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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.
LYUBIMKOV L.S., ROSTOPCHIN S.I. and LAMBERT D.L.
Abstract (from CDS):
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: 110
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