Work

Investigating r -process diversity in mildly metal-poor stars using the Th ii 5989 Å line

Year: 2026

Type: article

Abstract: The origin of the heaviest elements remains a fundamental mystery, with the “actinide-boost” (Thorium overabundance) serving as a critical diagnostic of r-process nucleosynthesis conditions. While this diversity is well documented in extremely metal-poor (EMP) stars, its frequency in the mildly metal-poor (MMP) regime ($-2.0 \lt [{\rm Fe/H}] \lt -0.5$) is poorly constrained due to blending issues in the commonly used 4019 Å line. We present a systematic abundance analysis of MMP stars, exploiting the relatively unblended Th ii 5989 Å line. High-dispersion spectra were obtained using the 1.5 m telescope at Gunma Astronomical Observatory with GAOES, complemented by archival data from the Subaru Telescope/HDS retrieved via Subaru Mitaka Okayama Kiso Archive (SMOKA). We successfully detected Th in 32 out of 42 sample stars. Our analysis demonstrates that the 5989 Å line is a useful diagnostic for Th abundance measurements in cool red giants. The mean $[{\rm Th/Eu}]$ ratio remains nearly constant over the MMP metallicity range, suggesting that progressive homogenization of the interstellar medium during Galactic chemical evolution would reduce abundance variations inherited from individual r-process events, or the dominant r-process events may share similar initial ${\rm Th/Eu}$ production ratios. While the $[{\rm Th/Eu}]$ ratio is nearly constant overall in the MMP regime, its dispersion appears to increase toward the metal-rich end of this regime and reaches approximately 1 dex near solar metallicity. This behavior may reflect the combined effects of variations in the initial ${\rm Th/Eu}$ production ratios of the progenitor objects, the mixing of r-process products in the interstellar medium, and the radioactive decay of Th over stellar lifetimes.

Source: Publications of the Astronomical Society of Japan

DOI: 10.1093/pasj/psag134

Authors: Kurumi Furutsuka, Satoshi Honda

Institutions: University of Hyogo, Nishi-Harima Astronomical Observatory

Language: English

Topic: Stellar, planetary, and galactic studies

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: green

Funders: Ministry of Education, Culture, Sports, Science and Technology, National Astronomical Observatory of Japan, Japan Society for the Promotion of Science

Awards: G2552440251, First stars nucleosyntheis and early galaxy formation explored by searches for bright metal-poor stars, Exploring the evolution and nucleosynthesis of massive stars and Galactic chemical evolution through nitrogen abundances in early generations of stars