Journal Article

Stratification of the elements in the atmospheres of blue horizontal-branch stars

F. LeBlanc, A. Hui-Bon-Hoa and V. R. Khalack

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 409, issue 4, pages 1606-1610
Published in print December 2010 | ISSN: 0035-8711
Published online December 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17404.x
Stratification of the elements in the atmospheres of blue horizontal-branch stars

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Blue horizontal-branch (BHB) stars with Teff approximately larger than 11 500 K show several observational anomalies. In globular clusters, they exhibit low rotational velocities, abundance anomalies (as compared to cluster abundances), photometric jumps and gaps and spectroscopic gravities lower than predicted by canonical models. It is commonly believed that the low rotational velocities of these stars permit atomic diffusion to be efficient in their atmosphere thereby causing the observed anomalies. Recent detections of vertical stratification of iron (and some other chemical elements) in several BHB stars concur with this framework. In this paper, improved model atmospheres that include the vertical stratification of the elements are applied to BHB stars to verify if they can explain their observational anomalies. The results from theoretical model atmospheres are consistent with the photometric jumps and gaps observed for BHB stars in globular clusters. It is found that iron stratification in the theoretical models and that obtained from observations have similar tendencies. Our results also show that the spectroscopic gravities obtained while using chemically homogeneous model atmospheres to fit observations are underestimated. These results significantly strengthen the belief that atomic diffusion is responsible for these BHB-star anomalies.

Keywords: diffusion; stars: abundances; stars: atmospheres; stars: horizontal branch

Journal Article.  3710 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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