Hot HB stars in globular clusters: physical parameters and consequences for theory

We present spectroscopic analyses of 17 faint blue stars in the globular cluster NGC 6752, using optical and UV spectrophotometric data and intermediate resolution optical spectra. Effective temperatures, surface gravities, and helium abundances of the stars are determined and compared to theoretical predictions. All stars are helium deficient by factors ranging from 3 to more than 100, indicative of gravitational settling of helium. Stars with effective temperatures above about 20000 K (sdBs) fit well to the evolutionary tracks, whereas the cooler stars show lower surface gravities than theoretically expected. This agrees with earlier findings by Moehler et al. (1995a) for the faint blue stars in M 15. Deriving masses from the atmospheric parameters and the cluster distance leads to ameanmass of the sdB stars of 0.50 M and a standard deviation of about 0.043 dex, which is below the value derived of the observational errors and therefore consistent with a very narrow mass distribution. The mean mass is in good agreement with the (0.49 ± 0.02) M expected if sdB stars are extreme horizontal branch stars with a helium core of 0.48 M and an extremely thin hydrogen layer. The cooler stars however show a significantly lower mean mass of 0.30 M with a standard deviation of the mean of 0.059 dex in agreement with the findings of Moehler et al. (1995a) and de Boer et al. (1995) for the BHB stars in M 15 and NGC 6397, respectively. The problems presented by the different mass distributions of BHB and sdB stars within one cluster are discussed.