Magnetic alignment of nonmagnetic silicates caused by paramagnetic anisotropy: Origin of polarization observed in planetary formation region

Abstract

Possible dust alignment in dense regions due to paramagnetic anisotropy χ PARA is discussed based on the results of a laboratory experiment on ortho-pyroxene grains containing a small amount of Fe 2+ ions (1.8 mol%). Ortho-pyroxene has been detected in circum-stellar regions by infrared emission spectroscopy. Our experiment was performed at room temperature using He gas as the dispersing medium. Although the grains do not contain strong magnetic moments, alignment was achieved at low field strength (<2000 G). The alignment efficiency of ortho-pyroxene was compared with those of various rock-forming minerals. The magnitude of diamagnetic anisotropy χ DIA , free of paramagnetic ions, is generally <10 −8 emu/g for various silicates. In contrast, χ PARA increases considerably with increasing Fe 2+ concentration, reaching 10 −5 emu/g for many of the silicates when the concentration of Fe 2+ is >1 mol%; this increasing χ PARA is the cause of the above-mentioned alignment obtained at the low field strength. Based on our observations, we infer the field intensity needed to obtain partial dust alignment of sufficient size to explain the observed polarization in astrophysical environments. Due to temperature dependences caused by a Curie Law and a rotational Brownian motion, the field intensity required to cause the alignment is expected to decrease considerably at low-temperature conditions assumed for a proto-planetary disk. The results of our experiment performed at room temperature provide a technical basis to reproduce grain alignment under such temperature conditions.

Attempt to detect diamagnetic anisotropy of dust-sized crystal orientated to investigate the origin of interstellar dust alignment

“…In the above function, θ is the angle between the magnetically stable axis of a grain and B. The value of B s was directly calculated for a Langevin process (Langevin and Curie, 1910) to be (Uyeda et al, 2005a(Uyeda et al, , 2010, and the possibility of grain alignment caused by χ PARA was discussed on the basis of the agreement. The dust alignment caused by χ DIA is considered to be less realistic compared with an alignment caused by χ PARA , since the calculated B S value based on χ DIA , obtained by bulk crystal measurement, is considerably large compared to the astronomical magnetic field.…”

“…The high angular momentum relative to that of the thermal equilibrium state assumed in the mechanism is realized only in diffuse interstellar regions. Hence, an alternative model was proposed that describes the origin of magnetic alignment based on the anisotropy of magnetic susceptibility (Uyeda et al, 2003b(Uyeda et al, , 2010. Table 1.…”