} \leq 32\,000$ K during four consecutive nights. Eighteen of our targets are extreme horizontal branch (EHB) stars with ${T_{\rm eff}}\ge 22\,000$ K. Radial velocity variations were measured with cross-correlation techniques, previously evaluated the statistical and systematic errors associated with them.
Results.No close binary system has been detected among our 51 targets. The data corrected for instrumental effects indicate that the radial velocity variations are always below $\approx$ 15 km s^-1 (3 $\sigma$ level). From a statistical analysis of our results, we conclude that (at 95% confidence level) the fraction of binaries with a ~ $0.5~{M_{\odot}}$ companion among EHB stars in NGC 6752 is smaller than 20%.
Conclusions.This empirical evidence sharply contrasts with what has been found for hot subdwarfs in the field, and opens new questions about the formation of EHB stars in globular clusters (and possibly in the field as well).

Key words: stars: horizontal-branch -- binaries: close -- binaries : spectroscopic -- globular clusters: individual: NGC 6752

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