The Transition from a Lognormal to a Powerlaw Column Density Distribution in Molecular Clouds: An Imprint of the Initial Magnetic Field and Turbulence
Abstract
We introduce a theory for the development of a transitional column density Σ_{TP} between the lognormal and the powerlaw forms of the probability distribution function in a molecular cloud. Our turbulent magnetohydrodynamic simulations show that the value of Σ_{TP} increases as the strength of both the initial magnetic field and turbulence increases. We develop an analytic expression for Σ_{TP} based on the interplay of turbulence, a (strong) magnetic field, and gravity. The transition value Σ_{TP} scales with {{ \mathcal M }}_{0}^{2}, the square of the initial sonic Mach number, and β _{0}, the initial ratio of gas pressure to magnetic pressure. We fit the variation of Σ_{TP} among different model clouds as a function of {{ \mathcal M }}_{0}^{2}{β }_{0} or, equivalently, the square of the initial Alfvénic Mach number {{ \mathcal M }}_{{{A}}0}^{2}. This implies that the transition value Σ_{TP} is an imprint of cloud initial conditions and is set by turbulent compression of a magnetic cloud. Physically, the value of Σ_{TP} denotes the boundary above which the masstoflux ratio becomes supercritical and gravity drives the evolution.
 Publication:

The Astrophysical Journal
 Pub Date:
 August 2019
 DOI:
 10.3847/20418213/ab3416
 arXiv:
 arXiv:1907.09783
 Bibcode:
 2019ApJ...881L..15A
 Keywords:

 ISM: clouds;
 magnetic fields;
 magnetohydrodynamics: MHD;
 stars: formation;
 Astrophysics  Solar and Stellar Astrophysics;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 7 pages, 2 figures, to appear in ApJL