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arXiv 1402.1907astro-ph.EPastro-ph.SR
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原行星盘系统中的星盘双星相互作用和初始自旋轨道不对准

Star-Disc-Binary Interactions in Protoplanetary Disc Systems and Primordial Spin-Orbit Misalignments

Dong Lai

摘要:

我们研究了在外部双星影响下原恒星和它的星周盘之间的相互作用,以确定相互恒星自旋盘失准角的演化。倾斜双星作用在星盘上的引力力矩使星盘绕双星轴进动,而旋转诱导的四极矩引起的星-盘相互作用力矩使恒星自旋和星盘角动量轴相互进动。当星盘系统随时间演变使得两个进动频率彼此交叉时,可以从小的初始值产生显著的星盘失准角。这种“长期共振”行为可以从自旋和圆盘角动量的进动动力学中以几何方式来理解。我们推导出这种共振发生的条件,并发现它们可以满足合理的原恒星盘二进制参数。星盘倾角的演变也受到质量吸积和磁星盘相互作用力矩的影响,这可以促进或减少星盘不对准。一般来说,只要初始的二元盘倾角大于几度,就可以在盘的寿命内产生各种星盘失准角。我们讨论了我们的研究结果的影响恒星的自旋方向的双星,原恒星盘和碎片盘的对齐/错位,以及恒星的行星系外系统。特别是,如果热彗星是由外部恒星伴星引起的Kozai效应产生的,那么很可能“原始”的星盘错位已经由星盘双星相互作用产生。即使在Kozai效应被抑制的系统中,在原行星盘阶段仍然可能产生未对准的行星。

英文摘要:

We study the interactions between a protostar and its circumstellar disc under the influence of an external binary companion to determine the evolution of the mutual stellar spin - disc misalignment angle. The gravitational torque on the disc from an inclined binary makes the disc precess around the binary axis, while the star-disc interaction torque due to the rotation-induced quadrupole makes the stellar spin and the disc angular momentum axes precess around each other. A significant star-disc misalignment angle can be generated from a small initial value as the star-disc system evolves in time such that the two precession frequencies cross each other. This ``secular resonance'' behavior can be understood in a geometric way from the precession dynamics of spin and disc angular momenta. We derive the conditions for such resonance to occur, and find that they can be satisfied for reasonable protostar-disc-binary parameters. The evolution of star-disc inclination is also affected by mass accretion and by magnetic star-disc interaction torques, which can either promote or reduce star-disc misalignment. In general, as long as the initial binary-disc inclination is greater than a few degrees, a variety of star-disc misalignment angles can be generated within the disc lifetimes. We discuss the implications of our results for the stellar spin orientations in binaries, for the alignments/misalignments of protostellar discs and debris discs, and for the stellar obliquities in exoplanetary systems. In particular, if hot Jupiters are produced by the Kozai effect induced by an external stellar companion, then it is likely that ``primordial'' star-disc misalignments are already generated by the star-disc-binary interactions. Even for systems where the Kozai effect is suppressed, misaligned planets may still be produced during the protoplanetary disc phase.

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