A study of polarized spectra of magnetic CP stars: Predicted vs. observed Stokes iQUV profiles for beta CrB and 53 Cam

We present, a comparison of observed and calculated Stokes IQUV spectra of two well-known magnetic chemically peculiar stars, beta Coronae Borealis an d 53 Camelopardalis. The observed Stokes spectra were recently described by Wade et al. (2000a), and have been complemented with additional circularly polarized spectra obtained at the Special Astrophysical Observatory. The c alculated spectra represent tile predictions of new and previously publishe d magnetic field models derived from the analysis of some surface averaged field estimates (e.g., longitudinal field, magnetic field modulus, etc.). W e find that these magnetic models are not sufficient to account fully for t he observed Stokes profiles particularly remarkable is the disagreement bet ween the predicted and observed Stokes Q and U profiles of 53 Cam. We sugge st that this should be interpreted in terms of magnetic morphologies which are significant-ly more complex than the second-order multipolar expansions assumed in the models. However, it is clear that some of our inability to reproduce the detailed shapes of the Stokes IQUV profiles is unrelated to t he magnetic models. For many metallic ions, for both stars, we found it imp ossible to account for the strengths and shapes of the observed spectral li ne profiles when we adopted a unique value for the individual ion abundance , We suggest that this results from strongly non-uniform distributions of t hese ions as a function of optical depth (i.e., chemical stratification), a hypothesis that is supported by comparison with simple chemically stratifi ed models.