Zeeman Doppler imaging of a cool star using line profiles in all four Stokes parameters for the first time

TL;DR

This paper presents the first successful Zeeman Doppler imaging of a cool star using all four Stokes parameters, revealing more comprehensive magnetic field topologies than previous methods relying solely on circular polarization.

Contribution

It demonstrates the first full Stokes vector magnetic mapping of a cool star, combining linear and circular polarization data for detailed magnetic topology reconstruction.

Findings

First full Stokes vector magnetic map of a cool star.

Detection of linear polarization signatures in a cool star.

Enhanced magnetic topology understanding through all four Stokes parameters.

Abstract

Magnetic fields in cool stars are ubiquitous but can still be challenging to characterize due to their complexity and relatively low strength. The polarization signature amplitudes are proportional to the field strength, and current studies of cool star magnetic fields are using circular polarization only since linear polarization is even weaker. However, circular polarization is only sensitive to the line-of-sight component of the magnetic field, meaning that many structural features are not recovered or may be misinterpreted when only circular polarization is used for reconstruction of stellar magnetic topologies. Linear polarization, on the other hand, is sensitive to the transverse component of the magnetic field and would provide a more complete picture of the magnetic field topology if combined with circular polarization. We have identified the first cool target, the RS CVn star II~Peg, suitable for full Stokes vector analysis. Using current instrumentation, we have succeeded in systematically detecting its linear polarization signatures with a precision and rotational phase coverage sufficient for magnetic mapping. Here we present the very first temperature and magnetic field maps reconstructed for a cool star using all four Stokes parameter spectra.