A Radially Resolved Magnetic Field Threading the Disk of TW Hya

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≈</mml:mo> <mml:mn>0</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> <mml:mn>5</mml:mn> </mml:math> 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 (H 2 ) ≳ 10 8 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 .