Magnetic Field Diagnosis in AP Stars through Analysis of Spectropolarimetric Observations with the Moment Technique
The moment technique for the diagnosis of the magnetic fields of Ap stars from spectropolarimetric observations is presented, and its strengths and limitations are discussed. The principle of the technique is as follows: the shapes of spectral line profiles recorded in the different Stokes parameters are characterized by their moments of various orders about their centre. These moments are in turn interpreted in terms of moments of the stellar magnetic field. In all the applications carried out so far, the interpretation has been based upon the weak-line approximation. Arguments supporting the validity of this approach are presented. The exploitation of the first-order line profile moment in the Stokes parameter V and of the second-order moment in all four Stokes parameters is described. Determinations of the corresponding magnetic field moments repeated throughout the stellar rotation cycle yield meaningful constraints on the field geometric structure and can be used as a starting point for modelling the latter.
Key wordsstars: chemically peculiar stars: magnetic fields methods: data analysis line profiles polarization
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