2013ApJ...763..135T

We present the first high-angular resolution (up to 0.''7, ∼5000 AU) polarization and thermal dust continuum images toward the massive star-forming region W51 North. The observations were carried out with the Submillimeter Array (SMA) in both the subcompact (SMA-SubC) and extended (SMA-Ext) configurations at a wavelength of 870 µm. W51 North is resolved into four cores (SMA1 to SMA4) in the 870 µm continuum image. The associated dust polarization exhibits more complex structures than seen at lower angular resolution. We analyze the inferred morphologies of the plane-of-sky magnetic field (B) in the SMA1 to SMA4 cores and in the envelope using the SMA-Ext and SMA-SubC data. These results are compared with the B archive images obtained from the Caltech Submillimeter Observatory (CSO) and James Clerk Maxwell Telescope (JCMT). The polarization percentage is about 1% to 4%, and it is found to decrease with higher intensity in our SMA images, which is a similar trend to that previously reported in the CSO and JCMT data. A correlation between dust intensity gradient position angles (φ_∇I) and magnetic field position angles (φ_B) is found in the CSO, JCMT, and both SMA data sets. This correlation is further analyzed quantitatively. A systematically tighter correlation between φ_∇I and φ_Bis found in the cores, whereas the correlation decreases in outside-core regions. Magnetic field-to-gravity force ratio (Σ_B) maps are derived using the newly developed polarization-intensity gradient method by Koch et al. We find that the force ratios tend to be small (Σ_B ≲ 0.5) in the cores in all four data sets. In regions outside of the cores, the ratios increase or the field is even dominating gravity (Σ_B> 1). This possibly provides a physical explanation of the tightening correlation between φ_∇I and φ_Bin the cores: The more the B field lines are dragged and aligned by gravity, the tighter the correlation is. Finally, we propose a schematic scenario for the magnetic field in W51 North to interpret the four polarization observations at different physical scales.