EDP Sciences
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Volume 445, Number 3, January III 2006
Page(s) 1031 - 1035
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20053873

A&A 445, 1031-1035 (2006)
DOI: 10.1051/0004-6361:20053873

Polarization of H $_\mathsf{2}$O masers in the presence of velocity and magnetic field gradients

W. H. T. Vlemmings

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)

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 $100\%$ 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.

Key words: masers -- radiative transfer -- polarization -- magnetic fields

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