THE STRUCTURE OF MAGNETIC-FIELDS IN DARK CLOUDS - INFRARED POLARIMETRY IN B216-217

We present a near-infrared polarization map of background starlight seen through the B216-217 dark cloud in Taurus. The mean direction and dispersion in direction of the polarization vectors observed in the near-infrared are indistinguishable from the direction and dispersion of optical polarization vectors around the periphery of the dark cloud. Measurements of J- and K-magnitudes of the stars observed in the near-infrared imply a range of extinctions 1 less than or similar to A(V) less than or similar to 10 mag, while the mean A(V) for the stars whose polarization has been measured optically is less than or similar to 1 mag. Assuming that grains in the high- and low-density regions are similar, and are similarly aligned by magnetic fields, then unless the field becomes significantly more nonuniform in denser regions, our results imply that the dark cloud has no effect on the projected direction of the magnetic field. If the field does become more nonuniform in the denser portions of the cloud, then the dark cloud is associated with a change in the total energy of the field, but not with any change in the mean direction of the field. Yet, we also find that the degree of polarization in B216-217 and other dark clouds increases more slowly with extinction than expected: if this reduction in polarization efficiency is caused by a drop in alignment and/or polarizing efficiency in denser regions, it is possible that near-infrared polarization may be relatively insensitive to the field direction in the dense interior of dark clouds.