ar X iv : a st ro - p h / 03 01 09 3 v 1 6 J an 2 00 3 Control of star formation by supersonic turbulence

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that support by supersonic turbulence rather than magnetic fields controls star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review begins with a historical overview of the successes and problems of both the classical dynamical theory, and the standard theory of magnetostatic support from both observational and theoretical perspectives. The outline of the new theory relying on support from driven supersonic turbulence is then presented. Numerical models demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. It suggests that individual star-forming cores are likely not quasi-static objects, but already dynamically collapsing. Accretion onto these objects will vary depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass function may be determined by the turbulent flow as well. Global star formation in galaxies is likely controlled by the balance between gravity and turbulence as well, possibly modulated by cooling and rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.