Do Be stars evolve from normal B?

Ronaldo Savarino Levenhagen¹
Nelson Vani Leister¹

(1) IAG/USP

Be stars are an enigmatic class of emission-line objects. Several important problems pertaining their structure and evolution are still unsolved, such as phase transitions, the nature of the circumstellar disk and characterization of photospheric physical condition.
Though it has long been known that these objects are fast rotators and that they rotate at least 1.5 to 2 times faster than normal B stars, it is still uncertain whether or not these objects are in average critical rotators, although recent interferometric observations on Achernar (a typical Be star) pointed it out to be a critical rotator (Domiciano de Souza et al 2003).
A closer look to the problem reveals that all physical photospheric parameters derived without gravity darkening correction can lead to significant differences of their main-sequence evolutionary stages. This arises from the fact that, due to their high rotation rates (of the order [EQUATION]), these stars suffer from geometrical distortions and non-uniform temperature distributions dependent on the stellar latitude.
In a preliminary work done in a catalog of 600 field B stars we observed at intermediary spectroscopic dispersion a total of 114 indistinguishable Be stars (Levenhagen & Leister 2004, Levenhagen 2004). Our goal is to constrain the occurrence of the Be phenomenon along the main-sequence in order to provide information to answer the question: Do Be stars evolve from normal B? We compare the evolutionary stages of these objects obtained through classical methods (i.e. non-corrected for gravity darkening) with those corrected for high rotation effects.