Excited-state hydroxyl maser polarimetry: Who ate all the {\pi}s?

TL;DR

This study presents the largest polarimetric survey of excited-state hydroxyl masers, revealing that {\pi} components are common and comparable to ground-state transitions, challenging previous assumptions about their suppression.

Contribution

It provides the first large-scale, full polarimetric analysis of excited-state OH masers, demonstrating the prevalence of {\pi} components in star-forming regions.

Findings

16% of Zeeman pairs exhibit {\pi} components.

{\pi} components are consistent across ground- and excited-state transitions.

Methanol masers show low linear polarization, no circular polarization.

Abstract

We present polarimetric maser observations with the Australia Telescope Compact Array (ATCA) of excited-state hydroxyl (OH) masers. We observed 30 fields of OH masers in full Stokes polarization with the Compact Array Broadband Backend (CABB) at both the 6030 and 6035 MHz excited-state OH transitions, and the 6668-MHz methanol maser transition, detecting 70 sites of maser emission. Amongst the OH we found 112 Zeeman pairs, of which 18 exhibited candidate {\pi} components. This is the largest single full polarimetric study of multiple sites of star formation for these frequencies, and the rate of 16% {\pi} components clearly indicates the {\pi} component exists, and is comparable to the percentage recently found for ground-state transitions. This significant percentage of {\pi} components, with consistent proportions at both ground- and excited-state transitions, argues against Faraday rotation suppressing the {\pi} component emission. Our simultaneous observations of methanol found the expected low level of polarisation, with no circular detected, and linear only found at the less than or equal to 10% level for the brightest sources.