Atomic alignment and Diagnostics of Magnetic Fields in Diffuse Media

TL;DR

This paper advances atomic alignment diagnostics to measure magnetic fields in diffuse astrophysical media, including strong field regimes and their effects on polarization, with applications to circumstellar environments and cosmic reionization.

Contribution

It extends previous models to include strong magnetic fields affecting emission lines, and demonstrates how polarimetry can probe magnetic fields in various astrophysical settings.

Findings

Predictions of polarization from aligned atoms like OI, SII, Ti II.

Demonstration of magnetic field probing via polarimetric studies.

Identification of atomic alignment effects on abundance measurements.

Abstract

We continue our studies of atomic alignment in diffuse media, in particularly, in interstellar and circumstellar media, with the goal of developing new diagnostics of magnetic fields in these environments. We understand atomic alignment as alignment of atoms or ions in their ground state. Such atoms are sensitive to weak magnetic fields. In particular, we provide predictions of the polarization that arises from astrophysically important aligned atoms (ions) with fine structure of the ground level, namely, OI and SII and Ti II. Unlike our earlier papers which dealt with weak fields only, a substantial 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, i.e. with the regime when the magnetic splitting is comparable to the line-width. This is a regime of Hanle effect modified by the atomic alignment. Using an example of emission and absorption lines of SII 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, the radio emission is polarized, provides a new way to study magnetic fields, e.g. at the epoch of Universe reionization.