POLARIZATION IN OH MASERS

We explore the origin of very high or 100% circular polarization in OH maser emission in star-forming regions. General expressions are devel oped describing the interaction of polarized maser radiation with the masing molecules, treating saturation of the molecular populations usi ng a semiclassical theory, in which the maser radiation is treated cla ssically and the response of the molecules quantum-mechanically. This theory is an extension of Field and Gray 1988, which ignored polarizat ion phenomena. The treatment of the transport of polarized maser radia tion is briefly set out, where the analysis is based on earlier work o f Goldreich et al. 1973, Landi Degli'Innocenti 1987, Rees 1987. Attent ion is drawn to the phenomenon of magnetic beaming (Gray and Field 199 4). A brief description is given of an extensive model for the calcula tion of OH magnetic hyperfine populations, involving 384 levels of OH, corresponding to energies up to 1100 K. This model uses the Sobolev o r Large Velocity Gradient approximation, substantially modified to inc lude effects of polarized radiation transfer. The model also includes a full treatment of FIR line overlap. The model is coupled to the theo ry of saturation and polarized radiation transfer. Assuming an ordered uniform magnetic field over the scale size of the maser zone, solutio ns yield the factor of amplification of maser rays as a function of ma ser gain length for any chosen angle, theta, of a maser ray to the dir ection of the magnetic field. Results are shown for two sets of physic al conditions appropriate to maser zones in star-forming regions. Comp etitive gain between magnetic substate transitions favours the emergen ce of sigma-rays, which yield elliptically or circularly polarized mas er radiation, and the suppression of pi-rays, which would yield linear ly polarized emission. A simplified analysis is given showing that sig ma-rays at low theta amplify over the shortest gain lengths and will t end, through saturation, to suppress amplification in all pi-rays, and also in sigma-rays at high theta. This semiquantitative argument show s that masers will form very highly or 100% circularly polarized emiss ion. We also show that maser beams may be less strongly polarized in t he line wings. The explanation of the origin of 100% circular polariza tion remains however incomplete in the absence of a quantitative beami ng model, taking account of the competition between rays both in the s ame and in different transitions, travelling at different angles to th e direction of the magnetic field. The frequent absence of Zeeman pair s for specific maser spots, or a general dissimilarity between Ihc and the spectra, also awaits the development of such a model, which may i nclude velocity gradients along the path of maser amplification.