2004MNRAS.351..745L

Non-local thermodynamic equilibrium analysis of HeI lines in spectra of 102 B stars is implemented in order to derive the helium abundance He/H, the microturbulent parameter V_tand the projected rotation velocity v sini. A simultaneous determination of He/H and V_tfor the stars is effected by analysing equivalent widths of the 4471- and 4922-Å lines primarily as indicators of He/H and the 4713-, 5016-, 5876- and 6678-Å lines primarily as indicators of V_t. The rotation velocities v sini are found from profiles of the same lines. It is shown that, when V_t> 7 km/s, the V_t(HeI) values determined from HeI lines are systematically overestimated as compared with the V_t(OII, NII) values derived from OII and NII lines. This discrepancy is especially appreciable for hot evolved B giants with V_t(HeI) = 16-23 km/s and may indicate a failure of classical model atmospheres to represent the strong HeI lines for these stars.

Two programme stars, HR 1512 and 7651, are found to be helium-weak stars. The remaining 100 stars are divided into three groups according to their masses M. The microturbulent parameter V_t(HeI) is low for all stars of group A (M= 4.1-6.9 M_☉) and for all stars with the relative ages t/t_MS< 0.8 of group B (M= 7.0-11.2 M_☉). Their V_t(HeI) values are within the 0 to 5 km/s range, as a rule; the mean value is V_t= 1.7 km/s. Only evolved giants of group B, which are close to the termination of the main-sequence (MS) evolutionary phase (t/t_MS> 0.8), show V_t(HeI) up to 11 km/s. The helium abundance He/H is correlated with the relative age t/t_MSin both groups; the averaged He/H enhancement during the MS phase is 26 per cent. For group C, containing the most massive stars (M= 12.4-18.8 M_☉), the V_t(HeI) values display a correlation with t/t_MS, varying from 4 to 23 km/s. The He/H determination for hot evolved B giants of the group with V_t(HeI) > 15 km/s depends on a choice between the V_t(HeI) and V_t(OII, NII) scales. The mean He/H enrichment by 67 per cent during the MS phase is found, if the abundances He/H are based on the V_t(OII, NII) scale; however, two evolved giants with especially high v sini, HR 7446 and 7993, show the He/H enhancement by about a factor of 2.5. When using the same V_tscale, we found a trend of He/H with projected rotational velocities v sini; a large dispersion for v sini > 150 km/s can result from differences in masses M.

A comparison with the stellar model computations with rotationally induced mixing shows that the observed helium enrichment during the MS phase can be explained by rotation with initial velocities 250-400 km/s. The He/H distribution on M and v sini based on the V_t(OII, NII) scale seems to be in better agreement with the theory than one based on the V_t(HeI) scale. The mean value He/H = 0.10 derived for stars in the zero age main sequence (ZAMS) vicinity can be adopted as the typical initial helium abundance for early B stars in the solar neighbourhood.