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Direct detection of a magnetic field in the innermost regions of an accretion disk

Abstract

Models1,2,3,4,5 predict that magnetic fields play a crucial role in the physics of astrophysical accretion disks and their associated winds and jets6,7. For example, the rotation of the disk twists around the rotation axis the initially vertical magnetic field, which responds by slowing down the plasma in the disk and by causing it to fall towards the central star. The magnetic energy flux produced in this process points away from the disk, pushing the surface plasma outwards, leading to a wind from the disk and sometimes a collimated jet. But these predictions have hitherto not been supported by observations. Here we report the direct detection of the magnetic field in the core of the protostellar accretion disk FU Orionis8. The surface field reaches strengths of about 1 kG close to the centre of the disk, and it includes a significant azimuthal component, in good agreement with recent models5. But we find that the field is very filamentary and slows down the disk plasma much more than models predict, which may explain why FU Ori fails to collimate its wind into a jet.

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Figure 1: Magnetic field detection in the protostellar accretion disk FU Ori.
Figure 2: Disk and wind contribution to the LSD profiles of FU Ori.
Figure 3: Modelling the LSD profiles of FU Ori.

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Acknowledgements

We thank the CFHT staff for help during the engineering runs of ESPaDOnS. We are grateful to F. Rincon and A. C. Cameron for comments on earlier versions of the manuscript. This Letter is based on observations obtained at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

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Correspondence to Jean-François Donati.

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Supplementary information

Supplementary Figure 1

LSD Stokes V profiles of FU Ori in the blue and red spectral regions, demonstrating that the disc regions producing the detected Zeeman signature are at the same temperature as those generating the bulk of optical lines. (PDF 14 kb)

Supplementary Figure 2

Portions of the ESPaDOnS spectra of FU Ori showing weak, relatively unblended, features, demonstrating that LSD captures well the average shape of weak spectral features. (PDF 25 kb)

Supplementary Figure Legends

Text to accompany the Supplementary Figures. (DOC 21 kb)

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Donati, JF., Paletou, F., Bouvier, J. et al. Direct detection of a magnetic field in the innermost regions of an accretion disk. Nature 438, 466–469 (2005). https://doi.org/10.1038/nature04253

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