Asymmetric shocks in $\chi$ Cyg observed with linear spectropolarimetry

TL;DR

This paper uses spectropolarimetry to analyze the stellar photosphere dynamics of Mira star $ ext{chi}$ Cyg, revealing non-uniform velocity fields during pulsation likely caused by convection and pulsation interactions.

Contribution

It introduces a novel spectropolarimetric analysis method to map velocity fields on the stellar disk, providing new insights into stellar pulsation and convection interactions.

Findings

Polarised spectrum dominated by intrinsic polarisation

Velocity fields are non-homogeneous over the stellar disk during pulsation

Developed an algorithm to analyze polarisation signals across spectral lines

Abstract

Aims: From a coherent interpretation of the linear polarisation detected in the spectral lines of the Mira star $\chi$ Cyg, we derive information about the dynamics of the stellar photosphere, including pulsation. Methods: From spectropolarimetric observations of $\chi$ Cyg, we perform careful analysis of the polarisation signals observed in atomic and molecular lines, both in absorption and emission, using the radiative transfer in the polarisation context, through two mechanisms: intrinsic polarisation and continuum depolarisation. We also explain the observed line doubling phenomenon in terms of an expanding shell in spherical geometry, which allows us to pinpoint the coordinates over the stellar disk with enhanced polarisation. Results: We find that the polarised spectrum of $\chi$ Cyg is dominated by intrinsic polarisation, with a negligible continuum depolarisation. The observed polarised signals can only be explained by assuming that this polarisation is locally enhanced by velocity fields. During the pulsation, radial velocities are not homogeneous over the disk. We map these regions of enhanced velocities. Conclusions: We have set an algorithm to distinguish in any stellar spectra of linear polarisation the origin of this polarisation and the way to increase signal by coherently adding many lines with an appropriated weight. Applied to the Mira star $\chi$ Cyg, we reached the unexpected result that during the pulsation, velocities are radial but not homogeneous over the disk. The reason for these local velocity enhancements are probably related to the interplay between the atmospheric pulsation dynamics and the underlying stellar convection.