The CNO Sum and Turnoff Ages in Clusters With Anomalous Light-Element Mixtures

In most clusters the light elements trade against each other — carbon down, nitrogen up — while their sum stays nearly constant, and stellar evolution, which cares chiefly about the sum through the CNO cycle, is unmoved. A few clusters break that rule: sub-populations exist whose total carbon, nitrogen, and oxygen is genuinely higher. This study computes the age consequence. Doubling the CNO sum shifts the age inferred at fixed turnoff luminosity by 0.85 billion years, and an enhancement of sixty percent — of the size reported in the best-studied anomalous cluster — shifts it by 0.58 billion years. A single age fitted to a mixed diagram of such a cluster averages two populations of genuinely different inferred age, and the split subgiant branches observed in these systems are the visible signature of exactly this effect. The response is computed from published CNO-enhanced model comparisons; no new abundances are measured here.