Intracluster Age Gradients In Numerous Young Stellar Clusters
  The pace and pattern of star formation leading to rich young stellar clusters
is quite uncertain. In this context, we analyze the spatial distribution of
ages within 19 young (median t<3 Myr on the Siess et al. (2000) timescale),
morphologically simple, isolated, and relatively rich stellar clusters. Our
analysis is based on young stellar object samples from the MYStIX and SFiNCs
surveys, and a new estimator of pre-main sequence (PMS) stellar ages, AgeJX,
derived from X-ray and near-infrared photometric data. Median cluster ages are
computed within four annular subregions of the clusters. We confirm and extend
the earlier result of Getman et al. (2014): 80% percent of the clusters show
age trends where stars in cluster cores are younger than in outer regions. Our
cluster stacking analyses establish the existence of an age gradient to high
statistical significance in several ways. Time scales vary with the choice of
PMS evolutionary model; the inferred median age gradient across the studied
clusters ranges from 0.75 Myr/pc to 1.5 Myr/pc. The empirical finding reported
in the present study -- late or continuing formation of stars in the cores of
star clusters with older stars dispersed in the outer regions -- has a strong
foundation with other observational studies and with the astrophysical models
like the global hierarchical collapse model of Vazquez-Semadeni et al. (2017).
