A MERLIN Study of 6 GHz Excited-state OH & 6.7 GHz Methanol Masers in ON1

TL;DR

This study uses high-resolution MERLIN observations to analyze excited-state OH and 6.7 GHz methanol masers in ON1, revealing their polarization, magnetic fields, and spatial distribution related to star formation processes.

Contribution

First polarization measurements of methanol masers with MERLIN, providing new insights into magnetic fields and maser distributions in ON1.

Findings

Masers are located at the southern edge of the ultra-compact HII region.

Linear polarization of OH masers ranges from 10% to 18.5%.

Magnetic fields inferred from Zeeman splitting are between -1.1 and -5.8 mG.

Abstract

MERLIN observations of 6.668-GHz methanol and both 6.031- and 6.035-GHz hydroxyl (OH) emission from the massive star-formation region ON1 are presented. These are the first methanol observations made in full polarization using 5 antennas of MERLIN, giving high resolution and sensitivity to extended emission. Maser features are found to lie at the southern edge of the ultra-compact HII region, following the known distribution of ground-state OH masers. The masers cover a region ~1 arcsec in extent, lying perpendicular to the H13CO+ bipolar outflow. Excited-state OH emission demonstrates consistent polarization angles across the strongest linearly polarized features which are parallel to the overall distribution. The linear polarizations vary between 10.0 and 18.5 per cent, with an average polarization angle of -60 deg +/- 28 deg. The strongest 6.668-GHz methanol features provide an upper limit to linear polarization of ~1 per cent. Zeeman splitting of OH shows magnetic fields between -1.1 to -5.8 mG, and a tentative methanol magnetic field strength of -18 mG is measured.