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.
Categories: astro-ph.SR