Magnetic fields in accretion disks play a dominant part during the star formation process1,2 but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars3 and possibly in the innermost part of their accretion disks4, 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 field5) imply average fields aligned with the disks6,7. Theoretically, the fields are expected to be largely toroidal, poloidal or a mixture of the two1,2,8,9,10, 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 long12) 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 magnetic field in the accretion of a T Tauri star is more complex than our current theoretical understanding.
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We thank R. L. Plambeck and C. L. H. Hull for consultation during the data reduction process and C. F. Gammie for discussions. This research made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com. Work at the Universities of Illinois and Maryland was supported by NSF AST-1139950 and AST-1139998, respectively. Support for CARMA construction was derived from the states of California, Illinois and Maryland, the James S. McDonnell Foundation, the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the University of Chicago, the Associates of the California Institute of Technology, and the National Science Foundation. Ongoing CARMA development and operations are supported under a cooperative agreement (NSF AST 08-38226) and by the CARMA partner universities.
The authors declare no competing financial interests.
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Stephens, I., Looney, L., Kwon, W. et al. Spatially resolved magnetic field structure in the disk of a T Tauri star. Nature 514, 597–599 (2014). https://doi.org/10.1038/nature13850
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