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

TL;DR

This study uses improved stratified model atmospheres to explain the observational anomalies of blue horizontal-branch stars, supporting atomic diffusion as the underlying cause.

Contribution

The paper introduces and applies stratified model atmospheres to BHB stars, demonstrating their effectiveness in explaining observed anomalies and confirming atomic diffusion's role.

Findings

Model atmospheres with element stratification match observed photometric features.

Iron stratification trends are consistent between models and observations.

Spectroscopic gravities are underestimated when using homogeneous models.

Abstract

Blue horizontal-branch (BHB) stars with $T_{\rm eff}$ approximately larger than 11500 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 tendancies. 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.