Astrophys. J., 759, L27 (2012/November-2)
A simple law of star formation.
PADOAN P., HAUGBOLLE T. and NORDLUND A.
Abstract (from CDS):
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, tff/tdyn, the Alfvénic Mach number, Ma, and the sonic Mach number, Ms. Using a very large set of deep adaptive-mesh-refinement simulations, we quantify the dependence of the SFR per free-fall time, εff, on the above parameters. Our main results are (1) that εff decreases exponentially with increasing tff/tdyn, but is insensitive to changes in Ms, for constant values of tff/tdyn and Ma. (2) Decreasing values of Ma (stronger magnetic fields) reduce εff, but only to a point, beyond which εff increases with a further decrease of Ma. (3) For values of Ma characteristic of star-forming regions, εff varies with Ma by less than a factor of two. We propose a simple star formation law, based on the empirical fit to the minimum εff, and depending only on tff/tdyn: εff ~ εwind exp (- 1.6 tff/tdyn). 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.
ISM: kinematics and dynamics - magnetohydrodynamics, MHD - stars: formation - turbulence
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