Determine the net effect of microscopic diffusion on RGB surface abundances

Determine the net influence of gravitational settling, thermal diffusion, and concentration diffusion on the observed surface abundances and inferred stellar parameters of the extremely metal-poor RGB giant BD-18°5550 after accounting for the counteracting effects of first dredge-up and convective-envelope mixing.

Background

Microscopic diffusion can alter stellar surface compositions through gravitational settling, thermal diffusion, and concentration diffusion over evolutionary timescales. In an evolved red giant, however, the convective envelope deepens during the first dredge-up and subsequent envelope mixing can partially or completely counteract these effects. The paper therefore treats the net impact of diffusion on the present-day surface composition and inferred properties of BD-18°5550 as unresolved.

The authors address this uncertainty by computing models with and without microscopic diffusion and comparing their effects on the stellar parameters and abundance evolution. Establishing the net contribution of diffusion is important for interpreting the nearly pristine surface abundances of this extremely metal-poor RGB star and for assessing the reliability of its mass and age determination.