Protostars and Planets VI, Heidelberg, July 15-20, 2013
MYStIX: Subclusters of Young Stars in Massive Star Forming Regions
Kuhn, Michael A. (Penn State University, USA)
Feigelson, Eric D. (Penn State University, USA)
Getman, Konstantin V. (Penn State University, USA)
Baddeley, Adrian (University of Western Australia, Australia)
Townsley, Leisa K. (Penn State University, USA)
Broos, Patrick S. (Penn State University, USA)
Povich, Matthew S. (California State Polytechnic University, USA)
Luhman, Kevin L. (Penn State University, USA)
Busk, Heather A. (Penn State University, USA)
Naylor, Tim (University of Exeter, UK)
King, Robert R. (University of Exeter, UK)
Garmire, Gordon P. (Penn State University, USA)
The MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray; Feigelson et al. 2013) project provides improved censuses of young stars in 20 nearby OB-dominated star-forming regions that were observed by the Chandra X-ray observatory, the Spitzer Space Telescope, and the UKIRT/UKIDSS and 2MASS surveys. The sample of >33,000 members reveals new details about the structure of clusters in these regions.
Clusters of young stars are identified using finite mixture models – the sums of isothermal ellipsoids used to model individual (sub)clusters. Maximum likelihood estimation is used to estimate the model parameters and the Akaike Information Criterion is used to detemine the number of subclusters. In the MYStIX star-forming regions, ∼150 subclusters are found (1 to >10 per region). The distribution of cluster core radii is log-normal, peaked at 0.18 pc (similar to the ONC) with a standard deviation of 0.4 dex. The locations of subclusters are often correlated with molecular-cloud clumps or cores. We also recover several well-known embedded subclusters such as the BN-KL region in Orion and the KW Object cluster in M 17.
MYStIX star-forming regions typically have one dominant cluster surrounded by smaller subclusters and filamentary groups of young stars. Some clusters are well fit by the ellipsoid model (e.g. Flame Nebula), but others have lumpy structure and are poorly fit (e.g. M 17). A few clusters have a core-halo structure modeled with two overlapping ellipsoids (e.g. RCW 36). Clumpy and core-halo structures could originate in the merger of subclusters. There is a power-law relation between the fitted cluster central density and core radius (index slightly shallower than -3), which may be an effect of cluster expansion. There is also a statistically significant negative relation between median gas/dust absorption of a subcluster and the subcluster’s size that can also be explained by cluster expansion if absorption acts as a proxy for age.
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