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We analyze age distributions of two nearby rich stellar clusters, the NGC2024 (Flame Nebula) and Orion Nebula Cluster (ONC) in the Orion molecular cloudcomplex. Our analysis is based on samples from the MYStIX survey and a newestimator of pre-main sequence (PMS) stellar ages, AgeJX, derived from X-rayand near-infrared photometric data. To overcome the problem of uncertainindividual ages and large spreads of age distributions for entire clusters, wecompute median ages and their confidence intervals of stellar samples withinannular subregions of the clusters. We find core-halo age gradients in both theNGC 2024 cluster and ONC: PMS stars in cluster cores appear younger and thuswere formed later than PMS stars in cluster peripheries. These findings arefurther supported by the spatial gradients in the disk fraction and K-bandexcess frequency. Our age analysis is based on AgeJX estimates for PMS stars,and is independent of any consideration of OB stars. The result has importantimplications for the formation of young stellar clusters. One basic implicationis that clusters form slowly and the apparent age spreads in young stellarclusters, which are often controversial, are (at least in part) real. Theresult further implies that simple models where clusters form inside-out areincorrect, and more complex models are needed. We provide several starformation scenarios that alone or in combination may lead to the observedcore-halo age gradients.

Core-Halo Age Gradients and Star Formation in the Orion Nebula and NGC~2024 Young Stellar Clusters