Faraday screens associated with local molecular clouds

Polarization observations at lambda21 cm and lambda18 cm towards the local Taurus molecular cloud complex were made with the Effelsberg 100-m telescope. Highly structured, frequency-dependent polarized emission features were detected. We discuss polarization minima with excessive rotation measures located at the boundaries of molecular clouds. These minima get less pronounced at the higher frequencies. The multi-frequency polarization data have been successfully modeled by considering magneto-ionic Faraday screens at the surface of the molecular clouds. Faraday rotated background emission adds to foreground emission towards these screens in a different way than in its surroundings. The physical size of the Faraday screens is of the order of 2 pc for 140 pc distance to the Taurus clouds. Intrinsic rotation measures between about -18 rad m(-2) to -30 rad m(-2) are required to model the observations. Depolarization of the background emission is quite small (compatible with zero), indicating a regular magnetic field structure with little turbulence within the Faraday screens. With observational constraints for the thermal electron density from Halpha observations of less than 0.8 cm(-3) we conclude that the regular magnetic field strength along the line of sight exceeds 20 muG, to account for the observed rotation measure. We discuss some possibilities for the origin of such strong and well ordered magnetic fields. The modeling also predicts a large-scale, regularly polarized emission in the foreground of the Taurus clouds which is of the order of 0.18 K at lambda21 cm. This in turn constrains the observed synchrotron emission in total intensity within 140 pc of the Taurus clouds. A lower limit of about 0.24 K, or about 1.7 K/kpc, is required for a perfectly ordered magnetic field with intrinsic (similar to75%) percentage polarization. Since this is rather unlikely to be the case, the fraction of foreground synchrotron emission is even larger. This amount of synchrotron emission is clearly excessive when compared to previous estimates of the local synchrotron emissivity.