Stellar magnetic fields from four Stokes parameter observations

Magnetic Doppler imagingfrom four Stokes parameterobservations has uncovered a new level of complexity of stellar magnetic fieldspreviously not known. This new information is of interest to the evolution and structure of magnetic fields of intermediate and high-mass stars. Recently, we performed magnetic Doppler imagingstudy of the chemically peculiar starHD 24712 using phase-resolved spectropolarimetric observations of line-profiles in all four Stokes parameters. The observations were obtained with the HARPSpol instrument on the 3.6-m ESO telescope. Our analysis shows that the magnetic field of HD 24712 has very strong dipolar component with weak small-scale contributions. The finding of dipole-like field for HD 24712 gives evidence for the hypothesis that old Ap stars have predominantly dipolar magnetic fields. We plan on performing magnetic Doppler imagingfrom four Stokes parameterobservations for other Ap/Bp stars as well, which will help us understand the nature of magnetic fields in these stars.