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Metal-poor stars preserve chemical abundance patterns imprinted by nucleosynthesis in the early Universe, providing key constraints on the evolution and explosions of massive stars at extremely low metallicity. Among these abundance ratios, [Na/Mg] exhibits a particularly large scatter (Ishigaki et al. 2026). Sodium is mainly produced during carbon burning, and its yield can be predicted more robustly than those of later-stage burning products, which are more affected by nonlinear late-stage evolution and supernova explosions. Although previous stellar evolution calculations have shown that sodium yields depend on initial conditions such as the zero-age main-sequence (ZAMS) mass and rotation velocity (Nomoto et al. 2013), the physical origin of the observed [Na/Mg] scatter remains unresolved.
To reveal this origin, we conducted a grid of stellar evolution calculations for massive stars with different ZAMS masses (10, 11, ..., 25 M_sun) and metallicities (10^-2, 10^-3, ..., 10^-5 Z_sun). We investigated how the nuclear burning history is connected to the final yields of carbon-burning products, including Na. We also compiled an observational dataset from two large catalogs of metal-poor stars (Roederer et al. 2014; Cohen et al. 2013), applying selection criteria to reduce uncertainties in stellar populations and abundance measurements, and compared it with our theoretical results.
Our calculations show that the final yields separate into two groups with high and low [Na/Mg] ratios, depending on the burning regime in the O+Ne shell during the final evolutionary stage. Models with stable shell burning retain relatively high [Na/Mg], whereas those experiencing dynamical carbon burning in the O+Ne shell exhibit low [Na/Mg]. In the latter case, alpha-capture reactions, particularly (Na(α,p)Mg) and (Ne(α, γ)Mg), are strongly activated, destroying Na and enhancing Mg production. This reaction pathway explains the observed [Na/Mg] scatter. By contrast, [Al/Mg] does not show a comparable separation, consistent with the observed abundance patterns.
This work identifies the nuclear-reaction origin of the observed [Na/Mg] scatter in metal-poor stars. We discuss its possible connection to shell merger during the final stage of stellar evolution. We also discuss the implication that most stars in our selected observational sample show low [Na/Mg], suggesting that alpha-capture reactions in the O+Ne shell commonly operated in their progenitors. Our findings demonstrate that not only abundance ratios themselves but also their scatter provide important constraints on stellar evolution in the early Universe. Expanding samples of metal-poor stars will therefore be essential for using abundance scatter as a more precise probe of early stellar evolution.
| Category | Theory |
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