On the evolutionary status of Be stars

We present a study of the incidence of Be stars in open clusters as a funct ion of the cluster age, using whenever possible ages determined through Str omgren uvby photometry. For the first time in studies of this kind we have considered separately classical and Herbig Be stars. The main results can be summarized as follows: Clusters associated to emitting nebulosities and undergoing stellar formati on are rich in emission line objects, which most likely are all pre main-se quence stars. No bona fide classical Be star has yet been identified among them. Clusters younger than 10 Myr and without associated nebulosity are almost c ompletely lacking Be stars, although they have a complete unevolved B main sequence. Classical Be stars appear at an age of 10 Myr, and reach the maximum abunda nce in the age interval 13-25 Myr. We interpret our results in the sense that the Be phenomenon is an evolutio nary effect which appears in the second half of the main sequence lifetime of a B star. We propose that it can be related to main structural changes h appening at this evolutionary phase, which also lead to the recently discov ered non-monotonic helium abundance enhancement. The semiconvection or turb ulent diffusion responsible of the surface helium enrichment, coupled with the high rotational velocity, can generate magnetic fields via the dynamo e ffect and thereby originate the Be phenomenon. Observational tests to this hypothesis are proposed.