Quantifying Systematic Age Discrepancies in Very Young Star Clusters

Abstract We infer the ages of three young stellar clusters—NGC 2004, NGC 7419, and NGC 2100—using Stellar Ages, a statistical algorithm designed to infer stellar population properties from color–magnitude diagrams. Recent studies have revealed emerging inconsistencies in the inferred ages of very young (≤50 Myr old) stellar clusters. Here, we identify and quantify two distinct discrepancies. First, ages inferred from red supergiants (RSGs) are systematically older—by about a factor of 4—than those inferred from bright blue stars, with a median age offset of Δ log 10 ( t / yr ) = 0.60 ± 0.06 dex ( t B ≈ t RSG /4.0). Second, given the observed numbers of RSGs and bright blue stars, there is a pronounced deficit of lower-mass main-sequence stars relative to expectations from a standard initial mass function. Although these discrepancies resemble those reported for intermediate-age clusters, their magnitude and character suggest that they are unique to the evolution of massive stars. Together, these results highlight population-level inconsistencies with single-star evolutionary models and underscore the need to consider multiple evolutionary tracers when age-dating young clusters. By combining individual stellar ages with population-wide constraints, our approach refines prior work on cluster age determinations and provides new insight into massive star evolution and the interpretation of cluster demographics.