Surface abundances of light elements for a large sample of early B‐type stars – III. An analysis of helium lines in spectra of 102 stars

Non‐local thermodynamic equilibrium analysis of He i lines in spectra of 102 B stars is implemented in order to derive the helium abundance He/H, the microturbulent parameter V t and the projected rotation velocity v sin  i . A simultaneous determination of He/H and V t for 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 sin  i are found from profiles of the same lines. It is shown that, when V t > 7 km s −1 , the V t (He i ) values determined from He i lines are systematically overestimated as compared with the V t (O  ii , N  ii ) values derived from O  ii and N  ii lines. This discrepancy is especially appreciable for hot evolved B giants with V t (He i ) = 16–23 km s −1 and may indicate a failure of classical model atmospheres to represent the strong He i 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 (He i ) 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 (He i ) values are within the 0 to 5 km s −1 range, as a rule; the mean value is V t = 1.7 km s −1 . 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 (He i ) up to 11 km s −1 . The helium abundance He/H is correlated with the relative age t / t MS in 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 (He i ) values display a correlation with t / t MS , varying from 4 to 23 km s −1 . The He/H determination for hot evolved B giants of the group with V t (He i ) > 15 km s −1 depends on a choice between the V t (He i ) and V t (O  ii , N  ii ) 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 (O  ii , N  ii ) scale; however, two evolved giants with especially high v sin  i , HR 7446 and 7993, show the He/H enhancement by about a factor of 2.5. When using the same V t scale, we found a trend of He/H with projected rotational velocities v sin  i ; a large dispersion for v sin  i > 150 km s −1 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 −1 . The He/H distribution on M and v sin  i based on the V t (O  ii , N  ii ) scale seems to be in better agreement with the theory than one based on the V t (He i ) 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.