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

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 () spectra of 51 hot HB stars with K during four consecutive nights. Eighteen of our targets are extreme horizontal branch (EHB) stars with 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 ≈15 km s^-1 (3σ level). From a statistical analysis of our results, we conclude that (at 95% confidence level) the fraction of binaries with a ~ 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).