OPTICAL POLARIZATION MAPS OF STAR-FORMING REGIONS IN PERSEUS, TAURUS, AND OPHIUCHUS

We present new optical linear polarization maps of the star-forming regions near L1506 in Taurus, L1755 in Ophiuchus, and the complex of dark clouds which extends from L1448 to B5 in Perseus. In the L1506 region, the polarization vectors show a systematic alignment with each other but are oriented at about 25° to the projected axis of the local dark cloud. This pattern fits in smoothly with a large-scale polarization map of the Taurus cloud complex. In the L1755 region, the individual polarization vectors are remarkably well aligned with each other and with the projected elongation of the cloud. As in Taurus, this observed alignment may be understood in view of a polarization map of the entire Ophiuchus cloud complex. In Perseus, no simple pattern can be discerned: the distribution of polarization vector orientation appears bimodal, with small vectors parallel to the overall projected cloud axis and larger vectors perpendicular. We propose that this bimodal distribution is due to polarization produced by magnetically aligned grains which are located in more than one cloud complex along the line of sight of Perseus. Molecular spectral line observations, which show gas in two distinct velocity ranges, support this hypothesis. The distributions of polarization vector angle, θ, can be decomposed into a peak direction, a dispersion about that peak, and a random component. We model the peak and its dispersion as arising from a uniform field with nonuniform perturbations about it. The dispersion in θ for an entire cloud complex is generally less than the variations in position angle of the long axis of filamentary clouds (observed in extinction or molecular lines) within the complex, suggesting that the magnetic field does not dominate the cloud structure on the size scale ≳ 1 pc.