Investigating r-Process Diversity in Mildly Metal-Poor Stars Using the Th II 5989 Angstrom Line

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 < $ [Fe/H] $< -0.5$) is poorly constrained due to blending issues in the commonly used 4019 angstrom line. We present a systematic abundance analysis of mildly metal-poor stars, exploiting the relatively unblended Th II 5989 angstrom 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 SMOKA. We successfully detected Th in 32 out of 42 sample stars. Our analysis demonstrates that the 5989 angstrom line is a useful diagnostic for Th abundance measurements in cool red giants. The mean [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 Th/Eu production ratios. While the [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 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.