Intracluster Age Gradients In Numerous Young Stellar Clusters

TL;DR

This study reveals that most young stellar clusters exhibit age gradients with younger stars in the core and older stars in the outskirts, supporting models of hierarchical star formation.

Contribution

It introduces a new age estimator, AgeJX, and provides the first comprehensive analysis of age gradients in numerous young stellar clusters.

Findings

80% of clusters show core regions younger than outer regions

Age gradients range from 0.75 to 1.5 Myr/pc

Supports hierarchical collapse models of star formation

Abstract

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).