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Magnetic Field Structure in Protostellar Envelopes

We have imaged the linearly polarized emission toward the young stellar objects NGC 1333/IRAS 4A and IRAS 16293-2422. We suggest that the polarized emission from the dense protostellar environment arises from magnetically aligned dust grains and use these observations to investigate the magnetic field structure. The observations were obtained using the new polarimetric capabilities at 3 mm of the six-element millimeter array of the Owens Valley Radio Observatory. Toward IRAS 4A, the average polarization observed is 4.4% ± 0.7%. The magnetic field direction implied by the observations is roughly parallel to the direction of the bipolar molecular outflow. Structure is seen at 3'' resolution in the linearly polarized image. The peak of the linear polarization is offset 1.″5 from the peak of the total intensity and elongated perpendicular to the implied field direction. Models calculated with an hourglass magnetic field morphology in a spherically symmetric dusty envelope are consistent with the observations. Toward IRAS 16293 the polarized emission is located between the two binary components, and the polarization percentage at the peak of the polarized emission is 2.7% ± 0.7%. The magnetic field direction implied from our measurements is parallel to the major axis of the circumbinary disk. The polarized emission could be produced by a toroidal magnetic field in the circumbinary disk. The high resolution of the interferometer provides a probe of the polarized emission at the high densities (n ≥ 108 cm-3) characteristic of protostellar envelopes and disks. The detection of polarized emission at these densities provides a test of grain alignment mechanisms. We conclude that alignment by paramagnetic relaxation of thermally rotating grains is unlikely, because of the well-coupled gas and grain temperatures, but alignment of suprathermally rotating grains is not ruled out.