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恆星形成

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A schematic drawing of the X-wind model

圖片來源: Mike Cai

A schematic drawing of the X-wind model.

It is well accepted that the observed optical jets of T-Tauri stars are products of lightly ionized material accelerated by the magnetic field from the accretion disk. One popular theoretical model that such explanation is based upon is the X-wind model, and many ASIAA members have participated in the development of this theory. As gaseous material in the disk drifts toward the central star, it also brings in magnetic field lines in the equatorial plane. Slightly above the surface of the disk, the magnetic field overwhelms inertia, and matter is forced to rotate with the magnetic field. If the magnetic field line forms a large enough angle with the disk normal, magnetocentrifugal effects would cause material to move outward along the field line. The material in the outflow gains angular momentum from the disk via the magnetic field, and the foot point of the field lines drift inward as a result. On the other hand, the accretion disk is truncated by the star's magnetic field according to the generalized X-wind model (Shu et. al. 1997). Interior to this truncation radius, matter is channeled by the magnetic field toward the stellar surface. In the funnel flow, the material loses angular momentum to Maxwell stress, which is then in turn deposited in the disk. If angular momentum transport inside the disk is inefficient, then the magnetic field lines would be pinched to within a small region of the truncation radius, forming a ring of split monopoles. This particular geometry of the field lines gives the X-wind theory its name.