Understanding the Characteristics of EMP Stars as Probes for the Early Universe: Stellar Evolution of Low‐ and Intermediate‐Mass EMP Stars

Abstract

The information on surface abundances in extremely metal‐poor (EMP) stars in the Galactic halo is a useful probe of the early universe. Therefore, it is very important to understand the modification of abundances in EMP stars by internal and/or external pollution through mixing processes and binary interactions in stellar evolution. We computed various sets of model stars for 0.8–9.0–M⊙ in mass and −5< [Fe/H] <−2.3 and Z = 0 in metallicity. Evolution is followed from zero‐age main sequence to the thermally pulsating AGB phase including the onset of hydrogen entrainment by the helium flash convection during the red giant branch phase and/or the asymptotic giant branch phase. We explored the evolutionary characteristics depending on the initial mass and metallicity. There exist various types of mixing episode of how the hydrogen mixing sets in and of how it affects the final abundances in the surface. For low mass models having M<1.2 M⊙, strong helium flash leads to the large enhancement of CN elements through the dredge‐up of materials produced in the helium flash convective region that injected the protons in the hydrogen burning shell. For more massive models, several episodes of mixing occur, with weak mixing events at AGB phase, which may allow longer timescale for nucleosynthesis in the helium flash convective region. We also found a difference in the efficiencies of dredge‐up during the thermally pulsating AGB phase.