1] Institute for Astrophysical Research, Boston University, Boston, Massachusetts 02215, USA [2] Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
Nature. 2014 Oct 30;514(7524):597-9. doi: 10.1038/nature13850. Epub 2014 Oct 22.
Magnetic fields in accretion disks play a dominant part during the star formation process but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars and possibly in the innermost part of their accretion disks, 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) imply average fields aligned with the disks. Theoretically, the fields are expected to be largely toroidal, poloidal or a mixture of the two, 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) 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.
在恒星形成过程中，吸积盘中的磁场起着主导作用，但迄今为止，对其的观测受到了很大的限制。磁场强度已在 T 型星和它们的吸积盘中的最内部被推断出来，但磁盘大部分区域中的磁场强度和形态尚未被观测到。对偏振发射的空间分辨率测量（源于垂直于磁场排列的拉长尘埃颗粒）表明平均磁场与磁盘一致。理论上，磁场预计主要是环形、极向或两者的混合，这意味着在 HL Tau 等活跃吸积的年轻恒星的磁盘中，角动量的输运机制不同（参考文献 11）。在这里，我们报告了 HL Tau 磁盘的偏振 1.25 毫米连续谱发射的分辨率测量结果。在 80 个天文单位的尺度上的磁场与磁盘的长轴（约 210 个天文单位）重合。由此我们得出结论，在这个尺度上，磁盘内部的磁场不可能由垂直分量主导，尽管纯粹的环形场也不太符合数据。这种意想不到的形态表明，在 T 型星的吸积过程中，磁场的作用比我们目前的理论理解更为复杂。
