Abstract
(Edited for length) The Zeeman effect is the only observational technique available to measure directly the strength of magnetic fields in regions of star formation. We review 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 ISM -- H~I, OH, and CN in extended gas and OH, CH$_3$OH, and H$_2$O in masers. These species span densities from $\sim10$ cm$^{-3}$ to $\sim10^{10}$ 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 is subcritical (gravity dominates magnetic support) at lower densities but supercritical for $N_H \gtrsim 10^{22}$ cm$^{-2}$. Above $n_H\sim 300$ cm$^{-3}$, which is roughly the density at which clouds typically become self-gravitating, the strength of magnetic fields increases approximately as $B \propto n^{2/3}$, 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.
Highlights
What governs or regulates star formation has been a crucial question in astrophysics for many decades
The first indication of an interstellar Zeeman effect came from observations of polarization in OH masers by Weinreb et al (1965); they suggested that the polarization might be due to the Zeeman effect, that interpretation was not certain because the standard pattern of the classical Zeeman effect was not seen in the polarized maser emission
In this chapter we review Zeeman observations in the interstellar medium and discuss how observations of the Zeeman effect can test models of star formation, the present state of such tests, and possible future developments
Summary
What governs or regulates star formation has been a crucial question in astrophysics for many decades. The Zeeman effect was discovered by Dutch physicist Pieter Zeeman in 1896 in a laboratory experiment In his discovery paper Zeeman suggested that the effect he had discovered could be important in measuring magnetic fields in astrophysics. Following intense observational work by several workers, Verschuur (1968) first detected the Zeeman effect in the extended interstellar medium in the 21 cm hyperfine line of H I. It was another 15 years before Zeeman splitting in extended molecular gas was detected, in OH by Crutcher and Kazès (1983), and yet another 15 years before detection in the third (and so far last) species, CN, by Crutcher et al (1996, 1999b). This article is specific to the Zeeman effect; it expands discussion of the effect itself, summarizes the discussion in the above review, adds the (very limited) new Zeeman data that have become available, and discusses some more recent controversies about the astrophysical interpretation of the observational results
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