Kinetic and Structural Evolution of Selfgravitating, Magnetized Clouds: 2.5dimensional Simulations of Decaying Turbulence
Abstract
The molecular component of the Galaxy is comprised of turbulent, magnetized clouds, many of which are selfgravitating and form stars. To develop an understanding of how these clouds' kinetic and structural evolution may depend on their level of turbulence, mean magnetization, and degree of selfgravity, we perform a survey of direct numerical MHD simulations in which three parameters are independently varied. Our simulations consist of solutions to the timedependent MHD equations on a twodimensional grid with periodic boundary conditions; an additional ``half'' dimension is also incorporated as dependent variables in the third Cartesian direction. Two of our survey parameters, the mean magnetization parameter β≡c^{2}_{sound}/v^{2}_{Alfven} and the Jeans number n_{J}≡L_{cloud}/L_{Jeans}, allow us to model clouds that either meet or fail conditions for magnetoJeans stability and magnetic criticality. Our third survey parameter, the sonic Mach number M≡σ_{velocity}/c_{sound}, allows us to initiate turbulence of either sub or superAlfvénic amplitude; we employ an isothermal equation of state throughout. We evaluate the times for each cloud model to become gravitationally bound and measure each model's kinetic energy loss over the fluidflow crossing time. We compare the evolution of density and magnetic field structural morphology and quantify the differences in the density contrast generated by internal stresses for models of differing mean magnetization. We find that the values of β and n_{J}, but not the initial Mach number M, determine the time for cloud gravitational binding and collapse: for mean cloud density n_{H2}=100 cm^{3}, unmagnetized models collapse after ~5 Myr, and magnetically supercritical models generally collapse after 510 Myr (although the smallest magnetoJeans stable clouds survive gravitational collapse until t~15 Myr), while magnetically subcritical clouds remain uncollapsed over the entire simulations; these cloud collapse times scale with the mean density as t_{g}~n^{1/2}_{H2}. We find, contrary to some previous expectations, less than a factor of 2 difference between turbulent decay times for models with varying magnetic field strength; the maximum decay time, for B~14 μG and n_{H2}=100 cm^{3}, is 1.4 flow crossing times t_{cross}=L/σ_{velocity} (or 8 Myr for typical giant molecular cloud parameters). In all models we find turbulent amplification in the magnetic field strength up to at least the level β_{pert}≡c^{2}_{sound}/δv^{2}_{Alfven}=0.1, with the turbulent magnetic energy between 25% and 60% of the turbulent kinetic energy after one flow crossing time. We find that for nonselfgravitating stages of evolution, when clouds have M=510, the massaveraged density contrast magnitudes <log(ρ/ρ¯)> are in the range 0.20.5, with the contrast increasing both toward low and high β. Although our conclusions about density statistics may be affected by our isothermal assumption, we note that only the more strongly magnetized models appear to be consistent with estimates of clump/interclump density contrasts inferred in Galactic giant molecular clouds.
 Publication:

The Astrophysical Journal
 Pub Date:
 March 1999
 DOI:
 10.1086/306842
 arXiv:
 arXiv:astroph/9810321
 Bibcode:
 1999ApJ...513..259O
 Keywords:

 ISM: GENERAL;
 ISM: KINEMATICS AND DYNAMICS;
 ISM: MAGNETIC FIELDS;
 MAGNETOHYDRODYNAMICS: MHD;
 SHOCK WAVES;
 ISM: General;
 ISM: Kinematics and Dynamics;
 ISM: Magnetic Fields;
 Magnetohydrodynamics: MHD;
 Shock Waves;
 Astrophysics
 EPrint:
 34 pages, 7 postscript figures, to appear in ApJ 3/1/99