Physical Parameters of Hot Horizontal-Branch Stars in NGC 6752: Deep Mixing and Radiative Levitation

Abstract

Atmospheric parameters (Teff, log g and log(He/H)) are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars MgII and FeII lines are detected indicating an iron enrichment by a factor 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster metallicity. This finding adds to the growing evidence that radiative levitation plays a significant role in determining the physical parameters of blue HB stars. Indeed, we find that iron enrichment can explain part, but not all, of the problem of anomalously low gravities along the blue HB. Thus the physical parameters of horizontal branch stars hotter than about 11,500K in NGC 6752, as derived in this paper, are best explained by a combination of helium mixing and radiative levitation effects.