A lack of close binaries among hot horizontal branch stars in globular clusters^*

M 80 and NGC 5986

¹ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile e-mail: cmbidin@astro-udec.cl
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Dipartimento di Astronomia, Università di Padova, Vicolo dell'osservatorio 3, 35122 Padova, Italy
⁴ INAF - Osservatorio Astronomico di Padova, Vicolo dell'osservatorio 2, 35122 Padova, Italy
⁵ Laboratoire Cassiopée UMR 6202, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d'Azur, France

Received: 12 July 2008
Accepted: 27 February 2009

Abstract

Context. Recent investigations have revealed a surprising lack of close binaries among extreme horizontal branch (EHB) stars in the globular cluster NGC 6752, at variance with the analogous sdB field stars. Another puzzling result concerns the derived spectroscopic masses for some EHB stars.

Aims. The present paper extends our study of NGC 6752 to M 80 and NGC 5986, to establish whether the unexpected properties of EHB stars in NGC 6752 are also present in other clusters.

Methods. Twenty-one horizontal branch stars (out of which 5 EHBs) in NGC 5986 and 31 in M 80 (11 EHBs) were observed during four consecutive nights. We measured radial velocity variations and evaluated statistical and systematic errors. Temperatures, gravities, and helium abundances were also measured.

Results. By means of a statistical analysis of the observed radial velocity variations, we detected one EHB close binary candidate per cluster. In M 80, the best estimate of the close binary EHB fraction is %, and even the lowest estimate of the binary fraction among field sdB stars can be ruled out within a 90% confidence level. Because of the small observed sample, no strong conclusions can be drawn on the close EHB binary fraction for NGC 5986, although our best estimate is rather low (%). For the discrepancy in spectroscopic derived masses with theoretical models observed in NGC 6752, our analysis of M 80 EHB stars shows a similar trend. For the first time, we report a clear trend in surface helium abundance with temperature, although the trend for the hottest stars is still unclear.

Conclusions. Our results show that the deficiency of close binaries among EHB stars is now confirmed in two, and possibly three, globular clusters. This feature is therefore not a peculiarity of NGC 6752. Our analysis also proves that the strangely high spectroscopic masses among EHB stars are now confirmed in at least a second cluster. Our results confirm that f could be a function of the age of the sdB star population, but we find that recent models have some problem reproducing all observations.