Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-05-06
Astronomy and Astrophysics
Astrophysics
11 pages, 5 figures, accepted by ApJ
Scientific paper
10.1086/589919
The observed rapid onset of star formation in molecular clouds requires rapid formation of dense fragments which can collapse individually before being overtaken by global gravitationally-driven flows. Many previous investigations have suggested that supersonic turbulence produces the necessary fragmentation, without addressing however the source of this turbulence. Motivated by our previous (numerical) work on the flow-driven formation of molecular clouds, we investigate the expected timescales of the dynamical and thermal instabilities leading to the rapid fragmentation of gas swept up by large-scale flows, and compare them with global gravitational collapse timescales. We identify parameter regimes in gas density, temperature and spatial scale within which a given instability will dominate. We find that dynamical instabilities disrupt large-scale coherent flows via generation of turbulence, while strong thermal fragmentation amplifies the resulting low-amplitude density perturbations, thus leading to small-scale, high-density fragments as seeds for {\em local} gravity to act upon. Global gravity dominates only on the largest scales; large-scale gravitationally-driven flows promote the formation of groups and clusters of stars formed by turbulence, thermal fragmentation, and rapid cooling.
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Burkert Andreas
Hartmann Lee
Heitsch Fabian
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