Grain Acceleration by MHD Turbulence: Gyroresonance Mechanism

We discuss a new mechanism of dust acceleration that acts in turbulent magnetized medium. The magnetohydrodynamic (MHD) turbulence includes both fluid motions and magnetic fluctuations. We show that while the fluid motions bring about grain motions through the drag, the electromagnetic fluctuations, can accelerate grains through resonant interactions. In this paper we discuss grain acceleration by gyroresonance. We consider both incompressible (Alfven) and compressible (fast and slow) MHD modes. We show that fast modes dominate grain acceleration. In magnetically dominated media, they can render supersonic velocities to grains that may shatter grains and enable efficient absorption of metals. Since the grains are preferentially accelerated with large pitch angles, the supersonic grains get aligned with long axes perpendicular to the magnetic field. Subject headings: dust, extinction–ISM: particle acceleration–kinematics and dynamics–magnetic fields