2024NatAs...8..216M

Age is one of the most fundamental parameters of a star, yet it is one of the hardest to determine as it requires modelling various aspects of stellar formation and evolution. When we compare the ages derived from isochronal and dynamical traceback methods for six young stellar associations, we find a systematic discrepancy. Specifically, dynamical traceback ages are consistently younger by an average of 〈Δ_Age〉 = 5.5 ± 1.1 Myr. We rule out measurement errors as the cause of the age mismatch and propose that Δ_Age indicates the time a young star remains bound to its parental cloud before moving away from its siblings. In this framework, the dynamical traceback 'clock' starts when a stellar cluster or association begins to expand after expelling most of the gas, whereas the isochronal 'clock' starts earlier when most stars form. The difference between these two age-dating techniques is a powerful tool for constraining evolutionary models, as isochronal ages cannot be younger than dynamical traceback ages. Measuring the Δ_Age accurately and understanding its variations across different environments will provide further information on the impact of local conditions and stellar feedback on the formation and dispersal of stellar clusters.