Spectroscopic search for binaries among EHB stars in globular clusters

Abstract

We performed a spectroscopic search for binaries among hot horizontal branch stars in globular clusters. We present final results for a sample of 51 stars in NGC 6752 and preliminary results for the first 15 stars analyzed in M 80. The observed stars are distributed along all the HBs in the range 8000 <= T-eff <= 32 000 K, and have been observed during four nights. Radial velocity variations were measured with the cross-correlation technique. We analyzed the statistical and systematic errors associated with the measurements in order to evaluate the statistical significance of the observed variations. No close binary system has been detected, neither among cooler stars nor among the sample of hot EHB stars (18 stars with T-eff >= 22 000 K in NGC 6752). The data corrected for instrumental effects indicate that the radial velocity variations are always below the 3 sigma, level of similar to 15 km s(-1). These results are in sharp contrast with those found for field hot subdwarfs, and open new questions about the formation of EHB stars in globular clusters and possibly of the field subdwarfs.