First detection of linear polarization in the line profiles of active cool stars

TL;DR

This study reports the first detection of linear polarization in the spectral lines of active cool stars, revealing new insights into stellar magnetic fields by using full Stokes spectropolarimetry.

Contribution

It presents the first systematic observations of all four Stokes parameters in cool active stars, expanding magnetic field diagnostics beyond circular polarization alone.

Findings

Detected linear polarization in active stars HR 1099 and epsilon Eri.

Linear polarization amplitude is about ten times lower than circular polarization.

No polarization detected in inactive star alpha Cen A, indicating weak or absent magnetic fields.

Abstract

The application of high-resolution spectropolarimetry has led to major progress in understanding the magnetism and activity of late-type stars. During the last decade, magnetic fields have been discovered and mapped for many types of active cool stars using spectropolarimetric data. However, these observations and modeling attempts are fundamentally incomplete since they are based on the interpretation of the circular polarization alone. Taking advantage of the newly built HARPS polarimeter, we have obtained the first systematic observations of several cool active stars in all four Stokes parameters. Here we report the detection of magnetically-induced linear polarization for the primary component of the very active RS CVn binary HR 1099 and for the moderately active K dwarf epsilon Eri. For both stars the amplitude of linear polarization signatures is measured to be ~10^{-4} of the unpolarized continuum, which is approximately a factor of ten lower than for circular polarization. This is the first detection of the linear polarization in line profiles of cool active stars. Our observations of the inactive solar-like star alpha Cen A show neither circular nor linear polarization above the level of 10^{-5}, indicating the absence of a net longitudinal magnetic field stronger than 0.2 G.