Reflection and dissipation of Alfvén waves in interstellar clouds

Abstract

Context. Supersonic nonthermal motions in molecular clouds are often interpreted as long-lived magnetohydrodynamic (MHD) waves. The propagation and amplitude of these waves is affected by local physical characteristics, most importantly the gas density and the ionization fraction. Aims.We study the propagation, reflection and dissipation of Alfvén waves in molecular clouds deriving the behavior of observable quantities such as the amplitudes of velocity fluctuations and the rate of energy dissipation. Methods.We formulated the problem in terms of Elsässer variables for transverse MHD waves propagating in a one-dimensional inhomogeneous medium, including the dissipation due to collisions between ions and neutrals and to a nonlinear turbulent cascade treated in a phenomenological way. We considered both steady-state and time-dependent situations and solved the equations of the problem numerically with an iterative method and a Lax-Wendroff scheme, respectively. Results. Alfvén waves incident on overdense regions with density profiles typical of cloud cores embedded in a diffuse gas suffer enhanced reflection in the regions of the steepest density gradient, and strong dissipation in the core's interior. These effects are especially significant when the wavelength is intermediate between the critical wavelength for propagation and the typical scale of the density gradient. For larger wave amplitudes and/or steeper input spectra, the effects of the perpendicular turbulent cascade result in a stronger energy dissipation in the regions immediately surrounding the dense core. Conclusions.The results may help to interpret the sharp decrease of line width observed in the environments of low-mass cloud cores in several molecular transitions. © 2012 ESO.