Polarization from aligned atoms as a diagnostics of circumstellar, AGN and interstellar magnetic fields: II. Atoms with Hyperfine Structure

Abstract

We show that atomic alignment presents a reliable way to study topology of astrophysical magnetic fields. The effect of atomic alignment arises from modulation of the relative population of the sublevels of atomic ground state pumped by anisotropic radiation flux. As such aligned atoms precess in the external magnetic field and this affects the properties of the polarized radiation arising from both scattering and absorption by the atoms. As the result the polarizations of emission and absorption lines depend on the 3D geometry of the magnetic field as well as the direction and anisotropy of incident radiation. We consider a subset of astrophysically important atoms with hyperfine structure. For emission lines we obtain the dependencies of the direction of linear polarization on the directions of magnetic field and the incident pumping radiation. For absorption lines we establish when the polarization is perpendicular and parallel to magnetic field. For both emission and absorption lines we find the dependence on the degree of polarization on the 3D geometry of magnetic field. We claim that atomic alignment provides a unique tool to study magnetic fields in circumstellar regions, AGN, interplanetary and interstellar medium. This tool allows studying of 3D topology of magnetic fields and establish other important astrophysical parameters. We consider polarization arising from both atoms in the steady state and also as they undergo individual scattering of photons. We exemplify the utility of atomic alignment for studies of astrophysical magnetic fields by considering a case of Na alignment in a comet wake.