The formation of a star cluster: predicting the properties of stars and brown dwarfs

Abstract

We present results from the largest numerical simulation of star formation to\nresolve the fragmentation process down to the opacity limit. The simulation\nfollows the collapse and fragmentation of a large-scale turbulent molecular\ncloud to form a stellar cluster and, simultaneously, the formation of\ncircumstellar discs and binary stars. This large range of scales enables us to\npredict a wide variety of stellar properties for comparison with observations.\nThe calculation clearly demonstrates that star formation is a highly-dynamic\nand chaotic process. Star-disc encounters form binaries and truncate discs.\nStellar encounters disrupt bound multiple systems. The cloud produces roughly\nequal numbers of stars and brown dwarfs, with masses down to the opacity limit\nfor fragmentation (~5 Jupiter masses). The initial mass function is consistent\nwith a Salpeter slope (Gamma=-1.35) above 0.5 Msun, a roughly flat distribution\n(Gamma=0) in the range 0.006-0.5 Msun, and a sharp cutoff below ~0.005 Msun.\nThis is consistent with recent observational surveys. The brown dwarfs form by\nthe dynamical ejection of low-mass fragments from dynamically unstable multiple\nsystems before the fragments have been able to accrete to stellar masses. Close\nbinary systems (with separations <10 AU) are not formed by fragmentation in\nsitu. Rather, they are produced by hardening of initially wider multiple\nsystems through a combination of dynamical encounters, gas accretion, and/or\nthe interaction with circumbinary and circumtriple discs. Finally, we find that\nthe majority of circumstellar discs have radii less than 20 AU due to\ntruncation in dynamical encounters. This is consistent with observations of the\nOrion Trapezium Cluster and implies that most stars and brown dwarfs do not\nform large planetary systems.\n

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Affiliated Institutions

• Institute of Astronomy RU
• Center for Astrophysics Harvard & Smithsonian US
• University of Cambridge GB
• University of St Andrews GB
• University of Exeter GB

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