Dust with Strong Magnetic Inclusions: Superparamagnetic Barnett Relaxation and Alignment by Radiative Torques

We report a substantial increase of the rate of internal relaxation in dust grains containing superparamagnetic inclusions. The effect arises from the increase of dissipation induced by the Barnett effect. The increase of the internal relaxation induces higher coupling of the rotational and vibrational degrees of freedom of grain with important consequences to thermal trapping of grains. In terms of the alignment of grains by radiative torques, which is currently is the prefered mechanism for alignment, first of all, the enhanced Barnett magnetization induces faster Larmor precession of grains, which affects, for instance, the moment when the radiative torque alignment occurs in respect to light direction rather than the direction of magnetic fields. More importantly, we show that the alignment of grains with superparamagnetic inclusions is different from that of ordinary paramagnetic grains. If for the latter the dissipation induced by magnetic field is negligible, for superparamagnetic grains, it changes the character of radiative torque alignment, inducing perfect alignment instead of a partial one. Subject headings: polarization -dust extinction -ISM: magnetic fields