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The formation of stars by gravitational collapse rather than competitive accretion


There are two dominant models of how stars form. Under gravitational collapse, star-forming molecular clumps, of typically hundreds to thousands of solar masses (M), fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems1,2,3. In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (0.5M), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump4,5,6,7,8. Competitive accretion models interpret brown dwarfs and free-floating planets as protostars ejected from star-forming clumps before they have accreted much mass; key predictions of this model are that such objects should lack disks, have high velocity dispersions, form more frequently in denser clumps9,10,11, and that the mean stellar mass should vary within the Galaxy8. Here we derive the rate of competitive accretion as a function of the star-forming environment, based partly on simulation12, and determine in what types of environments competitive accretion can occur. We show that no observed star-forming region can undergo significant competitive accretion, and that the simulations that show competitive accretion do so because the assumed properties differ from those determined by observation. Our result shows that stars form by gravitational collapse, and explains why observations have failed to confirm predictions of the competitive accretion model.

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We thank R. T. Fisher for discussions and P. Padoan for comments. This work was supported by grants from NASA through the Hubble Fellowship, GSRP and ATP programmes, by the NSF, and by the US DOE through the Lawrence Livermore National Laboratory. Computer simulations for this work were performed at the San Diego Supercomputer Center (supported by the NSF), the National Energy Research Scientific Computer Center (supported by the US DOE), and Lawrence Livermore National Laboratory (supported by the US DOE). M.R.K. is a Hubble Fellow.

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Correspondence to Mark R. Krumholz.

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Krumholz, M., McKee, C. & Klein, R. The formation of stars by gravitational collapse rather than competitive accretion. Nature 438, 332–334 (2005).

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