SIMBAD references

2004MNRAS.351..745L - Mon. Not. R. Astron. Soc., 351, 745-767 (2004/June-3)

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 (from CDS):

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 Vtand the projected rotation velocity v sini. A simultaneous determination of He/H and Vtfor 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 sini are found from profiles of the same lines. It is shown that, when Vt> 7 km/s, the Vt(HeI) values determined from HeI lines are systematically overestimated as compared with the Vt(OII, NII) values derived from OII and NII lines. This discrepancy is especially appreciable for hot evolved B giants with Vt(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 Vt(HeI) 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(HeI) values are within the 0 to 5 km/s range, as a rule; the mean value is Vt= 1.7 km/s. 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(HeI) up to 11 km/s. The helium abundance He/H is correlated with the relative age t/tMSin 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(HeI) values display a correlation with t/tMS, varying from 4 to 23 km/s. The He/H determination for hot evolved B giants of the group with Vt(HeI) > 15 km/s depends on a choice between the Vt(HeI) and Vt(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 Vt(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 Vtscale, 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 Vt(OII, NII) scale seems to be in better agreement with the theory than one based on the Vt(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.

Abstract Copyright: 2004 RAS

Journal keyword(s): stars: abundances - stars: atmospheres - stars: early-type - stars: rotation

Simbad objects: 109

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