THE HOT END OF EVOLUTIONARY HORIZONTAL BRANCHES

In this paper we investigate the hot end of the horizontal branch (HE) , presenting evolutionary constraints concerning the C-M diagram locat ion and the gravity of hot HE stars. According to the adopted evolutio nary scenario, we predict an upper limit for HE temperatures of about log T-e = 4.45, remarkably cooler than previous estimates. We find tha t such a theoretical prescription appears in good agreement with avail able observational data concerning both stellar temperatures and gravi ties. Finally, the problem of gaps in the HE distribution is discussed , suggesting that, at least in the case of the cluster NGC 6752, evolu tionary gaps, a bimodal distribution of CNO, and stellar rotation shou ld all play a negligible role.