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

Abstract

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 microturbuient parameter Vt and the projected rotation velocity v sin i. A simultaneous determination of He/H and Vt 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 Vt. The rotation velocities v sin i are found from profiles of the same lines. It is shown that, when Vt > 7 km s-1, the Vt(He I) values determined from He I lines are systematically overestimated as compared with the Vt(O II, Nil) values derived from OII and N II lines. This discrepancy is especially appreciable for hot evolved B giants with Vt(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 Vt(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/tMS < 0.8 of group B (M = 7.0-11.2 M⊙). Their Vt(He I) values are within the 0 to 5 km s-1 range, as a rule; the mean value is Vt = 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/tMS > 0-8), show Vt(He I) up to 11 km s -1. The helium abundance He/H is correlated with the relative age f/fMS 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 Vt(He I) values display a correlation with t/tMS, varying from 4 to 23 km s-1. The He/H determination for hot evolved B giants of the group with Vt(He I) > 15 km s-1 depends on a choice between the Vt(He I) and V t(0 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 Vt(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 Vt 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 Vt(O II, N II) scale seems to be in better agreement with the theory than one based on the Vt(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.