Infrared polarimetry of dark clouds. II : Magnetic field structure in the Rho Ophiuchi dark cloud

K-band polarimetry of 20 sources embedded within the densest region of the Rho Oph dark cloud has been obtained. The degree of polarization ranges from 1.1 to 7.4 percent, and the position angles have a bimodal distribution, centered at 50 deg and 150 deg. The 50 deg component, which permeates the surrounding region, persists in the region of the dense (C-18)O ridge, and is perpendicular to the elongation (NW-SE). The flattened shape of the condensation could be due to the effect of a magnetic field with matter contracting preferentially along the field lines. If the magnetic field is enhanced by at least one order of magnitude over the interstellar value, the cloud collapse could be retarded in the direction transverse to the magnetic field. The sources forming the 150 deg component are located around the densest region between the Rho Oph B1 and B2 clouds. This component, which is nearly perpendicular to the ambient field, could be caused by rotation of Rho Oph B with a rotation axis parallel to the existing magnetic field.