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, magnetized clouds, many of which are self-gravitating and form stars. To develop an u nderstanding of how these clouds' kinetic and structural evolution may depe nd on their level of turbulence, mean magnetization, and degree of self-gra vity, we perform a survey of direct numerical MHD simulations in which thre e parameters are independently varied. Our simulations consist of solutions to the time-dependent MI-ID equations on a two-dimensional grid with perio dic boundary conditions; an additional "half" dimension is also incorporate d as dependent variables in the third Cartesian direction. Two of our surve y parameters, the mean magnetization parameter beta = c(sound)(2)/upsilon(A lfven)(2) and the Jeans number n(J) = L-cloud/L-Jeans, allow us to model cl ouds that either meet or fail conditions for magneto-Jeans stability and ma gnetic criticality. Our third survey parameter, the sonic Mach number M = s igma(velocity)/c(sound), allows us to initiate turbulence of either sub- or super-Alfvenic amplitude; we employ an isothermal equation of state throug hout. We evaluate the times for each cloud model to become gravitationally bound and measure each model's kinetic energy loss over the fluid-flow cros sing time. We compare the evolution of density and magnetic held structural morphology and quantify the differences in the density contrast generated by internal stresses for models of differing mean magnetization. We find th at the values of beta and n(J), but not the initial Mach number M, determin e the time for cloud gravitational binding and collapse: for mean cloud den sity n(H2) = 100 cm(-3), unmagnetized models collapse after similar to 5 My r, and magnetically supercritical models generally collapse after 5-10 Myr (although the smallest magneto-Jeans stable clouds survive gravitational co llapse until t similar to 15 Myr), while magnetically subcritical clouds re main uncollapsed over the simulations; these cloud collapse times scale wit h the mean density as t(g) proportional to n(H2)(-1/2). We find, contrary t o some previous expectations, less than a factor of 2 difference between tu rbulent decay times for models with varying magnetic held strength; the max imum decay time, for B similar to 14 mu G and n(H2) = 100 cm(-3), is 1.4 fl ow crossing times t(cross) =L/sigma(velocity) (or 8 Myr for typical giant m olecular cloud cm parameters). In all models we find turbulent amplificatio n in the magnetic held strength up to at least the level beta(pert) c(sound )(2)/delta upsilon(Alfven)(2) = 0.1, With the turbulent magnetic energy bet ween 25% and 60% of the turbulent kinetic energy after one flow crossing ti me. We find that for non-self-gravitating stages of evolution, when clouds have M = 5-10, the mass-averaged density contrast magnitudes [log (rho/<(rh o)over bar>)] are in the range 0.2-0.5, with the contrast increasing both t oward low and high B. Although our conclusions about density statistics may be affected by our isothermal assumption, we note that only the more stron gly magnetized models appear to be consistent with estimates of clump/inter clump density contrasts inferred in Galactic giant molecular clouds.