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The effect of magnetic fields and ambipolar diffusion on the column density probability distribution function in molecular clouds

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Monthly Notices of the Royal Astronomical Society





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Simulations generally show that non-self-gravitating clouds have a lognormal column density (Σ) probability distribution function (PDF), while self-gravitating clouds with active star formation develop a distinct power-law tail at high column density. Although the growth of the power-law can be attributed to gravitational contraction leading to the formation of condensed cores, it is often debated if an observed lognormal shape is a direct consequence of supersonic turbulence alone, or even if it is really observed in molecular clouds. In this paper we run threedimensional magnetohydrodynamic simulations including ambipolar diffusion with different initial conditions to see the effect of strong magnetic fields and non-linear initial velocity perturbations on the evolution of the column density PDFs. Our simulations show that column density PDFs of clouds with supercritical mass-to-flux ratio, with either linear perturbations or non-linear turbulence, quickly develop a power-law tail such that dN/dlogΣ ∝ Σ -α with index α ≃ 2. Interestingly, clouds with subcritical mass-to-flux ratio also proceed directly to a power-law PDF, but with a much steeper index α ≃ 4. This is a result of gravitationally driven ambipolar diffusion. However, for non-linear perturbations with a turbulent spectrum (vk2 ∝ k -4), the column density PDFs of subcritical clouds do retain a lognormal shape for a major part of the cloud evolution, and only develop a distinct power-law tail with index α ≃ 2 at greater column density when supercritical pockets are formed.

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