THE FORMATION OF YOUNG DENSE STAR CLUSTERS THROUGH MERGERS

• Published in: The Astrophysical journal Vol. 753; no. 1; pp. 1 - 12
• Main Authors: FUJII, M. S, SAITOH, T. R, PORTEGIES ZWART, S. F
• Format: Journal Article
• Language: English
• Published: Bristol IOP 01-07-2012
• Subjects: Acquisitions; ASTRONOMY; ASTROPHYSICS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Clusters; COMPUTERIZED SIMULATION; DENSITY; Earth, ocean, space; Evolution; Exact sciences and technology; Formations; GALACTIC EVOLUTION; GALAXY CLUSTERS; MAGELLANIC CLOUDS; MASS; MASS DISTRIBUTION; Massive stars; MILKY WAY; RELAXATION; Relaxation time; SEGREGATION; STAR CLUSTERS; STARS; Runaway stars; Young cluster; Digital simulation; galaxies: star clusters: individual (R136); Mass distribution; N body system; Open clusters; Numerical method; Star clusters; Milky Way; methods: numerical; Relaxation time; Massive stars; Galaxy clusters; Young stars; Spatial distribution; Dynamical evolution; open clusters and associations: individual (NGC 3603, Westerlund 1, Westerlund 2); Large Magellanic Cloud
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637x/753/1/85

Young star clusters such as NGC 3603 and Westerlund 1 and 2 in the Milky Way and R136 in the Large Magellanic Cloud are dynamically more evolved than expected based on their current relaxation times. In particular, the combination of a high degree of mass segregation, a relatively low central density, and the large number of massive runaway stars in their vicinity are hard to explain with the monolithic formation of these clusters. Young star clusters can achieve such a mature dynamical state if they formed through the mergers of a number of less massive clusters. The shorter relaxation times of less massive clusters cause them to dynamically evolve further by the time they merge, and the merger product preserves the memory of the dynamical evolution of its constituent clusters. With a series of N-body simulations, we study the dynamical evolution of single massive clusters and those that are assembled through merging smaller clusters together. We find that the formation of massive star clusters through the mergers of smaller clusters can reproduce the currently observed spatial distribution of massive stars, the density, and the characteristics (number and mass distribution) of the stars ejected as runaways from young dense clusters. We therefore conclude that these clusters and possibly other young massive star clusters formed through the mergers of smaller clusters.