Book contents
- Frontmatter
- Contents
- Preface
- Turbulence in the Interstellar Medium: a Retrospective Review
- Mechanism of Formation of Atmospheric Turbulence Relevant for Optical Astronomy
- Properties of Atomic Gas in Spiral Galaxies
- Turbulence in the Ionized Gas in Spiral Galaxies
- Probing Interstellar Turbulence in the Warm Ionized Medium using Emission Lines
- The Spectrum & Galactic Distribution of MicroTurbulence in Diffuse Ionized Gas
- Small Scale Structure and Turbulence in the Interstellar Medium
- What is the Reynolds Number of the Reynolds' Layer?
- Photoionized Gas in the Galactic Halo
- Turbulent Heating of the Diffuse Ionized Gas
- Cosmic Rays in Interstellar Turbulence
- Turbulence in Line-Driven Stellar Winds
- An Introduction to Compressible MHD Turbulence
- Turbulence in Atomic Hydrogen
- Supershells in Spiral Galaxies
- The Size Distribution of Superbubbles in the Interstellar Medium
- Large-Scale Motions in the ISM of Elliptical and Spiral Galaxies
- Vortical Motions Driven by Supernova Explosions
- The Intermittent Dissipation of Turbulence: is it Observed in the Interstellar Medium?
- Chemistry in Turbulent Flows
- Supersonic Turbulence in Giant Extragalactic HII Regions
- Turbulence in HII regions: New results
- Hypersonic Turbulence of H2O Masers
- Water Masers Tracing Alfvenic Turbulence and Magnetic Fields in W51 M and W49 N
- Turbulence in the Ursa Major cirrus cloud
- The Collisions of HVCs with a Magnetized Gaseous Disk
- The Initial Stellar Mass Function as a Statistical Sample of Turbulent Cloud Structure
- The Structure of Molecular Clouds: are they Fractal?
- Diagnosing Properties of Turbulent Flows from Spectral Line Observations of the Molecular Interstellar Medium
- Centroid Velocity Increments as a Probe of the Turbulent Velocity Field in Interstellar Molecular Clouds
- High-Resolution C18O Mapping Observations of Heiles' Cloud 2 – Statistical Properties of the Line Width –
- Observations of Magnetic Fields in Dense Interstellar Clouds: Implications for MHD Turbulence and Cloud Evolution
- The Density PDFs of Supersonic Random Flows
- Turbulence as an Organizing Agent in the ISM
- Turbulence and Magnetic Reconnection in the Interstellar Medium
- The Evolution of Self-Gravitating, Magnetized, Turbulent Clouds: Numerical Experiments
- Super–Alfvénic Turbulent Fragmentation in Molecular Clouds
- Decay Timescales of MHD Turbulence in Molecular Clouds
- Numerical Magnetohydrodynamic Studies of Turbulence and Star Formation
- Direct Numerical Simulations of Compressible Magnetohydrodynamical Turbulence
- Fragmentation in Molecular Clouds: The Formation of a Stellar Cluster
- Accretion Disk Turbulence
- List of participants
Decay Timescales of MHD Turbulence in Molecular Clouds
Published online by Cambridge University Press: 04 August 2010
- Frontmatter
- Contents
- Preface
- Turbulence in the Interstellar Medium: a Retrospective Review
- Mechanism of Formation of Atmospheric Turbulence Relevant for Optical Astronomy
- Properties of Atomic Gas in Spiral Galaxies
- Turbulence in the Ionized Gas in Spiral Galaxies
- Probing Interstellar Turbulence in the Warm Ionized Medium using Emission Lines
- The Spectrum & Galactic Distribution of MicroTurbulence in Diffuse Ionized Gas
- Small Scale Structure and Turbulence in the Interstellar Medium
- What is the Reynolds Number of the Reynolds' Layer?
- Photoionized Gas in the Galactic Halo
- Turbulent Heating of the Diffuse Ionized Gas
- Cosmic Rays in Interstellar Turbulence
- Turbulence in Line-Driven Stellar Winds
- An Introduction to Compressible MHD Turbulence
- Turbulence in Atomic Hydrogen
- Supershells in Spiral Galaxies
- The Size Distribution of Superbubbles in the Interstellar Medium
- Large-Scale Motions in the ISM of Elliptical and Spiral Galaxies
- Vortical Motions Driven by Supernova Explosions
- The Intermittent Dissipation of Turbulence: is it Observed in the Interstellar Medium?
- Chemistry in Turbulent Flows
- Supersonic Turbulence in Giant Extragalactic HII Regions
- Turbulence in HII regions: New results
- Hypersonic Turbulence of H2O Masers
- Water Masers Tracing Alfvenic Turbulence and Magnetic Fields in W51 M and W49 N
- Turbulence in the Ursa Major cirrus cloud
- The Collisions of HVCs with a Magnetized Gaseous Disk
- The Initial Stellar Mass Function as a Statistical Sample of Turbulent Cloud Structure
- The Structure of Molecular Clouds: are they Fractal?
- Diagnosing Properties of Turbulent Flows from Spectral Line Observations of the Molecular Interstellar Medium
- Centroid Velocity Increments as a Probe of the Turbulent Velocity Field in Interstellar Molecular Clouds
- High-Resolution C18O Mapping Observations of Heiles' Cloud 2 – Statistical Properties of the Line Width –
- Observations of Magnetic Fields in Dense Interstellar Clouds: Implications for MHD Turbulence and Cloud Evolution
- The Density PDFs of Supersonic Random Flows
- Turbulence as an Organizing Agent in the ISM
- Turbulence and Magnetic Reconnection in the Interstellar Medium
- The Evolution of Self-Gravitating, Magnetized, Turbulent Clouds: Numerical Experiments
- Super–Alfvénic Turbulent Fragmentation in Molecular Clouds
- Decay Timescales of MHD Turbulence in Molecular Clouds
- Numerical Magnetohydrodynamic Studies of Turbulence and Star Formation
- Direct Numerical Simulations of Compressible Magnetohydrodynamical Turbulence
- Fragmentation in Molecular Clouds: The Formation of a Stellar Cluster
- Accretion Disk Turbulence
- List of participants
Summary
We compute 3D models of supersonic, sub-Alfvénic, and super-Alfvénic decaying turbulence, with initial rms Alfvénic and Mach numbers ranging up to five, and an isothermal equation of state appropriate for star-forming interstellar clouds of molecular gas. We find that in 3D the kinetic energy decays as t−η, with 0.85 < η < 1.2. In 1D magnetized turbulence actually decays faster than unmagnetized turbulence. We compared different algorithms, and performed resolution studies reaching 2563 zones or 703 particles. External driving must produce the observed long lifetimes and supersonic motions in molecular clouds, as undriven turbulence decays too fast.
Introduction
Molecular cloud lifetimes are of order 3 × 107 yr (Blitz & Shu 1980), while free-fall gravitational collapse times are only tff = (1.4 × 106 yr) (n/103 cm−3)−½. In the absence of non-thermal support, these clouds should collapse and form stars in a small fraction of their observed lifetime. Supersonic hydrodynamical (HD) turbulence is suggested as a support mechanism by the observed broad lines, but was dismissed because it would decay in times of order tff. A popular alternative has been sub- or trans-Alfvénic magnetohydrodynamical (MHD) turbulence, which was first suggested by Arons & Max (1975) to decay an order of magnitude more slowly. (Also see Gammie & Ostriker 1996).
However, analytic estimates and computational models suggest that incompressible MHD turbulence decays as t−η, with a decay rate 2/3 < η < 1.0 (Biskamp 1994; Hossain et al. 1995; Politano, Pouquet, & Sulem 1995; Galtier, Politano, & Pouquet 1997), while incompressible HD turbulence has been experimentally measured to decay with 1.2 < η < 2 (Comte-Bellot & Corrsin 1966; Smith et al. 1993; Warhaft & Lumley 1978).
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- Interstellar Turbulence , pp. 256 - 260Publisher: Cambridge University PressPrint publication year: 1999
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