Optical polarization map of the Polaris Flare with RoboPol

TL;DR

This study presents the first wide-field optical polarization map of the Polaris Flare, revealing the magnetic field structure in a quiescent molecular cloud using RoboPol, with detailed data analysis and comparison to Herschel and Planck observations.

Contribution

First application of RoboPol's wide-field imaging to map magnetic fields in a molecular cloud through optical polarization of background stars.

Findings

Magnetic field shows large-scale ordered pattern.

Alignment with Herschel dust emission striations at high longitudes.

Agreement with Planck dust polarization measurements.

Abstract

The stages before the formation of stars in molecular clouds are poorly understood. Insights can be gained by studying the properties of quiescent clouds, such as their magnetic field structure. The plane-of-the-sky orientation of the field can be traced by polarized starlight. We present the first extended, wide-field ($\sim$10 $\rm deg^2$) map of the Polaris Flare cloud in dust-absorption induced optical polarization of background stars, using the RoboPol polarimeter at the Skinakas Observatory. This is the first application of the wide-field imaging capabilities of RoboPol. The data were taken in the R-band and analysed with the automated reduction pipeline of the instrument. We present in detail optimizations in the reduction pipeline specific to wide-field observations. Our analysis resulted in reliable measurements of 641 stars with median fractional linear polarization 1.3%. The projected magnetic field shows a large scale ordered pattern. At high longitudes it appears to align with faint striations seen in the Herschel-SPIRE map of dust emission (250 $\mu m$), while in the central 4-5 deg$^2$ it shows an eddy-like feature. The overall polarization pattern we obtain is in good agreement with large scale measurements by Planck of the dust emission polarization in the same area of the sky.