APOGEE Survey of 100000 Red Giants Shows [Ce/Mg] Age Correlation Varies Sharply by Metallicity and Radius

Liam Dubay and colleagues measured cerium and magnesium abundances in roughly 100000 APOGEE red giants and derived ages from asteroseismology and isochrones. They binned the sample by guiding radius and [Fe/H] and fit linear age-[Ce/Mg] relations in each bin. The slope reaches its maximum value in the outer disk below solar metallicity and drops to zero for stars with [Fe/H] greater than zero inside 8 kpc. Thick-disk stars maintain a uniform positive slope across all radii. The authors conclude that local star-formation history and AGB yield variations both modulate the ratio, violating the assumption of a single universal clock. The result directly challenges galactic chemical-evolution models that treat [s/α] as a monotonic function of time. Earlier claims of a tight chemical clock relied on solar-neighborhood samples that missed the inner-disk turnover now visible in the larger APOGEE footprint. The new map implies that any age inference from [Ce/Mg] must incorporate both position and metallicity as additional variables. Future spectroscopic surveys such as 4MOST and WEAVE will test whether the same radial-metallicity dependence appears in other s-process elements. If confirmed, chemical-dating pipelines used for upcoming Gaia DR4 and Roman Space Telescope stellar populations will require spatially varying calibrations rather than a single linear relation.