Excited-state hydroxyl maser polarimetry: Who ate all the πs?

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 π components. This is the largest single full polarimetric study of multiple sites of star formation for these frequencies, and the rate of 16% π components clearly indicates the π component exists, and is comparable to the percentage recently found for ground-state transitions. This significant percentage of π components, with consistent proportions at both ground- and excited-state transitions, argues against Faraday rotation suppressing the π 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.

17 pages (72 pages including appendices), 9 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

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• Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
• A quantum mechanical approach to establishing the magnetic field orientation from a maser Zeeman profile
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• The Extraordinary Outburst in the Massive Protostellar System NGC6334I-MM1: Strong Increase in Mid-Infrared Continuum Emission
• Unusual shock-excited OH maser emission in a young Planetary Nebula
• A 6.7 GHz methanol maser survey of the central molecular zone
• The accretion burst of the massive young stellar object G323.46 -0.08
• MAGMO: Polarimetry of 1720-MHz OH Masers towards Southern Star Forming Regions
• A search for the OH 6035 MHz line in high-mass star-forming regions
• New ex-OH maser detections in the northern celestial hemisphere
• Physical conditions around high-mass young star-forming objects via simultaneous observations of excited OH and methanol masers
• Accurate polarization calibration of FAST spectral data for measurements of Zeeman splittings of OH megamasers in IRAS 02524+2046