Density, velocity, and magnetic field structure in turbulent molecular cloud models

We use three-dimensional (3D) numerical magnetohydrodynamic simulations to follow the evolution of cold, turbulent, gaseous systems with parameters ch osen to represent conditions in giant molecular clouds (GMCs). We present r esults of three model cloud simulations in which the mean magnetic field st rength is varied (B-0 = 1.4-14 muG for GMC parameters), but an identical in itial turbulent velocity field is introduced. We describe the energy evolut ion, showing that (1) turbulence decays rapidly, with the turbulent energy reduced by a factor 2 after 0.4-0.8 flow crossing times (similar to2-4 Myr for GMC parameters), and (2) the magnetically supercritical cloud models gr avitationally collapse after time approximate to6 Myr, while the magnetical ly subcritical cloud does not collapse. We compare density, velocity, and m agnetic field structure in three sets of model "snapshots" with matched val ues of the Mach number M approximate to 9,7,5. We show that the distributio ns of volume density and column density are both approximately log-normal, with mean mass-weighted volume density a factor 3-6 times the unperturbed v alue, but mean mass-weighted column density only a factor 1.1-1.4 times the unperturbed value. We introduce a spatial binning algorithm to investigate the dependence of kinetic quantities on spatial scale for regions of colum n density contrast (ROCs) on the plane of the sky. We show that the average velocity dispersion for the distribution of ROCs is only weakly correlated with scale, similar to mean size-line width distributions for clumps withi n GMCs. We find that ROCs are often superpositions of spatially unconnected regions that cannot easily be separated using velocity information; we arg ue that the same difficulty may affect observed GMC clumps. We suggest that it may be possible to deduce the mean 3D size-line width relation using th e lower envelope of the 2D size-line width distribution. We analyze magneti c field structure and show that in the high-density regime n(H2) greater th an or similar to 10(3) cm(-3), total magnetic field strengths increase with density with logarithmic slope similar to1/3-2/3. We find that mean line-o f-sight magnetic field strengths may vary widely across a projected cloud a nd are not positively correlated with column density. We compute simulated interstellar polarization maps at varying observer orientations and determi ne that the Chandrasekhar-Fermi formula multiplied by a factor similar to0. 5 yields a good estimate of the plane-of sky magnetic field strength, provi ded the dispersion in polarization angles is less than or similar to 25 deg rees.