Magnetic field topology of tau Sco. The uniqueness problem of Stokes V ZDI inversions

TL;DR

This study evaluates the robustness of Zeeman Doppler imaging (ZDI) in reconstructing tau Sco's complex magnetic field, highlighting the non-uniqueness of solutions and the importance of linear polarisation data for accurate magnetic topology mapping.

Contribution

It demonstrates that different ZDI parameterisations can produce similar Stokes V profiles but different magnetic topologies, emphasizing the need for linear polarisation observations.

Findings

Different ZDI models yield similar Stokes V but different magnetic maps.

Linear polarisation data can break degeneracies in magnetic field reconstructions.

Tau Sco's magnetic field comprises both poloidal and toroidal components.

Abstract

The early B-type star tau Sco exhibits an unusually complex, relatively weak surface magnetic field. Its topology was previously studied with the Zeeman Doppler imaging (ZDI) modelling of high-resolution circular polarisation (Stokes V) observations. Here we assess the robustness of the Stokes V ZDI reconstruction of the magnetic field geometry of tau Sco and explore the consequences of using different parameterisations of the surface magnetic maps. We succeeded in reproducing previously published magnetic field maps of tau Sco using both general harmonic expansion and a direct, pixel-based representation of the magnetic field. These maps suggest that the field topology of tau Sco is comprised of comparable contributions of the poloidal and toroidal magnetic components. At the same time, we also found that available Stokes V observations can be successfully fitted employing restricted harmonic expansions, by either neglecting the toroidal field altogether or linking the radial and horizontal components of the poloidal field as required by the widely used potential field extrapolation technique. These alternative modelling approaches lead to a stronger and topologically more complex surface field structure. The field distributions recovered with different ZDI options differ significantly, yielding indistinguishable Stokes V profiles but different linear polarisation (Stokes Q and U) signatures. Our investigation underscores the well-known problem of non-uniqueness of the Stokes V ZDI inversions. For the magnetic stars with properties similar to tau Sco (relatively complex field, slow rotation) the outcome of magnetic reconstruction depends sensitively on the adopted field parameterisation. Stokes Q and U spectropolarimetric observations represent the only way of breaking the degeneracy of surface magnetic field models.