Metal-poor stars—those formed in the early universe—offer clues about the nucleosynthesis processes of ancient supernovae (SNe). These stars have low levels of heavy elements, or "metals," compared to modern stars. This study by Yutaka Hirai and colleagues investigates "mono-enriched stars," a rare type of metal-poor star formed exclusively from the remnants of a single supernova. By understanding these stars, astronomers can decode the unique properties of their supernova origins. The research uses advanced simulations of a dwarf galaxy to explore the occurrence and distribution of mono-enriched stars across different metallicities.

Methods

The authors conducted cosmological "zoom-in" simulations to closely study the formation of individual stars in a dwarf galaxy. The simulations employed the asura+bridge code, which accounts for gravity, hydrodynamics, and chemical enrichment. Newly formed stars were classified based on their mass ranges, with the largest stars modeled individually to track their contributions to subsequent generations. This setup allowed the researchers to determine whether stars were mono-enriched or influenced by multiple supernovae. A critical parameter in identifying mono-enriched stars was the carbon-to-iron ratio ([C/Fe]), which is linked to specific supernova ejecta.

Key Results

The simulation revealed that the fraction of mono-enriched stars decreases as metallicity increases. At extremely low metallicities, such as [Fe/H] = -5.0, 11% of stars were mono-enriched, whereas this fraction dropped to just 1% at [Fe/H] = -2.5. This trend reflects the fact that as galaxies evolve, their interstellar medium becomes enriched by multiple supernovae, reducing the likelihood of single supernova imprints. Mono-enriched stars were also more centrally concentrated within the simulated galaxy, corresponding to regions where star formation began earliest and gas was densest. These stars primarily formed in the galaxy's early history, during periods of minimal chemical mixing.

Comparison with Observations

The simulation results align with observational trends, such as those suggested by machine-learning studies. However, the study predicts a lower fraction of mono-enriched stars compared to earlier estimates. This discrepancy is attributed to stricter criteria used in the simulation, such as exact matches in chemical ratios, which observational techniques often cannot resolve due to measurement uncertainties.

Implications and Future Directions

This work offers the first simulation-based estimate of mono-enriched star fractions, highlighting their rarity and unique role in tracing early supernovae. Future observational surveys, such as those using the Subaru Telescope's Prime Focus Spectrograph, will be pivotal in testing these predictions. Targeting extremely metal-poor stars in the centers of dwarf galaxies could maximize the chances of discovering mono-enriched stars.

Conclusion

Hirai and colleagues have advanced our understanding of early galaxy formation and stellar nucleosynthesis by identifying patterns in the distribution of mono-enriched stars. These findings provide a roadmap for future studies, bridging theoretical simulations and observational astronomy. Through such efforts, astronomers are uncovering the origins of the first stars and the supernovae that seeded the cosmos with elements essential for life.

Source: Hirai