A model of polarized OH maser emission in W75N

Abstract

Multi-element Radio-linked Interferometer Network (MERLIN) observations of 18-cm lines of OH from the star-forming region W75N indicate a rotating molecular disc orthogonal to the bipolar molecular outflow, with the maser polarization tracing twisted magnetic field lines. The observations are compared with synthetic maser features generated by a polarization-dependent model of maser amplification, which uses input physical conditions drawn from a numerical simulation of a star-forming disc/outflow source. The model helps to explain the complexity of the observed magnetic field structure on the basis of maser emission originating from several different depths within the protostellar disc. Line-ratios, spectral richness and polarization of ground-state (18-cm) masers agree broadly with observations. Although the detailed agreement is crude, the model is able to generate lines that are observed, whilst excluding those that are not: for example, the main lines and 1720 MHz are generated in the ground state, but not 1612 MHz; 4765 MHz is generated, but neither of the other 4.7-GHz lines appear as masers.