Alignment of Dust with Magnetic Inclusions: Radiative Torques and Superparamagnetic Barnett and Nuclear Relaxation

Abstract

We consider grains with superparamagnetic inclusions and report two new condensed matter effects that can enhance the internal relaxation of the energy of a wobbling grain, namely, superparamagnetic Barnett relaxation, as well as, an increase of frequencies for which nuclear relaxation becomes important. This findings extends the range of grain sizes for which grains are thermally trapped, i.e. rotate thermally, in spite of the presence of uncompensated pinwheel torques. In addition, we show that the alignment of dust grains by radiative torques gets modified for superparamagnetic grains, with grains obtaining perfect alignment with respect to magnetic fields as soon as the grain gaseous randomization time gets larger than that of paramagnetic relaxation. The same conclusion is valid for the mechanical alignment of helical grains. If observations confirm that the degrees of alignment are higher than radiative torques can produce alone, this may be a proof of the presence of superparamagentic inclusions.

5 pages, emulated ApJ style, 2 figures, submitted to ApJL (earlier version was corrupted)

References in corpus (6)

• Radiative torques: Analytical Model and Basic Properties
• Tracing Magnetic Fields with Aligned Grains
• The Efficiency of Grain Alignment in Dense Interstellar Clouds: A Reassessment of Constraints from Near Infrared Polarization
• Subsonic Mechanical Alignment of Irregular Grains
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Cited by in corpus (4)

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• Cassiopeia A: dust factory revealed via submillimetre polarimetry
• Radiative torques alignment in the presence of pinwheel torques
• Thermal Flipping of Interstellar Grains