A Radially Resolved Magnetic Field Threading the Disk of TW Hya

We present a new approach to detecting and characterizing a magnetic field in protoplanetary disks through the differential broadening of unpolarized molecular emission from CN. To demonstrate this technique, we apply it to new Atacama Large Millimeter/submillimeter Array observations of the full complement of hyperfine components from the N = 1–0 transition, achieving a spatial and spectral resolution of and 80 m s−1, respectively. By fitting a model that incorporates the velocity structure of the disk, the potential non-LTE excitation of the molecule, and the Zeeman effect, we recover a radially resolved magnetic field with a strength of ∼10 mG between 60 and 120 au. The morphology of the field is also inferred through azimuthal variations in the line broadening, revealing a predominantly poloidal field at 60 au, sharply transitioning to one within the disk plane outside of the gap at 82 au. The signal-to-noise ratio of the data meant that the planar component was unable to be decomposed into toroidal and radial components. Lower limits on the local gas density (n(H2) ≳ 108 cm−3) from the excitation analysis of the CN emission correspond to a lower limit between 0.1 and 0.01 for the plasma β.