## Abstract

The molecular component of the Galaxy is comprised of turbulent, magnetized clouds, many of which are self-gravitating 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 self-gravity, we perform a survey of direct numerical MHD simulations in which three parameters aie independently varied. Our simulations consist of solutions to the time-dependent MHD equations on a two-dimensional 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 magneto-Jeans stability and magnetic criticality. Our third survey parameter, the sonic Mach number Script M sign ≡ σ_{velocity}/c_{sound}, allows us to initiate turbulence of either sub-or super-Alfvénic 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 fluid-flow 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 Script M sign, 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 5-10 Myr (although the smallest magneto-Jeans 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_{2Alfven} = 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 non-self-gravitating stages of evolution, when clouds have Script M sign = 5-10, the mass-averaged density contrast magnitudes 〈log (ρ/ρ̄)〉 are in the range 0.2-0.5, 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.

Original language | English (US) |
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Pages (from-to) | 259-274 |

Number of pages | 16 |

Journal | Astrophysical Journal |

Volume | 513 |

Issue number | 1 PART 1 |

DOIs | |

State | Published - Mar 1 1999 |

## All Science Journal Classification (ASJC) codes

- Astronomy and Astrophysics
- Space and Planetary Science

## Keywords

- ISM: general
- ISM: kinematics and dynamics
- ISM: magnetic fields
- MHD shock waves