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

Abstract

The molecular component of the Galaxy is comprised of turbulent,magnetized clouds, many of which are self-gravitating and form stars. Todevelop an understanding of how these clouds' kinetic and structuralevolution may depend on their level of turbulence, mean magnetization,and degree of self-gravity, we perform a survey of direct numerical MHDsimulations in which three parameters are independently varied. Oursimulations consist of solutions to the time-dependent MHD equations ona two-dimensional grid with periodic boundary conditions; an additional``half'' dimension is also incorporated as dependent variables in thethird Cartesian direction. Two of our survey parameters, the meanmagnetization parameter beta=c^2_sound/v^2_Alfven and the Jeans numbern_J=L_cloud/L_Jeans, allow us to model clouds that either meet or failconditions for magneto-Jeans stability and magnetic criticality. Ourthird survey parameter, the sonic Mach number M=sigma_velocity/c_sound,allows us to initiate turbulence of either sub- or super-Alfvénicamplitude; we employ an isothermal equation of state throughout. Weevaluate the times for each cloud model to become gravitationally boundand measure each model's kinetic energy loss over the fluid-flowcrossing time. We compare the evolution of density and magnetic fieldstructural morphology and quantify the differences in the densitycontrast generated by internal stresses for models of differing meanmagnetization. We find that the values of beta and n_J, but not theinitial Mach number M, determine the time for cloud gravitationalbinding and collapse: for mean cloud density n_H_2=100 cm^-3,unmagnetized models collapse after ~5 Myr, and magneticallysupercritical models generally collapse after 5-10 Myr (although thesmallest magneto-Jeans stable clouds survive gravitational collapseuntil t~15 Myr), while magnetically subcritical clouds remainuncollapsed over the entire simulations; these cloud collapse timesscale with the mean density as t_g~n^-1/2_H_2. We find, contrary to someprevious expectations, less than a factor of 2 difference betweenturbulent decay times for models with varying magnetic field strength;the maximum decay time, for B~14 muG and n_H_2=100 cm^-3, is 1.4 flowcrossing times t_cross=L/sigma_velocity (or 8 Myr for typical giantmolecular cloud parameters). In all models we find turbulentamplification in the magnetic field strength up to at least the levelbeta_pert=c^2_sound/deltav^2_Alfven=0.1, with the turbulent magneticenergy between 25% and 60% of the turbulent kinetic energy after oneflow crossing time. We find that for non-self-gravitating stages ofevolution, when clouds have M=5-10, the mass-averaged density contrastmagnitudes are in the range 0.2-0.5, with thecontrast increasing both toward low and high beta. Although ourconclusions about density statistics may be affected by our isothermalassumption, we note that only the more strongly magnetized models appearto be consistent with estimates of clump/interclump density contrastsinferred in Galactic giant molecular clouds.