1994A&A...287..113D

Recently Gies & Lambert (1992) have reported on results of a composition analysis of a representative sample of early B-type stars. Their primary purpose was to verify a conjecture made earlier by Lyubimkov (1991) that almost every early B-type main sequence star displayed in its atmosphere CN-cycled material, i.e. an enhanced N abundance accompanied by a deficit of C. Though Gies & Lambert have revealed a source of systematic error in Lyubimkov's approach, nevertheless, they did confirm the presence of products of the CN-cycle in some of the B-stars. In the present study the evolution of a 10M_☉ star has been computed from the initial main sequence up to the first dredge-up phase. Hydrogen burning nuclear reactions have been taken into account in detail. To interpret the available observational data on the N overabundance in B-stars rotationally induced turbulent diffusive mixing has been assumed to operate in the radiative envelope of the stellar models. We have used a prescription for the diffusive mixing given by Zahn (1983). The main results obtained include a theoretical reproduction of the observational correlation between the N overabundance and the relative age of B-stars and a possible explanation of the nature of the microturbulence in atmospheres of B-stars proposed for the first time. It has also been shown that turbulent diffusion can transport Na synthesized in the convective core into the radiative envelope of the star, which could be responsible for the anomalous Na excesses observed in yellow supergiants. Besides that, the abundance ratio ²⁵Mg/²⁶Mg in the atmospheres of especially fast rotating stars has been found to decrease strongly enough to be detectable by means of isotopic composition analysis, and its determination could be an observational test for the considered turbulent diffusive mixing.