吸积磁性原恒星的自旋方向演化和系外行星系统中的自旋-轨道失调：II。翘曲圆盘
Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems: II. Warped Discs
摘要：
原恒星与其吸积盘之间的磁相互作用往往会导致吸积盘的翘曲，并使恒星的自旋方向发生长期变化，因此自旋轴可能并不总是垂直于吸积盘。这可能有助于解释最近在一些系外行星系统中观察到的自旋轨道不对准。我们研究了磁翘曲/进动扭矩和内部应力的联合作用下的翘曲原行星盘的动力学，包括翘曲的粘性阻尼和弯曲波的传播。我们表明，当外盘轴与恒星的自旋轴不一致，光盘的演变对一个扭曲的稳定状态的时间尺度上，取决于光盘的粘度或弯曲波的传播速度，但在所有情况下，是远远短于自旋演化的时间尺度（百万年的顺序）。此外，对于最有可能的物理参数表征磁性原恒星，拱星盘和它们的相互作用，稳态盘有一个相当小的翘曲，这样，整个磁盘近似位于一个单一的平面所确定的外盘边界条件，虽然更极端的参数可能会引起更大的磁盘翘曲。与我们最近基于平盘的分析（Lai et al. 2010）一致，我们发现恒星上轻微翘曲盘的反作用磁矩可以使恒星自旋轴与盘轴对齐，也可以将其推向不对齐，这取决于星盘系统的参数。这意味着新形成的行星系可能在恒星自转轴和行星轨道对称轴之间有一定的倾角范围。
英文摘要：
Magnetic interactions between a protostar and its accretion disc tend to induce warping in the disc and produce secular changes in the stellar spin direction, so that the spin axis may not always be perpendicular to the disc. This may help explain the recently observed spin-orbit misalignment in a number of exoplanetary systems. We study the dynamics of warped protoplanetary discs under the combined effects of magnetic warping/precession torques and internal stresses in the disc, including viscous damping of warps and propagation of bending waves. We show that when the outer disc axis is misaligned with the stellar spin axis, the disc evolves towards a warped steady-state on a timescale that depends on the disc viscosity or the bending wave propagation speed, but in all cases is much shorter than the timescale for the spin evolution (of order of a million years). Moreover, for the most likely physical parameters characterizing magnetic protostars, circumstellar discs and their interactions, the steady-state disc has a rather small warp, such that the whole disc lies approximately in a single plane determined by the outer disc boundary conditions, although more extreme parameters may give rise to larger disc warps. In agreement with our recent analysis (Lai et al. 2010) based on flat discs, we find that the back-reaction magnetic torques of the slightly warped disc on the star can either align the stellar spin axis with the disc axis or push it towards misalignment, depending on the parameters of the star-disc system. This implies that newly formed planetary systems may have a range of inclination angles between the stellar spin axis and the symmetry axis of the planetary orbits.
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