Interferometric mapping of magnetic fields in star-forming regions. I. W51e1/e2 molecular cores

We present the first interferometric polarization map of the W51 e1/e2 mole cular cores, obtained with the Berkeley-Illinois-Maryland Association (BIMA ) array at 1.3 mm wavelength, with approximately 3 " resolution. The polari zation angle varies smoothly across the double cores with an average positi on angle of 23 degrees +/- 5 degrees for W51 e1 and 15 degrees +/- 7 degree s for W51 e2. The inferred magnetic field direction is parallel to the mino r axis of the double cores, which is consistent with the theoretical pictur e that clouds collapse along the field lines. However, the magnetic field m ay not determine the axis of angular momentum of these two cores, since the field directions of the two cores significantly differ from the previously measured directions of rotational axes. The polarization percentage decrea ses toward regions with high intensity, suggesting that the dust alignment efficiency decreases toward high-density regions. The field directions are highly ordered, and the small dispersion of the polarization angles implies that magnetic fields are strong (greater than or similar to1 mG) and perha ps dominate turbulence in W51 e1/e2.