Doppler Imaging of stellar magnetic fields

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Abstract
We used the new magnetic Doppler Imaging code to reconstruct the magnetic field geometry and surface abundance distributions for the classical magnetic CP star α 2CVn. High-resolution spectropolarimetric observations in the Stokes I and V parameters were collected with the SOFIN échelle spectrograph at the Nordic Optical Telescope. This superb observational material in combination with the advanced modelling technique allowed to achieve the first simultaneous self-consistent mapping of the vector magnetic field and the abundance distributions of six chemical elements. In recovering the stellar magnetic distribution no prior assumptions about the field geometry or strength were made. Instead, we restricted possible solutions of the inverse problem by means of the multipolar regularization method which searches for the field map close to a general non-axisymmetric multipolar configuration but allows departures from this geometry if that is required by the observational data. We found that the magnetic field of α 2>CVn is dominated by a dipolar component and has a minor quadrupole contribution. The surface distributions of the chemical species form symmetric patterns which closely follow the magnetic geometry. This discovery constitutes one of the first direct observational constraints on the horizontal diffusion processes acting in the upper envelope of a strongly magnetized stellar atmosphere. In addition to the extensive magnetic Doppler Imaging analysis, we derived new accurate estimates of the atmospheric parameters and basic physical properties of α 2CVn using the energy distribution, hydrogen line profiles and the recent Hipparchos parallax.

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