Grain acceleration by magnetohydrodynamic turbulence: Gyroresonance mechanism

We discuss a new mechanism of dust acceleration that acts in a turbulent magnetized medium. The magneto-hydrodynamic (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 Letter, we calculate the grain acceleration by the gyroresonance in the cold neutral medium. We consider both incompressible and compressible MHD modes. We show that fast modes dominate the grain acceleration. For the parameters chosen, fast modes render to grains supersonic velocities that may shatter the grains and enable the efficient absorption of heavy elements. Since the grains are preferentially accelerated with large pitch angles, the supersonic grains get aligned with long axes perpendicular to the magnetic field.