The structure of magnetic fields in dark clouds - Infrared polarimetry in B216-217

A near infrared polarization map of background starlight seen through the B216-217 dark cloud in Taurus is presented. The mean direction and dispersion in the direction of the polarization vectors observed in the near-IR are indistinguishable from the direction and dispersion of optical polarization vectors around the periphery of the dark cloud. 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, the present results imply that the dark cloud has no effect on the projected direction of the magnetic field. It is suggested that near-infrared polarization is relatively insensitive to the field direction in the dense interior of dark clouds.