OBSERVATIONAL MANIFESTATIONS OF EARLY MIXING IN B-TYPE AND O-TYPE STARS

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 age t and the ste llar mass M. For example, the helium abundance He/H increases by 0.04 (60-70% of initial value) for stars with M = 8 - 13M. and by 0.025 (ab out 30%) for stars with M = 6M.. The nitrogen abundance rises by three times for M = 14M. and by two times for M = 10M.. According to the la test theoretical computations, the observed appearance of CNO-cycled m aterial in surface layers of the stars can be a result of the rotation ally induced mixing, in particular, of the turbulent diffusion. Carbon is in deficiency in B stars, but unexpectedly does not show any corre lation 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 st ars, as well as in components of binaries. As compared with early B st ars, the He abundance for more massive O stars and for components of b inaries show a different relation with the relative age t/t(MS). Namel y during the first half of the MS stage the normal value He/H = 0.08 - 0.10 is conserved, whereas in the short time between t/t(MS) approxim ate to 0.5 and 0.7 a sharp jump is observed up to He/H = 0.2 and more. In particular, such a jump is typical for fast rotating O stars (v si n i greater than or equal to 200 km s(-1)). Therefore the effect of mi xing depends on mass M, relative age t/t(Ms), rotational velocity v an d duplicity. The mass problem (the discrepancy between M-ev and M-sp) is also analysed, because some authors consider it as a possible evide nce of early mixing, too. It is shown that the accurate data for compo nents of binaries lead to the conclusion that the discrepancy is less than 30%. Such a difference can be removed at the expense of the M-ev lowering, if the displacement of evolutionary tracks owing to the rota tionally induced mixing is taken into consideration.