1996A&A...308L..13F

Recently, Venn et al. (1996) showed that the strength of the UV line of B II in five Galactic main sequence B stars -i.e. in 100% of their sample- can only be understood if the boron abundance in these stars is considerably less than the meteoritic/solar boron abundance of logε(B)=2.88 (Anders & Grevesse 1989). Furthermore, a spread of the boron abundances in these otherwise similar stars appears to indicate that the boron depletion occurs during the main sequence evolution. We analyze the results of recent stellar evolution calculations (Fliegner & Langer 1996) for stars of 10 and 15M_☉ which include the effects of rotation on the stellar structure as well as various mixing processes triggered thereby, and an appropriate nuclear network to follow the evolution of all elements from hydrogen to silicon. Mass loss, at the observed and predicted rates for B stars, is found to have little effect on the boron depletion. The rotating models predict a distinct correlation between nitrogen enhancement and boron depletion, as the thermonuclear destruction of boron occurs in the outer stellar envelope but the synthesis of nitrogen occurs in the deep interior of the stars. Thus, the N/B ratio reflects the efficiency of the mixing processes in quite different stellar layers. The qualitative agreement of this correlation with the observed boron and nitrogen abundances of main sequence R stars provides the strongest evidence for rotational mixing acting in massive main sequence stars so far.