Review of Zeeman Effect Observations of Regions of Star Formation

Abstract

The Zeeman effect is the only observational technique available to measure directly the strength of magnetic fields in regions of star formation. This chapter reviews the physics of the Zeeman effect and its practical use in both extended gas and in masers. We discuss observational results for the five species for which the Zeeman effect has been detected in the interstellar medium -H I, OH, and CN in extended gas and OH, CH 3 OH, and H 2 O in masers. These species cover a wide range in density, from ∼ 10 cm −3 to ∼ 10 10 cm −3 , which allows magnetic fields to be measured over the full range of cloud densities. However, there are significant limitations, including that only the line-of-sight component of the magnetic field strength can usually be measured and that there are often significant uncertainties about the physical conditions being sampled, particularly for masers. We discuss statistical methods to partially overcome these limitations. The results of Zeeman observations are that the mass to magnetic flux ratio, which measures the relative importance of gravity to magnetic support, is subcritical (gravity dominates magnetic support) at lower densities but supercritical for N H 10 22 cm −2 . Above n H ∼ 300 cm −3 , which is roughly the density at which clouds typically become self-gravitating, the strength of magnetic fields increases approximately as B ∝ n 2/3 , which suggest that magnetic fields do not provide significant support at high densities. This is consistent with high-density clouds being supercritical. However, magnetic fields have a large range in strengths at any given density, so the role of magnetic fields should differ significantly from one cloud to another. And for maser regions the dependence of field strength on density may have a slightly lower slope. Turbulent reconnection theory seems to best match the Zeeman observational results.

ALMA CN Zeeman Observations of AS 209: Limits on Magnetic Field Strength and Magnetically Driven Accretion Rate

“…The Zeeman effect offers a direct means of constraining the line-of-sight magnetic field strength without contamination from continuum dust scattering; however, it is possible to produce circular polarization in molecular lines through resonant scattering (Houde et al 2013). In the presence of a magnetic field, spectral lines split apart in frequency to a degree that depends on the magnetic field strength as ν = ν 0 ± eB 4πmec (e.g., Crutcher & Kemball 2019), where ν 0 is the line frequency in the absence of a magnetic field and B is the magnetic field strength. In astronomical sources with intrinsically weak magnetic fields compared to those in, for example, stellar photospheres, the observable of Zeeman splitting is circular polarization of the spectral line, which measures the line of sight magnetic field strength.…”

PORTAL: Three-dimensional polarized (sub)millimeter line radiative transfer

“…Magnetic fields permeate the Universe and often play an important role in the dynamics of astrophysical processes (Crutcher 2012;Vlemmings 2013;Crutcher & Kemball 2019). It is difficult to directly observe magnetic fields; one typically has to use the polarization properties of the observed light (e.g., Han 2017).…”

Magnetism Science with the Square Kilometre Array

“…The role of magnetic fields in forming and shaping these is not yet clear [151], but can be resolved by determining the strength and degree of order of magnetic fields and comparing with simulations. Within molecular clouds, field strength measurements are via the Zeeman effect [28], readily observable in maser emission, but requiring very deep, pointed observations with high spectral resolution at radio frequencies for thermal emission (e.g., OH at 1665 and 1667 MHz). The Zeeman effect can be prominent in paramagnetic molecules, particularly OH, but the nature of the emission can make measurements difficult, particularly thermal emission, for which it is small relative to the linewidth, and thus difficult to measure.…”

“…Three primary techniques are employed: (i) synchrotron radiation, including its degree of linear and circular polarization; (ii) the Faraday rotation effect induced by intervening magneto-ionic media; and (iii) Zeeman splitting of energy levels in atoms and molecules. A full review of these mechanisms is beyond the scope of this paper; we refer the reader to [27] for a detailed description of the insights that can be gained from the study of synchrotron emission and Faraday rotation and [28,29] for reviews of the power of Zeeman splitting.…”