Boron Abundances in B‐Type Stars: A Test of Rotational Depletion during Main‐Sequence Evolution

Boron abundances have been derived for seven main sequence B-type stars fromHST STIS spectra around the B III 2066 A line. In two stars, boron appears tobe undepleted with respect to the presumed initial abundance. In one star,boron is detectable but it is clearly depleted. In the other four stars, boronis undetectable implying depletions of 1 to 2 dex. Three of these four starsare nitrogen enriched, but the fourth shows no enrichment of nitrogen. Onlyrotationally induced mixing predicts that boron depletions are unaccompanied bynitrogen enrichments. The inferred rate of boron depletion from ourobservations is in good agreement with these predictions. Other boron-depletednitrogen-normal stars are identified from the literature. Also, severalboron-depleted nitrogen-rich stars are identified, and while all fall on theboron-nitrogen trend predicted by rotationally-induced mixing, a majority havenitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni)abundances. The seven B-type stars have near solar iron-group abundances, asexpected for young stars in the solar neighborhood. We have also analysed thehalo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absenceof the B III line gives the upper limit [B/H]<2.5. These and other publishedabundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap