2020ApJ...889..172K

We report high angular resolution observations of linearly polarized dust emission toward the Class 0 young stellar object NGC1333 IRAS4A (hereafter IRAS4A) using the Karl G. Jansky Very Large Array at K (11.5-16.7 mm), Ka (8.1-10.3 mm), and Q bands (6.3-7.9 mm), and using the Atacama Large Millimeter Array (ALMA) at Band 6 (1.3 mm) and Band 7 (0.85-0.89 mm). On 100-1000 au scales, all of these observations consistently trace the hourglass-shaped magnetic field topology as shown in the previous studies. In the innermost 100 au region of IRAS4A1, the polarization position angles (E field) detected at 6.3-16.7 mm are consistent; however, they are nearly 90° offset from those detected at 1.3 mm and 0.85-0.89 mm. Such a 90° offset may be explained by the inner ∼100 au area being optically thick at wavelengths shorter than ∼1.5 mm, whereby the observations probe the absorption of aligned dust against the weakly or unpolarized warm dust emission from the innermost region. This can also consistently explain why the highest angular resolution ALMA images at Band 7 show that the polarization percentage increases with dust brightness temperature in the inner ∼100 au region of IRAS4A1. Following this interpretation and assuming that the dust grains are aligned with the magnetic fields, the inferred magnetic field position angle based on the 90° rotated at 6.3-7.9 mm in the central peak of IRAS4A1 is ∼-22°, which is approximately consistent with the outflow direction of ∼-9°.