THE SPATIAL STRUCTURE OF YOUNG STELLAR CLUSTERS. I. SUBCLUSTERS

Abstract

The clusters of young stars in massive star-forming regions show a wide range\nof sizes, morphologies, and numbers of stars. Their highly subclustered\nstructures are revealed by the MYStIX project's sample of 31,754 young stars in\nnearby sites of star formation (regions at distances <3.6 kpc that contain at\nleast one O-type star.) In 17 of the regions surveyed by MYStIX, we identify\nsubclusters of young stars using finite mixture models -- collections of\nisothermal ellipsoids that model individual subclusters. Maximum likelihood\nestimation is used to estimate the model parameters, and the Akaike Information\nCriterion is used to determine the number of subclusters. This procedure often\nsuccessfully finds famous subclusters, such as the BN/KL complex behind the\nOrion Nebula Cluster and the KW-object complex in M17. A catalog of 142\nsubclusters is presented, with 1 to 20 subclusters per region. The subcluster\ncore radius distribution for this sample is peaked at 0.17 pc with a standard\ndeviation of 0.43 dex, and subcluster core radius is negatively correlated with\ngas/dust absorption of the stars -- a possible age effect. Based on the\nmorphological arrangements of subclusters, we identify four classes of spatial\nstructure: long chains of subclusters, clumpy structures, isolated clusters\nwith a core-halo structure, and isolated clusters well fit by a single\nisothermal ellipsoid.\n