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

TL;DR

This study investigates vertical iron abundance stratification in blue horizontal-branch stars, providing evidence that atomic diffusion influences their atmospheric composition, with some stars showing increased iron towards lower atmospheric layers.

Contribution

First direct evidence of iron abundance stratification in BHB star atmospheres, supporting atomic diffusion as a key process.

Findings

Iron is vertically stratified in two stars in M15.

One star in M13 shows signs of stratification.

Iron abundance increases towards lower atmosphere in affected stars.

Abstract

The observed slow rotation and abundance peculiarities of certain blue horizontal branch (BHB) stars suggests that atomic diffusion can be important in their stellar atmospheres and can lead to vertical abundance stratification of chemical species in the atmosphere. To verify this hypothesis, we have undertaken an abundance stratification analysis in the atmospheres of six BHB stars, based on recently acquired McDonald-CE spectra. Our numerical simulations show that the iron abundance is vertically stratified in the atmospheres of two stars in M15: B267 and B279. One star WF2-2541 in M13 also appears to have vertically stratified iron abundance, while for WF4-3085 the signatures of iron stratification are less convincing. In all cases the iron abundances increase towards the lower atmosphere. The other two stars in our sample, WF4-3485 and B84, do not show any significant variation of iron with atmospheric depth. Our results support the idea that atomic diffusion dominates other hydrodynamic processes in the atmospheres of BHB stars.