Mon. Not. R. Astron. Soc., 451, 74-92 (2015/July-3)
Excited-state hydroxyl maser polarimetry: who ate all the πs?
GREEN J.A., CASWELL J.L. and McCLURE-GRIFFITHS N.M.
Abstract (from CDS):
We present polarimetric maser observations with the Australia Telescope Compact Array of excited-state hydroxyl (OH) masers. We observed 30 fields of OH masers in full Stokes polarization with the Compact Array Broadband Backend 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 percent π components clearly indicates that 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 polarization, with no circular detected, and linear only found at the ≤ 10 percent level for the brightest sources.