Kinetic and Structural Evolution of Self‐gravitating, Magnetized Clouds: 2.5‐dimensional Simulations of Decaying Turbulence

The molecular component of the Galaxy is comprised of turbulent, magnetizedclouds, many of which are self-gravitating and form stars. To understand howthese clouds' evolution may depend on their level of turbulence, meanmagnetization, and degree of self-gravity, we perform a survey of directnumerical MHD simulations in 2.5 dimensions. Two of our independent surveyparameters allow us to model clouds which either meet or fail conditions formagneto-Jeans stability and magnetic criticality. Our third survey parameterallows us to initiate turbulence of either sub- or super-Alfv\'enic amplitude.We evaluate the times for each cloud model to become gravitationally bound, andmeasure each model's kinetic energy loss over the fluid flow crossing time. Wecompare the evolution of density and magnetic field structural morphology, andquantify the differences in the density contrast generated by internalstresses, for models of differing mean magnetization. We find that the valuesof (c_s/v_A)^2 and L/L_Jeans, but not the initial Mach number v/c_s, determinethe time for cloud gravitational binding and collapse. We find, contrary tosome previous expectations, less than a factor of two difference betweenturbulent decay times for models with varying magnetic field strength. In allmodels, we find turbulent amplification in the magnetic field strength, withthe turbulent magnetic energy between 25-60% of the turbulent kinetic energyafter one flow crossing time. We find that for non-self-gravitating stages ofevolution, the mass-averaged density contrast magnitudes are in the range 0.2-0.5; we note that only the more strongly-magnetized modelsappear consistent with estimates of clump/interclump density contrasts inferredin Galactic GMCs.Comment: 34 pages, 7 postscript figures, to appear in ApJ 3/1/9