Chemical clocks and their time zones: understanding the [s/Mg]–age relation with birth radii

The relative enrichment of s-process to α-elements ([s/α]) has been linked with age, providing a potentially useful avenue in exploring the Milky Way’s chemical evolution. However, the age–[s/α] relationship is non-universal, with dependencies on metallicity and current location in the Galaxy. In this work, we examine these chemical clock tracers across birth radii (Rbirth ), recovering the inherent trends between the variables. We derive Rbirth and explore the [s/α]–age–Rbirth relationship for 36 652 APOGEE DR17 red giant and 24 467 GALAH DR3 main-sequence turn-off and subgiant branch disc stars using [Ce/Mg], [Ba/Mg], and [Y/Mg]. We discover that the age–[s/Mg] relation is strongly dependent on birth location in the Milky Way, with stars born in the inner disc having the weakest correlation. This is congruent with the Galaxy’s initially weak, negative [s/Mg] radial gradient, which becomes positive and steep with time. We show that the non-universal relations of chemical clocks is caused by their fundamental trends with Rbirth over time, and suggest that the tight age–[s/Mg] relation obtained with solar-like stars is due to similar Rbirth for a given age. Our results are put into context with a Galactic chemical evolution model, where we demonstrate the need for data-driven nucleosynthetic yields.