Modelling s-process chemical clocks: insights from high-precision Kepler data

Abstract

We present Galactic chemical evolution (GCE) models for the chemical clocks [Zr/Ti] and [Ce/Ti], tracing first- and second-peak s-process nucleosynthesis, and compare them with a high-precision sample of 68 Kepler red giant stars with asteroseismic ages from individual-mode frequencies and high-resolution spectroscopy. Using a multi-zone GCE framework, we explore variations in metallicity-dependent asymptotic giant branch (AGB) nucleosynthetic yields, including proposed enhancements to high-metallicity Ce production. Our baseline model reproduces [Zr/Ti] and the high-α sequence in both age and metallicity space, but systematically underestimates [Ce/Ti] at young ages and intermediate metallicities, indicating a persistent deficit in second-peak s-process enrichment over the last ∼6 Gyr of Galactic disc evolution. Increasing second-peak yields from high-metallicity AGB stars only partially reduces this discrepancy, suggesting that simple yield rescaling is insufficient and more fundamental revisions to s-process nucleosynthesis at high metallicity, alongside a self-consistent treatment of stellar dynamics, may be required. In fact, models reproduce abundance trends more tightly in metallicity than in age space, with additional age scatter partly attributed to radial migration. This Letter highlights the diagnostic power of precise asteroseismic ages for GCE studies and the limitations of current models in capturing the complex interplay between s-process nucleosynthesis and stellar dynamics.