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

摘要

环形磁场被普遍认为是在恒星形成过程中在吸积盘中重新分配角动量的主导性机制，但缺乏对这一模型的观测支持。Ian Stephens等人报告了对来自“T Tauri星”HL Tau的吸积盘的1.25毫米偏振连续辐射的时间分辨测量结果。在80个天文单位尺度上的这个磁场与该吸积盘的主轴相一致。作者的结论是，一个纯粹的环形磁场或由某一垂直分量主导的一个磁场都不符合这些数据。其出乎意料的形态表明，磁场在一个“T Tauri星”的吸积过程中所起作用要比当前的理论认识更为复杂。%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.