Observations of the Goldreich-Kylafis effect in star-forming regions with XPOL at the IRAM 30m telescope

TL;DR

This study reports the detection of weak polarized molecular line emission in star-forming regions using the IRAM 30m telescope, exploring the Goldreich-Kylafis effect's potential to reveal magnetic field information along the line of sight.

Contribution

First observational evidence of the Goldreich-Kylafis effect in multiple star-forming regions using the XPOL polarimeter at IRAM 30m, including analysis of instrumental polarization effects.

Findings

Detected polarized signals in nearly all transitions, with one exceeding instrumental polarization expectations.

Tentative evidence of polarized line emission (pL<~1.5%) in CS(2-1) toward G34.3+0.2.

Instrumental effects can mimic polarization, requiring careful analysis to confirm astronomical signals.

Abstract

The Goldreich-Kylafis (GK) effect causes certain molecular line emission to be weakly linearly polarized, e.g., in the presence of a magnetic field. Compared to polarized dust emission, the GK effect has the potential to yield additional information along the line of sight through its dependence on velocity in the line profile. Our goal was to detect polarized molecular line emission toward the DR21(OH), W3OH/H2O, G34.3+0.2, and UYSO1 dense molecular cloud cores in transitions of rare CO isotopologues and CS. The feasibility of such observations had to be established by studying the influence of polarized sidelobes, e.g., in the presence of extended emission in the surroundings of compact sources. The observations were carried out with the IRAM 30m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers. We produced beam maps to investigate instrumental polarization. While in nearly all transitions toward all sources a polarized signal is found, its degree of polarization only in one case surpasses the polarization that can be expected due to instrumental effects. It is shown that any emission in the polarized sidelobes of the system can produce instrumental polarization, even if the source is unpolarized. Tentative evidence for astronomically polarized line emission with pL<~1.5% was found in the CS(2-1) line toward G34.3+0.2.