Basic Principles of Doppler Imaging

The goal of Doppler imaging is to invert line profile variations into 2-D map of stellar surface. Due to Doppler effect arising from the stellar rotation contributions of different surface zones are shifted in wavelength. Surface structures produce perturbations moving across line profiles as the star rotates. The longitude of a starspot can be determined directly from the position of the corresponding distortion in the line profile. The latitude can be inferred from the temporal behaviour of the distortion.

Below I illustrate typical line profile variation produced by different types of structures on the surfaces of rotating stars. I am doing Doppler Imaging analysis of all these different types of starspots.

Temperature Chemical abundance Magnetic field Non-radial pulsation

Magnetic field Active late-type stars possess small scale, structured magnetic fields. They can be detected and characterized using high-resolution spectroscopy combined with the analysis of circular polarization. The Stokes V signatures of magnetic spots move across the line profile and change in amplitude depending on the field orientation inside the spot (left).

Many hot stars exhibit very strong, globally-organized magnetic fields. Their magnetic geometries can be approximately described with a dipole inclined with respect to the stellar rotational axis. For such stars we can measure and interpret line profile variation in all four Stokes parameters. The Stokes V spectra provide information about the line of sight magnetic component while linear polarization spectra (Stokes Q and U) characterize the transverse magnetic field (right).