Measuring the continuum polarization with ESPaDOnS

TL;DR

This study demonstrates that high-resolution spectropolarimetric measurements of continuum polarization with ESPaDOnS are feasible and accurate within 0.2-0.3%, enabling detailed astrophysical investigations.

Contribution

The paper introduces a new pipeline, OPERA, and validates the capability of ESPaDOnS to measure continuum polarization accurately in both linear and circular modes.

Findings

Continuum linear polarization measurements are consistent with existing broad-band data.

The accuracy of polarization degree is around 0.2-0.3%.

The polarization position angle accuracy is better than 5 degrees.

Abstract

Our goal is to test the feasibility to obtain accurate measurements of the continuum polarization from high-resolution spectra using the spectropolarimetric mode of ESPaDOnS. We used the new pipeline OPERA to reduce recent and archived ESPaDOnS data. A couple of standard polarization stars and several science objects were tested for the linear mode. In addition, the circular mode was tested using several objects from the archive with expected null polarization. Synthetic broad-band polarization was computed from the ESPaDOnS continuum polarization spectra and compared with published values (when available) to quantify the accuracy of the instrument. The continuum linear polarization measured by ESPaDOnS is consistent with broad-band polarimetry measurements available in the literature. The accuracy in the degree of linear polarization is around 0.2-0.3% considering the full sample. The accuracy in polarization position angle using the most polarized objects is better than 5deg. Consistent with this, the instrumental polarization computed for the circular continuum polarization is also between 0.2-0.3%. Our results suggest that measurements of the continuum polarization using ESPaDOnS are viable and can be used to study many astrophysical objects.