Magnetic molecular clouds - Indirect evidence for magnetic support and ambipolar diffusion
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Abstract
Over 120 measurements of molecular cloud size R, velocity dispersion σ, and density n are compiled to investigate the well-known relations σ ∝ R1/2, n ∝ R-1, and GM/R ≈ σ2. For cloud sizes from 0.1 to 100 pc, a crude virial equilibrium model of thermal and magnetic support against gravity fits the observed trends, provided the one free parameter, magnetic field strength, is 15 - 40 μG. This model is consistent with observations of low-mass cloud cores if their ratio of magnetic to gravitational energy, or their ratio of flux to mass, is smaller than in larger clouds - a situation expected in models of core formation by ambipolar diffusion. Furthermore, for typical equilibrium field strengths, the time scale for ambipolar diffusion is shortest when the cloud size is about 0.1 pc, as is typical of low-mass cores.
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