Vertical stratification of iron in atmospheres of blue horizontal-branch stars

TL;DR

This study provides observational evidence of vertical iron stratification in the atmospheres of blue horizontal-branch stars, showing how iron abundance varies with atmospheric depth and temperature.

Contribution

First observational detection of iron stratification in BHB stars and analysis of its dependence on stellar effective temperature.

Findings

Iron abundance increases toward lower atmospheres in several BHB stars.

Stratification strongest at Teff around 11,500K and diminishes at higher temperatures.

Supports atomic diffusion as a key process in BHB stellar atmospheres.

Abstract

The aim of this study is to search for observational evidence of vertical iron stratification in the atmosphere of fourteen blue horizontal-branch (BHB) stars. We have found from our numerical simulations that five BHB stars: B22, B186 in the globular cluster NGC 288, WF2-820, WF2-2692 in M13 and B203 in M15 show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere. Two other BHB stars (B334 in M15 and B176 in M92) also show possible iron stratification in their atmosphere. A dependence of the slope of iron stratification on the effective temperature was also discovered. It is found that the vertical stratification of iron is strongest in BHB stars with Teff around 11,500K. The slope of iron abundance decreases as Teff increases and becomes negligible for the BHB stars with Teff= 14,000K. These results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with Teff > 11,500K.