Observational manifestations of early mixing in B- and O-type stars

Abstract

The helium and nitrogen enrichment of the atmospheres of early B-type stars during the main sequence (MS) evolutionary phase is re-analysed. It is confirmed that the effect depends on both the aget and the stellar massM. For example, the helium abundanceHe/H increases by 0.04 (60–70% of initial value) for stars withM=8–13M ⊙ and by 0.025 (about 30%) for stars withM=6M ⊙. The nitrogen abundance rises by three times forM=14M ⊙ and by, two times forM=10M ⊙. According to the latest theoretical computations, the observed appearance of CNO-cycled material in surface layers of the stars can be a result of the rotationally induced mixing, in particular, of the turbulent diffusion. Carbon is in deficiency in B stars, but unexpectedly does not show any correlation with the stellar age. However it is shown that the total C+N abundance derived for early B stars conflicts with the theory. Basing on modern data the helium enrichment is first examined in O-type MS stars, as well as in components of binaries. As compared with early B stars, the He abundance for more massive O stars and for components of binaries show a different relation with the relative aget/t MS . Namely during short time betweent/t MS ≈0.5 and 0.7 a sharp jump is observed up toHe/H=0.2 and more. In particular, such a jump is typical for fast rotating O stars (v sini≥200 km s−1),. Therefore the effect of mixing depends on massM, relative aget/t MS , rotational velocityv and duplicity. The mass problem (the discrepancy betweenM ev andM sp ) is also analysed, because some authors consider it as a possible evidence of early mixing, too. It is shown that the accurate data for components of binaries lead to the conclusion that the discrepancy is less than 30%. Such a difference can be removed at the expense of theM ev lowering, if the displacement of evolutionary tracks, owing to the rotationally induced mixing is taken into consideration.