ALFVEN WAVES IN INTERSTELLAR GASDYNAMICS

Magnetohydrodynamic (MHD) waves contribute a significant pressure in b oth the diffuse interstellar medium and in molecular clouds. Alfven wa ves are subject to less damping than compressive MHD waves and are the refore likely to be the dominant mode in astrophysical environments. P rovided that the medium in which the waves are propagating is slowly v arying, the dynamical effects of ideal MHD waves are governed by equat ions derived by Dewar. We show that these equations are similar in for m to the equations of radiation hydrodynamics to order v/c, provided t hat the radiation is nearly isotropic. For the case of Alfven waves, t he pressure due the waves, P-w, is isotropic. Furthermore, P-w is dire ctly observable through the nonthermal line width sigma(nt); for a ran domly oriented field, P-w = (3/2)rho sigma(nt)(2). In several simple c ases, including that in which the Alfven waves are isotropic, that in which the density is spatially uniform, and that in which the medium u ndergoes a self-similar contraction or expansion, undamped Alfven wave s behave like a gas with a ratio of specific heats of 3/2; i.e., press ure variations are related to density variations by Delta lnP(w) = gam ma(w) Delta ln rho with gamma(w) = 3/2. In a spatially nonuniform clou d, gamma(w) generally depends on position; an explicit expression is g iven. In the opposite limit of rapid variations, such as in a strong s hock, the wave magnetic field behaves like a static field and the wave pressure can increase as fast as rho(2), depending on the orientation of the shock and the polarization of the waves. The jump conditions f or a shock in a medium containing MI-ID waves are given. For strong no nradiative shocks, neither the wave pressure nor the static magnetic f ield pressure is significant downstream, but for radiative shocks thes e two pressures can become dominant. Alfven waves are essential in sup porting molecular clouds against gravitational collapse. In a static c loud with a nonuniform density rho(r), the spatial variation of the wa ve pressure is given by the polytropic relation P-w(r) proportional to rho(r)(gamma p) with gamma(p) = 1/2. This generalizes the result obta ined by Fatuzzo and Adams and is consistent with observations showing that molecular clouds have velocity dispersions that increase outward. The polytropic index gamma(p) for Alfven waves differs substantially from the adiabatic index gamma(w), which has implications for the grav itational stability of molecular clouds.