Control of star formation by supersonic turbulence

Abstract

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 supersonic turbulent flows rather than static magnetic fields control 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 gives 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 a new theory relying on control by 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 dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. We suggest that global star formation in galaxies is controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential 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.

Biases on Initial Mass Function Determinations. Iii. Cluster Masses Derived From Unresolved Photometry

“…Nowadays it is recognized that there is a continuum in the total (kinetic + potential) energy of stellar groups at birth that spans the zero energy point due to the dominating role of turbulence in the star formation process (Mac Low & Klessen 2004;Clark et al 2005). Furthermore, the cluster may become unbound soon after formation due to gas expulsion (Goodwin & Bastian 2006) or later on by intra-cluster stellar encounters or external tidal effects (Fall & Rees 1977).…”

Catalogue of far-infrared loops in the Galaxy

“…The idea that supersonic turbulence may be important in the control of star formation has been recognised in recent years (see MacLow & Klessen 2004, for a review). Turbulence -if dominant -can also rule the structure of the ISM, creating clouds and Appendices B-D are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/1227 cavities with regular properties of a fractal geometry, apparently similar to the structure formed via violent events.…”

Formation of Low-Mass Stars and Brown Dwarfs

“…Mac Low & Klessen 2004, McKee & Ostriker 2007. The general idea of this theory is that turbulence prevents most of the gas to collapse in a freefall time and regulates the star formation, though different aspects have been emphasized by various authors.…”