Bibliothek

Notizen: Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structure functions, the direction and locality of energy transfer and cascade, velocity probability distributions, and turbulent pressure. We emphasize expected differences between incompressible and compressible turbulence. Theories of magnetic turbulence on scales smaller than the collision mean free path are included, as are theories of magnetohydrodynamic turbulence and their various proposals for power spectra. Numerical simulations of interstellar turbulence are reviewed. Models have reproduced the basic features of the observed scaling relations, predicted fast decay rates for supersonic MHD turbulence, and derived probability distribution functions for density. Thermal instabilities and thermal phases have a new interpretation in a supersonically turbulent medium. Large-scale models with various combinations of self-gravity, magnetic fields, supernovae, and star formation are beginning to resemble the observed interstellar medium in morphology and statistical properties. The role of self-gravity in turbulent gas evolution is clarified, leading to new paradigms for the formation of star clusters, the stellar mass function, the origin of stellar rotation and binary stars, and the effects of magnetic fields. The review ends with a reflection on the progress that has been made in our understanding of the interstellar medium and offers a list of outstanding problems.

Notizen: Interstellar turbulence has implications for the dispersal and mixing of the elements, cloud chemistry, cosmic ray scattering, and radio wave propagation through the ionized medium. This review discusses the observations and theory of these effects. Metallicity fluctuations are summarized, and the theory of turbulent transport of passive tracers is reviewed. Modeling methods, turbulent concentration of dust grains, and the turbulent washout of radial abundance gradients are discussed. Interstellar chemistry is affected by turbulent transport of various species between environments with different physical properties and by turbulent heating in shocks, vortical dissipation regions, and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered and accelerated in turbulent magnetic waves and shocks, and they generate turbulence on the scale of their gyroradii. Radio wave scintillation is an important diagnostic for small-scale turbulence in the ionized medium, giving information about the power spectrum and amplitude of fluctuations. The theory of diffraction and refraction as well as the main observations and scintillation regions are reviewed.

Notizen: Motivated by the possible utility of studying turbulent astrophysical flows as dynamical systems, solutions to the incompressible two-dimensional Navier–Stokes equation in Fourier space are investigated employing a small subset of Fourier modes that span a range of nearly 3000:1 in size and that preserve the triadic nature of the nonlinear interactions. These solutions thus preserve the energy and enstrophy conservation properties of the full solutions. The statistical similarity of the reduced solutions to the full solutions is tested by comparing their respective energy spectra in the energy and enstrophy inertial ranges, as well as the energy and enstrophy cascade directions. The behavior of the reduced solutions is consistent with full solutions having similar physical conditions (rates of injection and dissipation of the energy and enstrophy), while providing a computational gain of about a factor ∼1000 for kmax∼104 in two dimensions (2D) and ∼106 at kmax∼103.5 in 3D. Also, the effect and importance of the forcing on the outcome of the calculations are discussed, namely the slope of the energy spectrum and the duration of the net energy and enstrophy transfer.

Notizen: Abstract Some bursts of star formation are thought to be associated with situations in which a galaxy's density is increasing. Examples include protogalaxy collapse, mergers, inflow of gas into a galactic nucleus, or accretion of intergalactic gas. We have examined the evolution of the star formation rate (SFR) and other properties of galaxies with increasing density using one-zone cloud fluid equations describing an extension of the Oort cycle, for which the equilibrium state would give an SFR which increases monotonically with density. However, the calculations show that the energy input associated with the density increase generally dominates the evolution, and forces the system far from its normal equilibrium to a state in which cloud collisions are disruptive rather than coalescent. The calculations predict that starbursts associated with collapse, accretion, or inflow events should be preceded by a long incubation period with a very small SFR. For example, the initial star formation burst in a protogalaxy may be delayed for several billion years until nearly all the infalling material has been accreted onto the growing central object.