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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 351; no. 2; pp. 745 - 767
• Main Authors: Lyubimkov, L. S., Rostopchin, S. I., Lambert, D. L.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 21.06.2004; Blackwell Science
• Subjects: Abundances, chemical composition; Astronomy; Earth, ocean, space; Exact sciences and technology; Main-sequence: early-type stars (o and b); Normal stars (by class): general or individual; Stars; stars: abundances; stars: atmospheres; stars: early-type; stars: rotation; Stellar characteristics and properties; Stellar spectra; stars: atmospheres; B stars; Mixing; Atmosphere model; Helium stars; Equivalent width; Rotation speed; Stellar abundance; M stars; Star models; stars: abundances; Thermodynamic equilibrium; Massive stars; Stellar atmospheres; Early type stars; stars: early-type; Element abundance; Light element; Correlations; Solar neighborhood; Stellar rotation; stars: rotation; Age
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2004.07825.x

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 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, N ii) values derived from O ii 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 t/tMS 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 Vt(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 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.