Magnetic Doppler imaging of alpha^2 Canum Venaticorum in all four Stokes parameters. Unveiling the hidden complexity of stellar magnetic fields

TL;DR

This study uses advanced magnetic Doppler imaging with all four Stokes parameters to reveal complex magnetic field structures on the Ap star alpha^2 CVn, highlighting the importance of linear polarization data for accurate magnetic field characterization.

Contribution

The paper presents the first detailed magnetic Doppler imaging of alpha^2 CVn using all four Stokes parameters, uncovering small-scale magnetic structures that are essential for accurate modeling.

Findings

Magnetic maps show a combination of dipolar and localized spots.

Linear polarization spectra are crucial to detect small-scale magnetic features.

Complex magnetic fields are common in upper main sequence stars.

Abstract

Strong organized magnetic fields have been studied in the upper main sequence chemically peculiar stars for more than half a century. However, only recently have observational methods and numerical techniques become sufficiently mature to allow us to record and interpret high-resolution four Stokes parameter spectra, leading to the first assumption-free magnetic field models of these stars. Here we present a detailed magnetic Doppler imaging analysis of the spectropolarimetric observations of the prototypical magnetic Ap star alpha^2 CVn. The surface abundance distributions of Fe and Cr and a full vector map of the stellar magnetic field are reconstructed in a self-consistent inversion using our state-of-the-art magnetic Doppler imaging code Invers10. We succeeded in reproducing most of the details of the available spectropolarimetric observations of alpha^2 CVn with a magnetic map which combines a global dipolar-like field topology with localized spots of higher field intensity. We demonstrate that these small-scale magnetic structures are inevitably required to fit the linear polarization spectra; however, their presence cannot be inferred from the Stokes I and V observations alone. Our magnetic Doppler imaging analysis of alpha^2 CVn and previous results for 53 Cam support the view that the upper main sequence stars can harbour fairly complex surface magnetic fields which resemble oblique dipoles only at the largest spatial scales. Spectra in all four Stokes parameters are absolutely essential to unveil and meaningfully characterize this field complexity in Ap stars. We therefore suggest that understanding magnetism of stars in other parts of the H-R diagram is similarly incomplete without investigation of their linear polarization spectra.