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

摘要

We show that atomic alignment presents a reliable way to study topology ofastrophysical magnetic fields. The effect of atomic alignment arises frommodulation of the relative population of the sublevels of atomic ground statepumped by anisotropic radiation flux. As such aligned atoms precess in theexternal magnetic field and this affects the properties of the polarizedradiation arising from both scattering and absorption by the atoms. As theresult the polarizations of emission and absorption lines depend on the 3Dgeometry of the magnetic field as well as the direction and anisotropy ofincident radiation. We consider a subset of astrophysically important atomswith hyperfine structure. For emission lines we obtain the dependencies of thedirection of linear polarization on the directions of magnetic field and theincident pumping radiation. For absorption lines we establish when thepolarization is perpendicular and parallel to magnetic field. For both emissionand absorption lines we find the dependence on the degree of polarization onthe 3D geometry of magnetic field. We claim that atomic alignment provides aunique tool to study magnetic fields in circumstellar regions, AGN,interplanetary and interstellar medium. This tool allows studying of 3Dtopology of magnetic fields and establish other important astrophysicalparameters. We consider polarization arising from both atoms in the steadystate and also as they undergo individual scattering of photons. We exemplifythe utility of atomic alignment for studies of astrophysical magnetic fields byconsidering a case of Na alignment in a comet wake.