The Spatial Structure of Young Stellar Clusters. I. Subclusters

Abstract

The clusters of young stars in massive star-forming regions show a wide range of sizes, morphologies, and numbers of stars. Their highly subclustered structures are revealed by the MYStIX project's sample of 31,754 young stars in nearby sites of star formation (regions at distances <3.6 kpc that contain at least one O-type star.) In 17 of the regions surveyed by MYStIX, we identify subclusters of young stars using finite mixture models -collections of isothermal ellipsoids that model individual subclusters. Maximum likelihood estimation is used to estimate the model parameters, and the Akaike Information Criterion is used to determine the number of subclusters. This procedure often successfully finds famous subclusters, such as the BN/KL complex behind the Orion Nebula Cluster and the KW-object complex in M 17. A catalog of 142 subclusters is presented, with 1 to 20 subclusters per region. The subcluster core radius distribution for this sample is peaked at 0.17 pc with a standard deviation of 0.43 dex, and subcluster core radius is negatively correlated with gas/dust absorption of the stars -a possible age effect. Based on the morphological arrangements of subclusters, we identify four classes of spatial structure: long chains of subclusters, clumpy structures, isolated clusters with a core-halo structure, and isolated clusters well fit by a single isothermal ellipsoid.Subject headings: methods: statistical; open clusters and associations: general; stars: formation; stars: pre-main sequence; H ii regions; ISM: structure 7 These libraries can be installed from the R session by install.packages ("spatstat",dependencies=T)

The Structure of the Young Star Cluster NGC 6231. II. Structure, Formation, and Fate

“…To account for elongation of subclusters, Kuhn et al (2014) introduced additional parameters to the model for ellipticity, , and orientation, φ, in projection on the sky. Equations 1-4 can be redefined to describe ellipsoidal distributions if we redefine R as…”

“…This method of cluster analysis was used by Kuhn et al (2014) to identify subclusters in MYStIX. For NGC 6231, two subclusters are suspected: the main cluster and the possible subcluster to the north-west -the smaller subcluster is designated Subcl.…”

“…The identification of multiple statistical clusters follows the same method as Kuhn et al (2014). Models with G = 0, 1, 2, 3, 4, 5, and 6 are fit, and the AIC and BIC calculated.…”

“…These elongated clouds may imprint their structure on the star clusters that form within them. For example, in the MYStIX study, several examples of "linear chains of subclusters" are noted (Kuhn et al 2014, their Figure 5). In some of these, like DR 21, the young stars are deeply embedded in a massive infrared-dark filament.…”

“…These include methods for identifying and modeling subclusters of stars (Kuhn et al 2014), methods for analyzing the intrinsic densities of stars in clusters (Kuhn et al 2015b), methods for analyzing mass segregation (M. A. Kuhn et al, in preparation), and methods for identifying gradients in stellar ages (Getman et al 2014a,b).…”