Polarization of HO masers in the presence of velocity and magnetic field gradients

Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire, SK11 9DL, UK e-mail: wouter@jb.man.ac.uk

Received: 21 July 2005
Accepted: 12 September 2005

Abstract

Through polarization observations H₂O masers are excellent probes of magnetic fields in the maser region. Magnetic field strengths, such as those in the H₂O masers regions of the envelopes of late-type stars and star-forming regions, are typically determined using a direct relation between the field strength and the observed circular polarization. Here it is shown that velocity and magnetic field gradients along the maser have a significant effect on the field strengths obtained from circular polarization observations. Due to velocity gradients the actual magnetic field strength could be up to higher than the field strength derived from the observations. Additionally, when a magnetic field gradient is present, the resulting circular polarization derived is caused predominantly by the average magnetic field in the unsaturated maser core. Measurements of the fractional linear polarization are not affected by velocity or magnetic field strength gradients, though changes in the magnetic field angle along the maser do quench the linear polarization intensity when the maser saturates.