MAGNETIC-FIELD STRUCTURE IN PROTOSTELLAR ENVELOPES

We have imaged the linearly polarized emission toward the young stella r 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 invest igate the magnetic held structure. The observations were obtained usin g the new polarimetric capabilities at 3 mm of the six-element millime ter array of the Owens Valley Radio Observatory.Toward IRAS 4A, the av erage polarization observed is 4.4% +/- 0.7%. The magnetic field direc tion 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 i s offset 1''.5 from the peak of the total intensity and elongated perp endicular to the implied field direction. Models calculated with an ho urglass magnetic field morphology in a spherically symmetric dusty env elope are consistent with the observations. Toward IRAS 16293 the pola rized emission is located between the two binary components, and the p olarization 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 em ission could be produced by a toroidal magnetic field in the circumbin ary disk. The high resolution of the interferometer provides a probe o f the polarized emission at the high densities (n greater than or equa l to 10(8) cm(-3)) characteristic of protostellar envelopes and disks. The detection of polarized emission at these densities provides a tes t of grain alignment mechanisms. We conclude that alignment by paramag netic relaxation of thermally rotating grains is unlikely, because of the well-coupled gas and grain temperatures, but alignment of suprathe rmally rotating grains is not ruled out.