TURBULENT DIFFUSIVE MIXING IN EARLY B-TYPE MAIN-SEQUENCE STARS

Recently Gies & Lambert (1992) have reported on results of a compositi on analysis of a representative sample of early B-type stars. Their pr imary purpose was to verify a conjecture made earlier by Lyubimkov (19 91) that almost every early B-type main sequence star displayed in its atmosphere CN-cycled material, i.e. an enhanced N abundance accompani ed by a deficit of C. Though Gies & Lambert have revealed a source of systematic error in Lyubimkov's approach, nevertheless, they did confi rm the presence of products of the CN-cycle in some of the B-stars. In the present study the evolution of a 10 M. star has been computed fro m the initial main sequence up to the first dredge-up phase. Hydrogen burning nuclear reactions have been taken into account in detail. To i nterpret 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 u sed a prescription for the diffusive mixing given by Zahn (1983). The main results obtained include a theoretical reproduction of the observ ational correlation between the N overabundance and the relative age o f B-stars and a possible explanation of the nature of the microturbule nce in atmospheres of B-stars proposed for the first time. It has also been shown that turbulent diffusion can transport Na synthesized in t he convective core into the radiative envelope of the star, which coul d be responsible for the anomalous Na excesses observed in yellow supe rgiants. Besides that, the abundance ratio Mg-25/Mg-26 in the atmosphe res of especially fast rotating stars has been found to decrease stron gly enough to be detectable by means of isotopic composition analysis, and its determination could be an observational test for the consider ed turbulent diffusive mixing.