We explore the origin of very high or 100% circular polarization in OH maser emission in star-forming regions. General expressions are developed describing the interaction of polarized maser radiation with the masing molecules, treating saturation of the molecular populations using a semiclassical theory, in which the maser radiation is treated classically and the response of the molecules quantum-mechanically. This theory is an extension of Field and Gray 1988MNRAS.234..353F
, which ignored polarization phenomena. The treatment of the transport of polarized maser radiation is briefly set out, where the analysis is based on earlier work of Goldreich et al. 1973ApJ...179..111G
, Landi Degl'Innocenti 1987 (in Numerical Radiative Transfer), Rees 1987 (in Numerical Radiative Transfer). Attention is drawn to the phenomenon of magnetic beaming (Gray & Field 1994A&A...292..693G
). A brief description is given of an extensive model for the calculation of OH magnetic hyperfine populations, involving 384 levels of OH, corresponding to energies up to 1100K. This model uses the Sobolev or Large Velocity Gradient approximation, substantially modified to include effects of polarized radiation transfer. The model also includes a full treatment of FIR line overlap. The model is coupled to the theory of saturation and polarized radiation transfer. Assuming an ordered uniform magnetic field over the scale size of the maser zone, solutions yield the factor of amplification of maser rays as a function of maser gain length for any chosen angle, θ, of a maser ray to the direction of the magnetic field. Result
. are shown for two sets of physical conditions appropriate to maser zones in star-forming regions. Competitive gain between magnetic substate transitions favours the emergence of σ-rays, which yield elliptically or circularly polarized maser radiation, and the suppression of π-rays, which would yield linearly polarized emission. A simplified analysis is given showing that σ-rays at low θ amplify over the shortest gain lengths and will tend, through saturation, to suppress amplification in all π-rays, and also in σ-rays at high θ . This semiquantitative argument shows that masers will form very highly or 100% circularly polarized emission. We also show that maser beams may be less strongly polarized in the line wings. The explanation of the origin of 100% circular polarization remains however incomplete in the absence of a quantitative beaming model, taking account of the competition between rays both in the same and in different transitions, travelling at different angles to the direction of the magnetic field. The frequent absence of Zeeman pairs for specific maser spots, or a general dissimilarity between lhc and rhc spectra, also awaits the development of such a model, which may include velocity gradients along the path of maser amplification.