Saturday, October 22, 2005

How do stars form?

Stars Form By Gravitational Collapse, Not Competitive Accretion
There are now two dominant models of how stars form: gravitational collapse theory holds that star-forming molecular clumps, typically hundreds to thousands of solar masses in mass, fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems. In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (~0.5 solar masses), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump. Competitive accretion models explain brown dwarfs and free-floating planets as protostars ejected from star-forming clumps before accreting much mass, predicting that they should lack disks, have high velocity dispersions, and form more frequently in denser clumps. They also predict that mean stellar mass should vary within the Galaxy. Here we derive a simple estimate for the rate of competitive accretion as a function of the star-forming environment, based partly on simulations, and determine in what types of environments competitive accretion can occur. We show that no observed star-forming region produces significant competitive accretion, and that simulations that show competitive accretion do so because their 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 scenario
This sounds significant... Out of two plausible alternative models of star formation, only one is determined to be consistent with observation.

It's a top-down vs. bottom-up question. Does a large mass of matter first collapse to a denser state and then fragment, or do numerous small regions of the matter collapse indpendently directly to protostars? The latter possibility, here called "competitive accretion" makes predictions that don't agree with observations, so the other possibility, known as "gravitational collapse", is more likely.

More: How do stars form?

Press release: Astrophysicists put kibosh on alternative theory of star formation

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