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Astron. Astrophys. 357, 451-459 (2000)


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On the evolutionary status of Be stars

J. Fabregat 1 and J.M. Torrejón 2

1 Universidad de Valencia, Departamento de Astronomía, 46100 Burjassot, Valencia, Spain
2 Universidad de Alicante, Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Spain

Received 18 June 1999 / Accepted 23 March 2000

Abstract

We present a study of the incidence of Be stars in open clusters as a function of the cluster age, using whenever possible ages determined through Strömgren 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 formation are rich in emission line objects, which most likely are all pre main-sequence stars. No bona fide classical Be star has yet been identified among them.

  • Clusters younger than 10 Myr and without associated nebulosity are almost completely 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 abundance in the age interval 13-25 Myr.

We interpret our results in the sense that the Be phenomenon is an evolutionary 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 happening at this evolutionary phase, which also lead to the recently discovered non-monotonic helium abundance enhancement. The semiconvection or turbulent diffusion responsible of the surface helium enrichment, coupled with the high rotational velocity, can generate magnetic fields via the dynamo effect and thereby originate the Be phenomenon. Observational tests to this hypothesis are proposed.

Key words: stars: emission-line, Be – stars: evolution – Galaxy: open clusters and associations: general

Send offprint requests to: J. Fabregat

Correspondence to: juan@pleione.uv.es

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000

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