Speaker
Description
We present a detailed chemical-abundance analysis of an actinide-boost (log ϵ (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 ∼ 55,000) taken with Gemini-S/GHOST, we determine abundances for 43 elements, including thorium. The abundance pattern of J1222 is consistent with predicted nucleosynthetic yields from neutron star mergers (NSMs) or black hole-neutron star mergers (BH-NSMs), under specific ejecta conditions. Our kinematic analysis of J1222 indicates that it belongs to 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] ≤ +0.3), moderate enhancement (+0.3 < [Eu/Fe] ≤ +0.7), strong enhancement ([Eu/Fe] > +0.7), and actinide-boost stars (including an additional candidate actinide-boost star with a high upper limit on thorium 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. We also report on an ongoing analysis of the uranium abundance in this star.
| Career stage | Tenured mid-to-late-career researcher |
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