Spin-Polarized Electrons from Magnetically Aligned Grains and Chiral Symmetry Breaking: Effects of Cosmic Rays in Protostellar Environments

Abstract

Low-energy spin-polarized electrons (SPEs) are thought to cause symmetry breaking and could explain the origin of homochirality of prebiotic molecules such as amino acids and sugars. Here we study the effect of cosmic rays (CRs) on the emission of SPEs from aligned grains in dense protostellar environments and explore their effects on chiral asymmetry of prebiotic molecules. We first show that icy grains in protostellar environments can align with magnetic fields due to magnetically enhanced radiative torque mechanism. We then study the production of thermal electrons by CR ionization of H$_2$ and the CR-induced UV radiation using the attenuated CR spectra in dense cores obtained from a continuous slowing down model. Next, we show that thermal electrons with initial random spins captured by aligned grains will become spin-polarized due to the Barnett effect, converting unpolarized electrons into SPEs. We calculate the rate of photoemission of such SPEs by CRs-induced UV radiation and secondary electron emission from aligned grains and find that the photoemission by CRs-induced UV radiation is dominant. Finally, we calculate the total production rate of SPEs inside aligned dust grains by CRs. We estimate the alignment degree of SPEs from superparmagnetic (SPM) grains and find that it is only significant for SPM grains having large iron clusters and fast rotation. We suggest that low-energy secondary SPEs from aligned superparamagnetic grains with large iron inclusions induced by CRs might cause the chiral asymmetry of chiral prebiotic molecules formed in the ice mantle of aligned grains, in analogous to UV circularly polarized light.