The number of Be stars compared with the number of B stars, taking into account the specific physical characteristics of Be stars

It is very important for the understanding of the Be phenomenon, and particularly for locating a possible Be phase in the evolutionary track of B stars, to accurately determine the proportion of Be stars among all B stars. This type of study was already made several times in the past. Results obtained generally show a maximum Be frequency around spectral type B2 then a decrease towards late spectral types. Actually Be stars do not have the same characteristics as “normal” B stars and we have to take this into account in the determination of the ratio : number of Be stars / number of B stars. We use the Bright Star Catalogue (Hoffleit & Jaschek 1982) and the Supplement to the Bright Star Catalogue (Hoffleit, Saladyga & Wlasuk 1983) containing stars V= 7.10 and brighter. This study needed to be made separately for the different spectral types because: - Physical parameters of B stars are very different from B0 to B9; - Emission characteristics of Be stars vary very much, with a decrease from B0e to B9e. We successively consider three effects which can influence the frequency of Be stars: - The over-luminosity of Be stars as compared with B stars; - Spectral type changes during constant mass evolution; - Spectral type changes due to the fast rotation of Be stars.