The Actinide-Boost Star LAMOST J122216.85-063345.2: A Detailed R-process Abundance Study with Gemini-S/GHOST
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2026
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| author | Jeong, Miji Lee, Young Sun Placco, Vinicius M. Hirai, Yutaka Beers, Timothy C. Heo, Jeong-Eun Chiboucas, Kristin Deibert, Emily Gamen, Roberto Kalari, Venu M. Martioli, Eder Diaz, Ruben Gomez-Jimenez, Manuel Ruiz-Carmona, Roque Burley, Greg Robertson, J. Gordon Labrie, Kathleen Thomas-Osip, Joanna |
| author_facet | Jeong, Miji Lee, Young Sun Placco, Vinicius M. Hirai, Yutaka Beers, Timothy C. Heo, Jeong-Eun Chiboucas, Kristin Deibert, Emily Gamen, Roberto Kalari, Venu M. Martioli, Eder Diaz, Ruben Gomez-Jimenez, Manuel Ruiz-Carmona, Roque Burley, Greg Robertson, J. Gordon Labrie, Kathleen Thomas-Osip, Joanna |
| contents | We present a detailed chemical-abundance analysis of an actinide-boost ($\logε$(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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_29246 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | The Actinide-Boost Star LAMOST J122216.85-063345.2: A Detailed R-process Abundance Study with Gemini-S/GHOST Jeong, Miji Lee, Young Sun Placco, Vinicius M. Hirai, Yutaka Beers, Timothy C. Heo, Jeong-Eun Chiboucas, Kristin Deibert, Emily Gamen, Roberto Kalari, Venu M. Martioli, Eder Diaz, Ruben Gomez-Jimenez, Manuel Ruiz-Carmona, Roque Burley, Greg Robertson, J. Gordon Labrie, Kathleen Thomas-Osip, Joanna Astrophysics of Galaxies Solar and Stellar Astrophysics We present a detailed chemical-abundance analysis of an actinide-boost ($\logε$(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. |
| title | The Actinide-Boost Star LAMOST J122216.85-063345.2: A Detailed R-process Abundance Study with Gemini-S/GHOST |
| topic | Astrophysics of Galaxies Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2603.29246 |