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arXiv · 2509.05015

s-process Enriched Post-AGB Star J003643.94-723722.1 in the SMC with an Extreme C/O Ratio and the First Precise Detection of Lead

Meghna Menon·Devika Kamath·Maksym Mohorian·Anish M. Amarsi·Diego Vescovi·Sergio Cristallo·Amanda Karakas·Hans Van Winckel·Paolo Ventura
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Abstract

Post-asymptotic giant branch (post-AGB) stars are exquisite tracers of s-process nucleosynthesis via their surface abundances. We present a comprehensive analysis of J003643.94$-$723722.1 (J003643), a single SMC post-AGB star, using high-resolution UVES/VLT spectra analysed with E-iSpec. We find C/O = 16.21 and $[\mathrm{s}/\mathrm{Fe}]=2.09\pm0.20 \mathrm{dex}$. We also report the first direct Pb detection in a post-AGB star from the Pb II 5608.853 A line, with $[\mathrm{Pb}/\mathrm{Fe}]=3.18\,\mathrm{dex}$. Comparison with post-AGB samples in the Galaxy and Magellanic Clouds reveals that J003643 has an unusually high C/O ratio. J003643's $[\mathrm{C}/\mathrm{Fe}]=1.33\pm0.14 \mathrm{dex}$ and $[\mathrm{s}/\mathrm{Fe}]=2.09\pm0.20\,\mathrm{dex}$ are consistent with third dredge-up enrichment, but its $[\mathrm{O}/\mathrm{Fe}]=-0.08\pm0.20\,\mathrm{dex}$ is low relative to objects of similar $[\mathrm{C}/\mathrm{Fe}]$ and $[\mathrm{Fe}/\mathrm{H}]$. Together with $[\alpha/\mathrm{Fe}]\approx0\,\mathrm{dex}$ at $[\mathrm{Fe}/\mathrm{H}]\approx-1\,\mathrm{dex}$, consistent with SMC chemical evolution, this indicates the high C/O chiefly reflects oxygen deficiency rather than exceptional carbon enrichment. Additionally, we compare the full abundance pattern with yields from ATON, MONASH and FRUITY (the latter two with post-processing nucleosynthesis). Most elements are reproduced, but Pb is strongly underpredicted, highlighting a persistent gap in models of heavy-element production in AGB stars. The photospheric chemistry of J003643 adds to the growing diversity among post-AGB stars and underscores the complexity of AGB nucleosynthesis.

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Meghna Menon, Devika Kamath, Maksym Mohorian, Anish M. Amarsi, Diego Vescovi, Sergio Cristallo, Amanda Karakas, Hans Van Winckel, Paolo Ventura. 2025-09-05. s-process Enriched Post-AGB Star J003643.94-723722.1 in the SMC with an Extreme C/O Ratio and the First Precise Detection of Lead. https://doi.org/10.1017/pasa.2025.10085

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