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A simple law of star formation

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We show that supersonic MHD turbulence yields a star formation rate (SFR) as low as observed in molecular clouds, for characteristic values of the free-fall time divided by the dynamical time, t ff/t dyn, the Alfvénic Mach number, {\cal M}_a, and the sonic Mach number, {\cal M}_s. Using a very large set of deep adaptive-mesh-refinement simulations, we quantify the dependence of the SFR per free-fall time, epsilonff, on the above parameters. Our main results are (1) that epsilonff decreases exponentially with increasing t ff/t dyn, but is insensitive to changes in {\cal M}_s, for constant values of t ff/t dyn and {\cal M}_a. (2) Decreasing values of {\cal M}_a (stronger magnetic fields) reduce epsilonff, but only to a point, beyond which epsilonff increases with a further decrease of {\cal M}_a. (3) For values of {\cal M}_a characteristic of star-forming regions, epsilonff varies with {\cal M}_a by less than a factor of two. We propose a simple star formation law, based on the empirical fit to the minimum epsilonff, and depending only on t ff/t dyn: epsilonff ˜ epsilonwindexp (– 1.6 t ff/t dyn). Because it only depends on the mean gas density and rms velocity, this law is straightforward to implement in simulations and analytical models of galaxy formation and evolution.
OriginalsprogEngelsk
ArtikelnummerL27
TidsskriftThe Astrophysical Journal Letters
Vol/bind759
Antal sider5
ISSN0004-637X
DOI
StatusUdgivet - 2012

ID: 42036930