Generation of high circular polarization of interstellar Lyman $\alpha$ radiation triggering biological homochirality

Abstract

The homochirality of biological molecules on the Earth is a long-standing mystery regarding the origin of life. Circularly polarized ultraviolet (UV) light could induce the enantiomeric excess of biological molecules in the interstellar medium, leading to the homochirality on the earth. By performing 3D radiation transfer simulations with multiple scattering processes in interstellar dusty slabs, we study the generation of circular polarization (CP) of ultraviolet light at Lyman $\alpha$ ($\lambda = 0.1216~{\rm \mu m}$) as well as in the near-infrared (NIR, $\lambda = 2.14~{\rm \mu m}$) wavelengths. Our simulations show that the distributions of CP exhibit a symmetric quadrupole pattern, regardless of wavelength and viewing angle. The CP degree of scattered light from a dusty slab composed of aligned grains is $\sim 15$ percent for Ly$\alpha$ and $\sim 3$ percent at NIR wavelengths in the case of oblate grains with an MRN size distribution. We find that the CP degree of Ly$\alpha$ is well correlated with that in the NIR regardless of viewing angles, whilst being a factor of $\sim 5$ higher. Thus, high CP of Ly$\alpha$ is expected in sites where NIR CP is detected. We suggest that such circularly polarized Ly$\alpha$ may initiate the enantiomeric excess of biological molecules in space.