Strong variable linear polarization in the cool active star II Peg

TL;DR

This study demonstrates the detection of strong linear polarization in cool active star II Peg, enabling more comprehensive magnetic field mapping through Zeeman Doppler imaging using all four Stokes parameters.

Contribution

It initiates systematic observations of active stars in all four Stokes parameters and identifies II Peg as a promising target for advanced magnetic field reconstruction.

Findings

Linear polarization detected in all four stars in at least one observation.

II Peg shows exceptionally strong and variable linear polarization signatures.

First promising target for full Stokes Zeeman Doppler imaging in active cool stars.

Abstract

Magnetic fields of cool active stars are currently studied polarimetrically using only circular polarization observations. Including linear polarization in the reconstruction of stellar magnetic fields allows more information about the magnetic field to be extracted and significantly improves the reliability of stellar magnetic field maps. The goal of this study is to initiate systematic observations of active stars in all four Stokes parameters and to identify cool stars for which linear polarization can be detected at a level sufficient for Zeeman Doppler imaging (ZDI). Four active RS CVn binaries, II Peg, HR 1099, IM Peg, and sigma Gem, were observed with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope during a time period from February 2012 to January 2013. The least-squares deconvolution procedure was applied to derive mean polarization profiles of all four Stokes parameters. Linear polarization was detected in all four stars in at least one observation. At the same time, II Peg showed an exceptionally strong and highly variable linear polarization signature throughout all observations. This establishes II Peg as the first promising target for ZDI in all four Stokes parameters and suggests the feasibility of such an analysis with existing equipment for at least a few of the most active cool stars.