ZeemanDopplerimaging

Zeeman-Doppler imaging is a stellar astrophysical technique used to reconstruct maps of magnetic fields on the surfaces of stars. It fuses the Zeeman effect, which splits and polarizes spectral lines in the presence of magnetic fields, with Doppler broadening caused by stellar rotation. By monitoring polarized light over the star’s rotation, Zeeman-Doppler imaging infers the distribution of magnetic-field vectors across the visible stellar hemisphere, and in some implementations also maps brightness features such as spots.

Data for Zeeman-Doppler imaging come from high-resolution spectropolarimetry, typically recording Stokes I (intensity) and Stokes V (circular polarization), and sometimes linear polarization (Stokes Q and U). Time-series spectra are gathered across different rotational phases. Inversion algorithms reconstruct a surface magnetic-field map—often decomposed into radial, azimuthal, and meridional components—sometimes alongside a brightness map, to reproduce observed line-profile variations. Regularization methods, such as maximum entropy or Tikhonov regularization, help constrain the solution. Least-squares deconvolution is frequently used to boost the effective signal-to-noise by combining many spectral lines.

Applications span a wide range of stars, from rapidly rotating young stars to solar-like dwarfs and evolved giants. Zeeman-Doppler imaging provides insights into large-scale magnetic topology (polodial and toroidal components), differential rotation, and magnetic cycles, contributing to studies of stellar dynamos and activity.

Limitations include limited spatial resolution set by rotation speed and data quality, and intrinsic degeneracies in the inversion. ZDI is most sensitive to large-scale fields and may miss small-scale structures; reliable maps require good phase coverage and high signal-to-noise, especially in Stokes Q/U data when used.