Annu. Rev. Astron. Astrophys. 2004. 42: 211-273
Copyright © 2004 by . All rights reserved

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INTERSTELLAR TURBULENCE I: OBSERVATIONS AND PROCESSES

Bruce G. Elmegreen

IBM Research Division, Yorktown Heights, New York 10598

John Scalo

Department of Astronomy, University of Texas, Austin, Texas 78712


Abstract. Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium, and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher order structure functions, the direction and locality of energy transfer and cascade, velocity probability distributions, and turbulent pressure. We emphasize expected differences between incompressible and compressible turbulence. Theories of magnetic turbulence on scales smaller than the collision mean free path are included, as are theories of magnetohydrodynamic turbulence and their various proposals for power spectra. Numerical simulations of interstellar turbulence are reviewed. Models have reproduced the basic features of the observed scaling relations, predicted fast decay rates for supersonic MHD turbulence, and derived probability distribution functions for density. Thermal instabilities and thermal phases have a new interpretation in a supersonically turbulent medium. Large-scale models with various combinations of self-gravity, magnetic fields, supernovae, and star formation are beginning to resemble the observed interstellar medium in morphology and statistical properties. The role of self-gravity in turbulent gas evolution is clarified, leading to new paradigms for the formation of star clusters, the stellar mass function, the origin of stellar rotation and binary stars, and the effects of magnetic fields. The review ends with a reflection on the progress that has been made in our understanding of the interstellar medium, and offers a list of outstanding problems.


Table of Contents

INTRODUCTION

DIAGNOSTICS OF TURBULENCE IN THE DENSE INTERSTELLAR MEDIUM

POWER SOURCES FOR INTERSTELLAR TURBULENCE

THEORY OF INTERSTELLAR TURBULENCE
What is Turbulence and Why Is It So Complicated?
Basic Equations
Statistical Closure Theories
Solenoidal and Compressible Modes
Global Inviscid Quadratic Invariants are Fundamental Constraints on the Nature of Turbulent Flows
Scale-invariant Kinetic Energy Flux and Scaling Arguments for the Power Spectrum
Intermittency and Structure Function Scaling
Details of the Energy Cascade: Isotropy and Independence of Large and Small Scales
Velocity Probability Distribution
Turbulent "Pressure"
Below the Collision Mean Free Path
MHD Turbulence Theory: Power Spectra
The Anisotropic Kolmogorov Model

SIMULATIONS OF INTERSTELLAR TURBULENCE
Introduction
Scaling Relations
Decay of Supersonic MHD Turbulence
The Density Probability Distribution
Energy Cascades
Compressible versus Solenoidal Motions
Filamentary Structure
Thermal Instability and Thermal Phases
Supernova-Driven Turbulence
The Role of Self-Gravity in the ISM
Formation of Star Clusters and the IMF
Rotation and Binary Star Formation
Effects of Magnetic Fields on Interstellar Turbulence
Turbulent Rotating Galaxy Disk Simulations

SUMMARY AND REFLECTIONS

REFERENCES

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