Spatially resolved magnetic field structure in the disk of a T Tauri star

Measurements of polarized 1.25-mm continuum emission from the accretion disk of the T Tauri star HL Tau show that the magnetic field inside the disk cannot be dominated by a vertical component, and that a purely toroidal field also does not fit the data; this suggests that the role of the magnetic field in the accretion of a T Tauri star is more complex than the current theoretical understanding. Magnetic moments in star formation Toroidal magnetic fields are widely thought to be the dominant mechanism for redistributing angular momentum during star formation in accretion disks, but observational support for this model is lacking. Ian Stephens et al . report resolved measurements of the polarized 1.25-millimetre continuum emission from the disk of the T Tauri star HL Tau. The magnetic field on a scale of 80 astronomical units is coincident with the major axis of the disk. The authors conclude that neither a purely toroidal field, nor a magnetic field dominated by a vertical component are good fits to the data. The unexpected morphology suggests that the magnetic field's role during the accretion of a T Tauri star is more complex than the current theoretical understanding. Magnetic fields in accretion disks play a dominant part during the star formation process 1 , 2 but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars 3 and possibly in the innermost part of their accretion disks 4 , but the strength and morphology of the field in the bulk of a disk have not been observed. Spatially unresolved measurements of polarized emission (arising from elongated dust grains aligned perpendicularly to the field 5 ) imply average fields aligned with the disks 6 , 7 . Theoretically, the fields are expected to be largely toroidal, poloidal or a mixture of the two 1 , 2 , 8 , 9 , 10 , which imply different mechanisms for transporting angular momentum in the disks of actively accreting young stars such as HL Tau (ref. 11 ). Here we report resolved measurements of the polarized 1.25-millimetre continuum emission from the disk of HL Tau. The magnetic field on a scale of 80 astronomical units is coincident with the major axis (about 210 astronomical units long 12 ) of the disk. From this we conclude that the magnetic field inside the disk at this scale cannot be dominated by a vertical component, though a purely toroidal field also does not fit the data well. The unexpected morphology suggests that the role of the magne