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Boron in main sequence B stars: a critical test for rotational mixing.
Recently, Venn et al. (1996) showed that the strength of the UV line ofB II in five Galactic main sequence B stars -i.e. in 100% of theirsample- can only be understood if the boron abundance in these stars isconsiderably less than the meteoritic/solar boron abundance oflogɛ(B)=2.88 (Anders & Grevesse 1989). Furthermore, a spreadof the boron abundances in these otherwise similar stars appears toindicate that the boron depletion occurs during the main sequenceevolution. We analyze the results of recent stellar evolutioncalculations (Fliegner & Langer 1996) for stars of 10 and15Msun_ which include the effects of rotation on the stellarstructure as well as various mixing processes triggered thereby, and anappropriate nuclear network to follow the evolution of all elements fromhydrogen to silicon. Mass loss, at the observed and predicted rates forB stars, is found to have little effect on the boron depletion. Therotating models predict a distinct correlation between nitrogenenhancement and boron depletion, as the thermonuclear destruction ofboron occurs in the outer stellar envelope but the synthesis of nitrogenoccurs in the deep interior of the stars. Thus, the N/B ratio reflectsthe efficiency of the mixing processes in quite different stellarlayers. The qualitative agreement of this correlation with the observedboron and nitrogen abundances of main sequence R stars provides thestrongest evidence for rotational mixing acting in massive main sequencestars so far.
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