CONSTRAINTS ON BE STAR WIND GEOMETRY BY LINEAR-POLARIZATION AND IR EXCESS

We have investigated the correlation between optical linear polarisation and infra-red excess in classical Be stars in order to derive constraints on the geometry of the circumstellar material. We use a simple disc model to calculate the expected degree of polarisation and IR excess, and derive analytical expressions for the relation between polarisation and IR excess as a function of model parameters (density gradient, opening angle of the disc) for edge-on discs that are optically thin for free-bound and free-free radiation in the optical. We also investigate the energy distribution and degree of polarisation numerically, solving first for the degree of ionisation in the disc. Comparison between analytical and numerical results shows good agreement for small polarisation, but large differences in the case of high densities, i.e. when optical depth effects become important. When compared to the observations, our calculations indicate that the combination of polarisation and IR excess does not put strong constraints on the geometry of the disc. We are able to rule out density distributions with a small density gradient in the equatorial plane because they produce too much IR excess. We find that a wide range in opening angles can produce a polarisation and IR excess in agreement with observations.