The Spatial Structure of Young Stellar Clusters

Open Access
Author:
Kuhn, Michael Ascher
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 25, 2014
Committee Members:
  • Eric D Feigelson, Dissertation Advisor
  • Konstantin V Getman, Dissertation Advisor
  • G Jogesh Babu, Committee Member
  • Leisa K Townsley, Committee Member
  • Kevin Luhman, Committee Member
  • Richard Alan Wade, Committee Member
Keywords:
  • Astronomy
  • Star Formation
  • Star Clusters
  • X-ray Astronomy
  • Infrared Astronomy
Abstract:
Star formation is an extremely active area of astronomical research, and young stellar clusters in our Galaxy offer a useful laboratory where star-formation processes can be studied. Young stars form from the the gravitational collapse of molecular clouds that have a hierarchical spatial structure. This leads to stars forming in clustered environments, often with thousands of other young stars in environments that are strongly affected by feedback from massive O-type stars. The environments in these massive star-forming regions (MSFR) can affect how stars form and whether the young stellar clusters remain bound after star formation ends, both of which are questions that have received considerable attention from researchers. Studies of stellar populations in Galactic MSFRs are made difficult due to large numbers of fields stars in the Galactic Plane, large areas of the sky that must be surveyed, high optical extinction from dust, and nebulosity in the the optical and infrared. The Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) uses multiwavelength observations to overcome some of these difficulties, providing some of the most complete, clean membership lists for 20 MSFRs within 3.6 kpc of the Sun. I described X-ray catalogs and mid-infrared catalogs that were used in this survey. The spatial distribution of young stars in 17 MYStIX regions are used to probe the origin and dynamics of the young stellar clusters. Intrinsic stellar surface-density maps are made for each region, which reveal complex structures with dense subclusters. I examine in detail one of the nearest MYStIX young stellar clusters, W~40 (d=500 pc), which has properties similar to many of the subclusters in more massive and more distant star-forming regions. The cluster in W~40 has a simple structure with mass segregation, indicating that it has undergone dynamical evolution, even though its young age (~0.8 Myr) is insufficient for relaxation from two-body interactions. This apparent contradiction may be evidence of more rapid dynamical evolution accelerated by the merger of subclusters. Overall, 142 subclusters of young stars are found in the 17 MSFRs using the statistical "finite-mixture model" cluster analysis method, and the intrinsic stellar populations for these clusters are inferred using "initial mass functions" and "X-ray luminosity functions." Four structural classes are seen in MSFR: linear chains of subclusters, clumpy structures, core-halo structures, and simple isolated clusters. The subclusters do follow the structure of the molecular clouds, but do not appear to be coeval with each other. There is strong evidence in the subcluster properties for gas expulsion and subcluster expansion (e.g., the density~radius and age~radius relations), and evidence that is consistent with subcluster mergers (e.g., the ellipticity distribution and the number~density}$ relation). The cluster analysis provides evidence to support hierarchical models of stellar cluster formation, which have been theorized to explain mass segregation and dynamical relaxation in very young clusters. The ~1 Myr age spreads in the subclusters of a MSFR appear to require slower star-formation in giant molecular clouds with continually driven turbulence, rather than clouds with rapidly decaying turbulence. And, the diverse range of stellar surface density environment in MSFRs will have implications for models of cluster survival after gas removal.