Magnetic Doppler imaging of α2 Canum Venaticorum in all four Stokes parameters - Unveiling the hidden complexity of stellar magnetic fields

摘要

Context. Strong organized magnetic fields have been studiedin the upper main sequence chemically peculiar stars for more than halfa century. However, only recently have observational methods andnumerical techniques become sufficiently mature to allow us to recordand interpret high-resolution four Stokes parameter spectra, leading tothe first assumption-free magnetic field models of these stars. Aims. Here we present a detailed magnetic Doppler imaginganalysis of the spectropolarimetric observations of the prototypicalmagnetic Ap star CVn.This is the second star for which the magnetic field topology andhorizontal chemical abundance inhomogeneities have been inferreddirectly from phase-resolved observations of line profiles in all fourStokes parameters, free from the traditional assumption of a low-ordermultipolar field geometry. Methods. We interpret the rotational modulation of the circular and linear polarization profiles of the strong Fe II and Cr II lines in the spectra of CVn recorded with the M US IC OSspectropolarimeter. The surface abundance distributions of the twochemical elements and a full vector map of the stellar magnetic fieldare reconstructed in a self-consistent inversion using ourstate-of-the-art magnetic Doppler imaging code I NVERS10.Results. We succeeded in reproducing most of the details of the available spectropolarimetric observations of CVnwith a magnetic map which combines a global dipolar-like field topologywith localized spots of higher field intensity. We demonstrate thatthese small-scale magnetic structures are inevitably required to fitthe linear polarization spectra; however, their presence cannot beinferred from the StokesI and V observations alone.We also found high-contrast surface distributions of Fe and Cr, withboth elements showing abundance minima in the region of weaker andtopologically simpler magnetic field.Conclusions. Our magnetic Doppler imaging analysis of CVnand previous results for 53Cam support the view that the uppermain sequence stars can harbour fairly complex surface magnetic fieldswhich resemble oblique dipoles only at the largest spatial scales.Spectra in all four Stokes parameters are absolutely essential tounveil and meaningfully characterize this field complexity in Ap stars.We therefore suggest that understanding magnetism of stars in otherparts of the H-R diagram is similarly incomplete without investigationof their linear polarization spectra. Key words: polarization - stars: atmospheres - stars: chemically peculiar - stars: individual: CVn - stars: magnetic field