ALFVEN WAVES IN INTERSTELLAR GASDYNAMICS

摘要

Magnetohydrodynamic (MHD) waves contribute a significant pressure in both the diffuse interstellar medium and in molecular clouds. Alfven waves are subject to less damping than compressive MHD waves and are therefore likely to be the dominant mode in astrophysical environments. Provided that the medium in which the waves are propagating is slowly varying, the dynamical effects of ideal MHD waves are governed by equations derived by Dewar. We show that these equations are similar in form to the equations of radiation hydrodynamics to order v/c, provided that the radiation is nearly isotropic. For the case of Alfven waves, the pressure due the waves, P_w, is isotropic. Furthermore, P_w is directly observable through the non-thermal line width σ_(nt); for a randomly oriented field, P_w = (3/2)ρσ_(nt)~2. In several simple cases, including that in which the Alfven waves are isotropic, that in which the density is spatially uniform, and that in which the medium undergoes a self-similar contraction or expansion, undamped Alfven waves behave like a gas with a ratio of specific heats of 3/2; i.e., pressure variations are related to density variations by ΔlnP_w = γ_wΔlnρ with γ_w = 3/2. In a spatially nonuniform cloud, γ_w generally depends on position; an explicit expression is given. In the opposite limit of rapid variations, such as in a strong shock, the wave magnetic field behaves like a static field and the wave pressure can increase as fast as ρ~2, depending on the orientation of the shock and the polarization of the waves. The jump conditions for a shock in a medium containing MHD waves are given. For strong nonradiative shocks, neither the wave pressure nor the static magnetic field pressure is significant downstream, but for radiative shocks these two pressures can become dominant. Alfven waves are essential in supporting molecular clouds against gravitational collapse. In a static cloud with a nonuniform density ρ(r), the spatial variation of the wave pressure is given by the polytropic relation P_w(r) ∝ ρ(r)~(γp) with γ_p = 1/2. This generalizes the result obtained by Fatuzzo & Adams and is consistent with observations showing that molecular clouds have velocity dispersions that increase outward. The polytropic index γ_p for Alfven waves differs substantially from the adiabatic index γ_w, which has implications for the gravitational stability of molecular clouds.