2026-03-16 シカゴ大学（UChicago）
Stars in the ultra-faint dwarf galaxy, Pictor II, which is more than ten billion years old. Scientists have determined that one of the stars in this image, PicII-503, likely dates back to the second generation of stars in the universe.Image by CTIO/NOIRLab/DOE/NSF/AURA
＜関連情報＞
- https://news.uchicago.edu/story/ancient-star-opens-window-early-days-universe
- https://www.nature.com/articles/s41550-026-02802-z
矮小銀河の遺存星による初期星形成 Enrichment by the first stars in a relic dwarf galaxy
Anirudh Chiti,Vinicius M. Placco,Andrew B. Pace,Alexander P. Ji,Deepthi S. Prabhu,William Cerny,Guilherme Limberg,Guy S. Stringfellow,Alex Drlica-Wagner,Kaia R. Atzberger,Yumi Choi,Denija Crnojević,Peter S. Ferguson,Nitya Kallivayalil,Noelia E. D. Noël,Alexander H. Riley,David J. Sand,Joshua D. Simon,Alistair R. Walker,Clecio R. Bom,Julio A. Carballo-Bello,David J. James,Clara E. Martínez-Vázquez,Gustavo E. Medina & A. Katherina Vivas
Nature Astronomy Published:16 March 2026
DOI:https://doi.org/10.1038/s41550-026-02802-z
Abstract
Stars that contain trace amounts of elements heavier than helium (that is, ‘metallicity’) preserve the chemical fingerprints of the first stars. In the Milky Way, nearly all of the lowest-metallicity stars show an extreme over-abundance of carbon. The origin of this signature has remained a mystery owing to the lack of observational constraints on the environments in which it originates. Here we present observations of a star in the >10-billion-year-old ultrafaint dwarf galaxy Pictor II, showing the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with >3,000× relative carbon enhancement. The star’s exceptional paucity in iron and calcium make it clearly preserve enrichment from the first stars in a relic dwarf galaxy; Pictor II is one of the smallest, most chemically primordial systems known. This star supports the hypothesis that extreme carbon enhancement results from low-energy supernovae from the first stars, as the yields of energetic supernovae are harder to retain in small-scale environments. This signature of enrichment by the first stars may trace a regime inaccessible to current high-redshift observations, which can hardly detect the initial enrichment of the smallest galaxies.
