The Actinide-Boost Star LAMOST J122216.85-063345.2: A Detailed R-process Abundance Study with Gemini-S/GHOST

TL;DR

This study analyzes the detailed chemical composition of the actinide-boost star J1222, revealing its r-process element enrichment likely originated from neutron star mergers and black hole-neutron star mergers, with implications for understanding nucleosynthesis and galactic chemical evolution.

Contribution

First detailed high-resolution abundance analysis of an actinide-boost star using Gemini-S/GHOST, linking its r-process pattern to specific astrophysical merger events.

Findings

Abundance pattern consistent with neutron star merger yields.

J1222 is part of the I'itoi substructure with diverse r-process enrichment.

Multiple r-process events and inhomogeneous mixing shaped the enrichment history.

Abstract

We present a detailed chemical-abundance analysis of an actinide-boost ($\log\epsilon$(Th/Dy) = -0.74) star, LAMOST J122216.85-063345.2 (J1222), a very metal-poor ([Fe/H] = -2.45) halo star with moderate enhancement in rapid neutron-capture ($r$-)process elements ([Eu/Fe] = +0.61). From high-resolution spectra (R $\sim$ 55,000) taken with Gemini-S/GHOST, we determine the abundances for 47 elements, including thorium. The abundance pattern of J1222 is consistent with predicted nucleosynthetic yields from neutron star mergers (NSMs) and black hole-neutron star mergers (BH-NSMs), under specific ejecta conditions. Our kinematic analysis of J1222 indicates that it is a member of the I'itoi substructure. A comparative analysis of J1222 and seven other stars from the literature with similar dynamics to the I'itoi substructure exhibits a broad dispersion in $r$-process enrichment - spanning non-enhancement ([Eu/Fe] $\leq$ +0.3), moderate enhancement (+0.3 $<$ [Eu/Fe] $\leq$ +0.7), strong enhancement ([Eu/Fe] $>$ +0.7), and actinide-boost stars (including one additional actinide-boost candidate newly recognized to be associated with I'itoi) - suggesting a complex enrichment history shaped by multiple $r$-process events and inhomogeneous mixing. After exploring several astrophysical scenarios to explain the observed $r$-process abundances, we find that NSMs and BH-NSMs were likely the main contributors to the enrichment, while magneto-rotational supernovae (MR-SNe) may have played a secondary role in enriching some light $r$-process element-rich stars in the I'itoi substructure.