ALIGNMENT OF DUST WITH MAGNETIC INCLUSIONS: RADIATIVE TORQUES AND SUPERPARAMAGNETIC BARNETT AND NUCLEAR RELAXATION

摘要

We consider grains with superparamagnetic inclusions and report two new condensed matter effects that enhance the internal relaxation of the energy of a wobbling grain, namely, superparamagnetic Barnett relaxation, and an extension of the range where nuclear relaxation is important to higher frequencies. In addition, we report a new effect related to grain rotation: in situations where grains otherwise rotate subthermally subject to radiative torques, they get into a state of fast rotation in the presence of superparamagnetic inclusions. This effect enhances alignment, as fast rotating grains, unlike slowly rotating ones, exhibit the perfect alignment of grain axes with respect to the ambient astrophysical, e.g., interstellar, magnetic fields. Thus measurements of the polarization degree provide a possibility of testing whether grains have superparamagnetic inclusions.