Interstellar polarization at high galactic latitudes from distant stars .2. Evidence for depolarization starting at Z approximate to 600 pc

We have measured linear polarization for 20 distant high latitude A and F stars, as a first step to extend our previous study (Berdyugin et al. 1995: Paper I) of the North Galactic polar cap to distances larger than 600 pc. The results give tentative evidence for depolarization starting to be seen around Z approximate to 500 pc, possibly indicating that there is a significant change in the magnetic field direction. Depolarization requires that there is still light-polarizing dust in the Z range 500-1000 pc and the lower limit to extinction inferred in Paper I (A(B) > 0.11 mag) must be increased by a yet undetermined amount.