MAGNETIC-FIELD IN A LOW-MASS PROTOSTAR DISK - MILLIMETER POLARIMETRY OF IRAS 16293-2422

We have measured the millimeter polarization of IRAS 16293-2422, a low-mass protostar candidate embedded in the east streamer of the rho Ophiuchi dark cloud. The source has a linear polarization at 1.1 mm of (2.8 +/- 0.5)% at a position angle of 144-degrees +/- 5-degrees with a 19'' beam. If this polarization is due to thermal emission from magnetically aligned nonspherical grains in a dust disk around the central (proto)star, then the magnetic field threads across the disk at a position angle of 54-degrees +/- 5-degrees projected on the sky. This direction is the same as that of the minor axis of the elongated circumstellar gas disk (of scale 1600 AU) and of the direction of one of the quadratic outflows. The direction of the disk magnetic field is also in good agreement with that of the large-scale (1 to 10 pc) magnetic field of the molecular cloud as inferred from previous near-infrared and optical polarimetry of background field stars. There is, therefore, no positive evidence for a twisted magnetic field in the dust disk, which constrains the hydromagnetic models for molecular outflows, at least for this source.