Spectral Properties of Compressible Magnetohydrodynamic Turbulence from Numerical Simulations

We analyze the spectral properties of driven, supersonic compressible magnetohydrodynamic (MHD) turbulence obtained via high-resolution numerical experiments, for application to understanding the dynamics of giant molecular clouds. Via angle-averaged power spectra, we characterize the transfer of energy from the intermediate, driving scales down to smaller dissipative scales, and also present evidence for inverse cascades that achieve modal-equipartition levels on larger spatial scales. Investigating compressive versus shear modes separately, we evaluate their relative total power, and find that as the magnetic field strength decreases, (1) the shear fraction of the total kinetic power decreases, and (2) slopes of power-law fits over the inertial range steepen. To relate to previous work on incompressible MHD turbulence, we present qualitative and quantitative measures of the scale-dependent spectral anisotropy arising from the shear-Alfv\'{e}n cascade, and show how these vary with changing mean magnetic field strength. Finally, we propose a method for using anisotropy in velocity centroid maps as a diagnostic of the mean magnetic field strength in observed cloud cores.Comment: 22 pages, 11 figures; Ap.J., accepte

The generation of low-energy cosmic rays in molecular clouds

It is argued that if cosmic rays penetrate into molecular clouds, the total energy they lose can exceed the energy from galactic supernovae shocks. It is shown that most likely galactic cosmic rays interacting with the surface layers of molecular clouds are efficiently reflected and do not penetrate into the cloud interior. Low-energy cosmic rays ($E<1$ GeV) that provide the primary ionization of the molecular cloud gas can be generated inside such clouds by multiple shocks arising due to supersonic turbulence.Comment: 11 pages, no figure

ANISOTROPIC FORMATION OF MAGNETIZED CORES IN TURBULENT CLOUDS

In giant molecular clouds (GMCs), shocks driven by converging turbulent flows create high-density, strongly-magnetized regions that are locally sheetlike. In previous work, we showed that within these layers, dense filaments and embedded self-gravitating cores form by gathering material along the magnetic field lines. Here, we extend the parameter space of our three-dimensional, turbulent MHD core formation simulations. We confirm the anisotropic core formation model we previously proposed, and quantify the dependence of median core properties on the pre-shock inflow velocity and upstream magnetic field strength. Our results suggest that bound core properties are set by the total dynamic pressure (dominated by large-scale turbulence) and thermal sound speed c_s in GMCs, independent of magnetic field strength. For models with Mach number between 5 and 20, the median core masses and radii are comparable to the critical Bonnor-Ebert mass and radius defined using the dynamic pressure for P_ext. Our results correspond to M_core = 1.2 c_s^4/sqrt(G^3 rho_0 v_0^2) and R_core = 0.34 c_s^2/sqrt(G rho_0 v_0^2) for rho_0 and v_0 the large-scale mean density and velocity. For our parameter range, the median M_core ~ 0.1-1 M_sun, but a very high pressure cloud could have lower characteristic core mass. We find cores and filaments form simultaneously, and filament column densities are a factor ~2 greater than the surrounding cloud when cores first collapse. We also show that cores identified in our simulations have physical properties comparable to those observed in the Perseus cloud. Superthermal cores in our models are generally also magnetically supercritical, suggesting that the same may be true in observed clouds.Comment: 32 pages, 15 figures, 4 tables, accepted for publication in Ap