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

Abstract

The young cluster NGC 6231 (stellar ages ∼2-7 Myr) is observed shortly after star-formation activity has ceased. Using the catalog of 2148 probable cluster members obtained from Chandra, VVV, and optical surveys (Paper I), we examine the cluster's spatial structure and dynamical state. The spatial distribution of stars is remarkably well fit by an isothermal sphere with moderate elongation, while other commonly used models like Plummer spheres, multivariate normal distributions, or power-law models are poor fits. The cluster has a core radius of 1.2 ± 0.1 pc and a central density of ∼200 stars pc −3 . The distribution of stars is mildly mass segregated. However, there is no radial stratification of the stars by age. Although most of the stars belong to a single cluster, a small subcluster of stars is found superimposed on the main cluster, and there are clumpy non-isotropic distributions of stars outside ∼4 core radii. When the size, mass, and age of NGC 6231 are compared to other young star clusters and subclusters in nearby active star-forming regions, it lies at the high-mass end of the distribution but along the same trend line. This could result from similar formation processes, possibly hierarchical cluster assembly. We argue that NGC 6231 has expanded from its initial size but that it remains gravitationally bound.

Kinematics in Young Star Clusters and Associations with Gaia DR2

“…Subclusters in the star-forming regions studied here were cataloged by Kuhn et al (2014) for MYStIX, Getman et al (2018) for SFiNCs, and Kuhn et al (2017a) for NGC6231. Subcluster identification in these papers was based on mixture models, a statistical cluster analysis method that is well adapted to cases where the size and density of clusters can vary and the number of clusters is uncertain (McLachlan & Peel 2000;Kuhn & Feigelson 2017).…”

“…In the plots for these three systems, we use the cluster core radius r c as a length scale in order to better compare with theoretical models for gravitationally bound clusters. The core radii were measured by Hillenbrand & Hartmann (1998) and Kuhn et al (2017aKuhn et al ( , 2014 for these three clusters, who updated them with the new distance estimates (Section 8.4). For the ONC and NGC 2362, velocity is nearly constant out to a radius of eight times r c , while for NGC 6231 it decreases steeply with radius-by a factor of ∼2 at a distance of 4r c from the center (Figure 13).…”

“…We find the bulk motions of subclusters by calculating the weighted-median velocities of their stellar members. In a few cases, subclusters have been combined when overlapping subclusters represent core-halo structures (Kuhn et al 2017a)-these are indicated in Table 4. The projected subcluster velocities, relative to the association rest frame, range up to ∼8 km s −1 , with a median value of ∼2 km s −1 and an interquartile range of 0.9-3.5 km s −1 .…”

On the spatial distributions of dense cores in Orion B

“…The spatial distribution of the most massive stars in star-forming regions has been the topic of numerous observational (Hillenbrand & Hartmann 1998;Raboud & Mermilliod 1998;de Grijs et al 2002;Littlefair et al 2004;Allison et al 2009;Wright et al 2014;Kuhn et al 2017;Parker & Alves de Oliveira 2017) and theoretical studies (Bonnell & Davies 1998;Moeckel & Bonnell 2009a,b;Allison et al 2010;Maschberger & Clarke 2011;Olczak, Spurzem & Henning 2011;Girichidis et al 2012;Parkeretal.2014;Kuznetsova, Hartmann & Ballesteros-Paredes 2015;D o ḿ ınguez et al 2017), with the goal of understanding if the formation channel of massive stars produces a different spatial distribution to that of low-mass starsso-called mass segregation. Initially, mass segregation was thought to be a natural outcome of the competitive accretion theory for star formation (Zinnecker 1982;Bonnell, Bate & Zinnecker 1998;Bonnell et al 2001), where the most massive stars would from in the most gas-rich regions of the cluster, which, in turn, would likely be the more central regions.…”

The low-mass pre-main sequence population of Scorpius OB1

“…to the East from the Tr 24 density peak: this group is named C 1651-408 in SIMBAD, and was also recently recovered in the study by Kuhn et al (2017b, their Figure 11). It is somewhat puzzling that this small cluster is not seen among Hα-emission or IR-excess stars (Figure 10d), nor in X-rays (Figure 12 below), and therefore increased extinction does not seem responsible for its non-detection as a distinct peak in the M-star sample; we conclude that this cluster is unlikely to be very young, despite lying very close to Tr 24 on the sky.…”