A new code for astrophysical magnetohydrodynamics (MHD) is described. The code has been designed to be easily extensible for use with static and adaptive mesh refinement. It combines higher-order Godunov methods with the constrained transport (CT) technique to enforce the divergence-free constraint on the magnetic field. Discretization is based on cell-centered volume-averages for mass, momentum, and energy, and face-centered area-averages for the magnetic field. Novel features of the algorithm include (1) a consistent framework for computing the time- and edge-averaged electric fields used by CT to evolve the magnetic field from the time- and area-averaged Godunov fluxes, (2) the extension to MHD of spatial reconstruction schemes that involve a dimensionally-split time advance, and (3) the extension to MHD of two different dimensionally-unsplit integration methods. Implementation of the algorithm in both C and Fortran95 is detailed, including strategies for parallelization using domain decomposition. Results from a test suite which includes problems in one-, two-, and three-dimensions for both hydrodynamics and MHD are given, not only to demonstrate the fidelity of the algorithms, but also to enable comparisons to other methods. The source code is freely available for download on the web.Comment: 61 pages, 36 figures. accepted by ApJ

We use three-dimensional (3D) numerical magnetohydrodynamic simulations to follow the evolution of cold, turbulent, gaseous systems with parameters chosen to represent conditions in giant molecular clouds (GMCs). We present results of three model cloud simulations in which the mean magnetic field strength is varied (B 0 = 1.4− 14 µG for GMC parameters), but an identical initial turbulent velocity field is introduced. We describe the energy evolution, showing that (i) turbulence decays rapidly, with the turbulent energy reduced by a factor two after 0.4-0.8 flow crossing times (∼ 2 − 4 Myr for GMC parameters), and (ii) the magnetically supercritical cloud models gravitationally collapse after time ≈ 6 Myr, while the magnetically subcritical cloud does not collapse.We compare density, velocity, and magnetic field structure in three sets of model "snapshots" with matched values of the Mach number M ≈ 9, 7, 5. We show that the distributions of volume density and column density are both approximately log-normal, with mean mass-weighted volume density a factor 3 − 6 times the unperturbed value, 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 column 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-linewidth distributions for clumps within GMCs. We find that ROCs are often superpositions of spatially unconnected regions that cannot easily be separated using velocity information; we argue that the same difficulty may affect observed GMC clumps. We suggest that it may be possible to deduce the mean 3D size-linewidth relation using the lower envelope of the 2D sizelinewidth distribution. We analyze magnetic field structure, and show that in the high density regime n H 2 > ∼ 10 3 cm −3 , total magnetic field strengths increase with density with logarithmic slope ∼ 1/3 − 2/3. We find that mean line-of-sight magnetic field strengths may vary widely across a projected cloud, and are not positively correlated with column density. We compute simulated interstellar polarization maps at varying observer orientations, and determine that the Chandrasekhar-Fermi formula multiplied by a factor ∼ 0.5 yields a good estimate of the plane-of sky magnetic field strength, provided the dispersion in polarization angles is < ∼ 25 • .

We present the results of axisymmetric time-dependent hydrodynamic calculations of line-driven winds from accretion disks in active galactic nuclei (AGN). We assume the disk is flat, Keplerian, geometrically thin, and optically thick, radiating according to the α-disk prescription. The central engine of the AGN is a source of both ionizing X-rays and wind-driving ultraviolet (UV) photons. To calculate the radiation force, we take into account radiation from the disk and the central engine. The gas temperature and ionization state in the wind are calculated self-consistently from the photoionization and heating rate of the central engine.We find that a disk accreting onto a 10 8 M ⊙ black hole at the rate of 1.8 M ⊙ yr −1 can launch a wind at ∼ 10 16 cm from the central engine. The X-rays from the central object are significantly attenuated by the disk atmosphere so they cannot prevent the local disk radiation from pushing matter away from the disk. However in the supersonic portion of the flow high above the disk, the X-rays can overionize the gas and decrease the wind terminal velocity. For a reasonable X-ray opacity, e.g., κ X = 40 g −1 cm 2 , the disk wind can be accelerated by the central UV radiation to velocities of up to 15000 km s −1 at a distance of ∼ 10 17 cm from the central engine. The covering factor of the disk wind is ∼ 0.2. The wind is unsteady and consists of an opaque, slow vertical flow near the disk that is bounded on the polar side by a high-velocity stream. A typical column density through the fast stream is a few 10 23 cm −2 so the stream is optically thin to the UV radiation. This low column density is precisely why gas can be accelerated to high velocities. The fast stream contributes nearly 100% to the total wind mass loss rate of 0.5 M ⊙ yr −1 .

We have carried out a hydrodynamical code comparison study of interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of the SPH smoothing kernel over which information is not transferred.Comment: 15 pages, 13 figures, to be submitted to MNRAS. For high-resolution figures, please see http://www-theorie.physik.unizh.ch/~agertz

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