A map of the magnetic field in milligauss in the W3(OH) star-forming region, derived from Zeeman splittings of OH maser features.
Atoms and molecules with net electronic angular momentum have large Zeeman splittings, proportional to the Bohr magneton. Examples are hydrogen atoms and the OH and CH molecules, and species of this type typically have splittings between magnetic components of their spectral lines of several kHz per mG of magnetic field.
This type of Zeeman effect influences, among other transitions, the 21-cm line of the H-atom, and the OH maser transitions close to 1.7 and 6.0GHz. Closed-shell species, such as H2O and CH3OH have Zeeman splittings proportional to the nuclear magneton, which is smaller than the Bohr magneton by the electron to proton mass ratio.
If the Zeeman splitting significantly exceeds the line width of the individual spectral components, as is often the case for OH masers in star-forming regions, the magnetic field strength in the source can be recovered directly from the observed splitting. The sense of the field (towards or away from the observer) is revealed by observing whether magnetic components in the spectra with left-hand elliptical polarization lie at higher or lower frequency than their right-handed counterparts. Linear polarization can also be detected, but the relationship with the magnetic field direction can be complex. Numerical modelling is required to recover the full vector magnetic field.
We cannot observe the Zeeman splitting directly when the splitting is weak (vastly smaller than the spectral line width). However, we can still use spectra in the Stokes parameters to recover the line-of-sight component of the magnetic field in this case.