2008ApJ...677.1401Y

We continue our studies of alignment of atoms by radiation in diffuse media and their realignment by the ambient magnetic field. We understand atomic alignment as the alignment of atoms or ions in their ground or metastable states that have more than two fine or hyperfine sublevels. In particular, we consider the alignment in interstellar and circumstellar media, with the goal of developing new diagnostics of magnetic fields in these environments. We provide predictions of the polarization that arises from astrophysically important aligned atoms (ions) with fine structure of the ground level, namely, O I, S II, and Ti II. Unlike our earlier papers which dealt with weak fields only, a part of our current paper is devoted to the studies of atomic alignment when magnetic fields get strong enough to affect the emission from the excited level. This is a regime of the Hanle effect, but modified by the atomic alignment of the atomic ground state. We also discuss the ground Hanle effect where the magnetic splitting is comparable to the pumping rate. Using an example of emission and absorption lines of the S II ion we demonstrate how polarimetric studies can probe magnetic fields in circumstellar regions and accretion disks. In addition, we show that atomic alignment induced by anisotropic radiation can induce substantial variations of magnetic dipole transitions within the ground state, thus affecting abundance studies based on this emission. Moreover, we show that the radio emission arising this way is polarized, which provides a new way to study magnetic fields, e.g., at the epoch of universe reionization.