Theoretical Study of i -process Contribution to the Post-AGB Star IRAS 14325-6428

Abstract The post-asymptotic giant branch (post-AGB) star IRAS 14325-6428 exhibits a peculiar heavy-element abundance pattern, characterized by a high [Ba/La] ratio and an anomalous odd-isotope Ba ratio ( f odd,Ba = 0.25 ± 0.08), which is difficult to explain via the classical slow neutron-capture process ( s -process). We theoretically investigate this abundance pattern by considering the intermediate neutron-capture process ( i -process) nucleosynthesis. The i -process nucleosynthesis calculations are performed with a one-zone network (NucNet Tools) at constant temperature and density, using intershell abundances obtained from a stellar model (1.5 M ⊙ , Z = 0.003) evolved with MESA to the thermally pulsing AGB phase. The observed heavy-element abundances are best reproduced by a model with neutron density N n = 5 × 10 13 cm −3 and neutron exposure τ = 1.4 mbarn −1 , followed by dilution with a scaled-solar composition. This model reproduces the high [Ba/La] and [Ba/Ce] ratios and yields a postdilution odd-isotope fraction f odd,Ba ≈ 0.51, which is supported by the observed value being substantially above the expectations of the pure s -process. The abundance pattern arises from the i -process path approaching the neutron magic number N = 82, where the flow slows, and the material accumulates at 135 I, which subsequently β − decays to 135 Ba, enhancing Ba production and reducing the synthesis of heavier species. Combined with its multielement i -process classification, our results provide quantitative support for a dominant i -process origin of the heavy elements in IRAS 14325−6428, likely triggered by a relatively short-lived proton-ingestion episode in its AGB progenitor.