Molecular Cloud Turbulence and Star Formation

Abstract

We review the properties of turbulent molecular clouds (MCs), focusing on the physical processes that influence star formation (SF). MC formation appears to occur during large-scale compression of the diffuse ISM driven by supernovae, magnetorotational instability, or gravitational instability in galactic disks of stars and gas. The compressions generate turbulence that can accelerate molecule production and produce the observed morphology. We then review the properties of MC turbulence, including density enhancements observed as clumps and cores, magnetic field structure, driving scales, the relation to observed scaling relations, and the interaction with gas thermodynamics. We argue that MC cores are dynamical, not quasistatic, objects with relatively short lifetimes not exceeding a few megayears. We review their morphology, magnetic fields, density and velocity profiles, and virial budget. Next, we discuss how MC turbulence controls SF. On global scales turbulence prevents monolithic collapse of the clouds; on small scales it promotes local collapse. We discuss its effects on the SF efficiency, and critically examine the possible relation between the clump mass distribution and the initial mass function, and then turn to the redistribution of angular momentum during collapse and how it determines the multiplicity of stellar systems. Finally, we discuss the importance of dynamical interactions between protostars in dense clusters, and the effect of the ionization and winds from those protostars on the surrounding cloud. We conclude that the interaction of self-gravity and turbulence controls MC formation and behavior, as well as the core and star formation processes within them.