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

Possible dust alignment in dense regions due to paramagnetic anisotropy ΔχPARA is discussed based on the results of a laboratory experiment on ortho-pyroxene grains containing a small amount of Fe2+ 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 ΔχDIA, free of paramagnetic ions, is generally <10-8 emu/g for various silicates. In contrast, ΔχPARA increases considerably with increasing Fe2+ concentration, reaching 10-5 emu/g for many of the silicates when the concentration of Fe2+ is >1 mol%; this increasing ΔχPARA 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.