The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars

Abstract

Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. For the subset of post-AGB stars with low metallicities the determined upper limits based on detailed chemical abundance studies are much lower than what is predicted. Recent theoretical studies have pointed to the occurrence of the i-process to explain the observed chemical patterns, especially of Pb. A major development, in the observational context, is the release of the GAIA EDR3 parallaxes of the post-AGBs in the Galaxy, which has opened the gateway to systematically studying the sample of stars as a function of current luminosities (which can be linked to their initial masses). In this paper, we succinctly review the Pb discrepancy in post-AGB stars and present the latest observational and theoretical developments in this research landscape.

Post-AGB Stars as Tracers of AGB Nucleosynthesis: An Update

“…The observationally derived Pb upper limits are significantly lower than what are predicted by models [55]. We refer to [28] for a full description of what we call the Pb problem. This is likely linked to the lack of correlation between the hs/ls ratio and metallicity.…”

“…This sheds the entire outer envelope, and for an astronomically brief moment (<10,000 years)referred to as the post-AGB phase-the chemically enriched surface of the star is exposed [23]. This makes post-AGB stars formidable probes to examine the elements produced by the star during and prior to the AGB phase (e.g., [24][25][26][27][28]).…”

“…To date, chemical abundance studies of post-AGB stars in the Galaxy, LMC and SMC (see [27,28,45] and references therein) have shown an intriguing chemical diversity that ranges from stars that are extremely enriched in carbon and s-process elements [25,26,45] to the discovery of the first post-AGB star with no traces of carbon nor s-process elements [45,48]. Additionally, stellar nucleosynthesis is significantly affected by a binary companion [43,49,50].…”

“…While a review on the i-process is beyond the scope of this paper, both the Pb problem and the presence of a wide variety of neutron exposures within the post-AGB sample are indicative of the presence of irradiation at intermediate neutron densities (e.g., [55,72]). For a more elaborate discussion on the i-process within post-AGB stars, we refer to [28].…”