Polarization models of bipolar reflection nebulae .1.

We have developed a model for optically thin reflection nebulae surrounding young stellar objects. We present the results of a parameter-space study in which we have elucidated several diagnostic characteristics which we can apply to the interpretation of polarization measurements in the outer regions of optically thin bipolar reflection nebulae. The possibility of scattering from randomly oriented, small ellipsoidal particles is also discussed. In this paper the model has been applied specifically to R Monocerotis (R Mon), for which optical and near-infrared polarization maps exist. The modelling suggests that scattering occurs at the cavity walls and at an inner optical jet. Some reasonable constraints have been placed upon the nature of the scattering particles based upon the appearance of polarization maps of R Mon at optical and infrared wavelengths.