Be星衰变盘的潮汐翘曲
Tidal Warping of Be Star Decretion Discs
摘要：
快速旋转的Be星被观察到是壳星，当从边缘观察递减盘时，两者之间的过渡意味着盘可能是弯曲和进动的。II型X射线爆发被认为发生在翘曲的盘与快速的恒星风相互作用时。我们认为，潮汐从一个错位的伴星中子星可以导致观察到的效果。我们建立了Be星衰变盘的数值模型，其中Be星的自旋与中子星伴星的轨道轴不一致。来自中子星的潮汐力矩在一个足够小的半径处截断圆盘，使得中子星的轨道不与圆盘相交，除非偏心率或未对准非常大。一个磁力矩从Be星是最大的赤道，旋转最快的地方，近似的内边界条件。当圆盘向稳定状态移动时，圆盘的质量和倾角会有很大的振荡。这些大的变化可以解释从Be星到Be壳星的变化，反之亦然，也可以解释II型X射线爆发。我们发现，潮汐的时间尺度上的磁盘扭曲，进动，并达到稳定的状态是大约一年到几百年。如果存在的话，质量和盘倾角的振荡发生在这个时间尺度的一小部分上，这取决于双星的轨道参数。与观测到的Be星双星的潮汐扭矩的时间尺度表明，这些影响是重要的，但最长的周期二进制文件。
英文摘要：
Rapidly rotating Be stars are observed as shell stars when the decretion disc is viewed edge on. Transitions between the two implies that the discs may be warped and precessing. Type II X-ray outbursts are thought to occur when the warped disc interacts with the fast stellar wind. We suggest that tides from a misaligned companion neutron star can cause the observed effects. We make numerical models of a Be star decretion disc in which the spin of the Be star is misaligned with the orbital axis of a neutron star companion. Tidal torques from the neutron star truncate the disc at a radius small enough that the neutron star orbit does not intersect the disc unless the eccentricity or misalignment is very large. A magnetic torque from the Be star that is largest at the equator, where the rotation is fastest, is approximated by an inner boundary condition. There are large oscillations in the mass and inclination of the disc as it moves towards a steady state. These large variations may explain the observed changes from Be star to Be shell star and vice-versa and also the Type II X-ray outbursts. We find the tidal timescale on which the disc warps, precesses and reaches a steady state to be around a year up to a few hundred years. If present, the oscillations in mass and disc inclination occur on a fraction of this timescale depending on the orbital parameters of the binary. The timescales associated with the tidal torque for observed Be star binaries suggest that these effects are important in all but the longest period binaries.
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