Hot HB stars in globular clusters - physical parameters and consequences for theory. III. NGC 6752 and its long blue vertical branch

Abstract

We present spectroscopic analyses of 17 faint blue stars in the globular cluster NGC 6752. Effective temperatures, surface gravities, and helium abundances are determined and compared to theoretical predictions. All stars are helium deficient by factors of 3 to more than 100, indicative of gravitational settling of helium. Stars with Teff > 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. (1995, A&A 294, 65) for the faint blue stars in M 15. Deriving masses from the atmospheric parameters and the cluster distance leads to a mean mass of the sdB stars of 0.50 M0 and a standard deviation of about 0.043 dex, which is below the value derived of the observational errors and thus consistent with a very narrow mass distribution. The mean mass is in good agreement with the (0.49 +/- 0.02) M0 expected if sdB stars are extreme horizontal branch stars. The cooler stars show a significantly lower mean mass of 0.30 M0 in agreement with the findings of Moehler et al. (1995, M 15) and de Boer et al. (1995, A&A 303, 95; NGC 6397). The problems presented by the different mass distributions of BHB and sdB stars within one cluster are discussed.