Be星周围产生Keplerian的脉动机制
A Pulsational Mechanism for Producing Keplerian Disks around Be Stars
摘要：
经典Be星是一个神秘的快速旋转的热星子类，其特征是密集的赤道气体盘，被推断为以开普勒速度运行。虽然这些圆盘似乎是从恒星中喷射出来的，而不是吸积的，但有大量的观测证据表明，恒星的旋转速度比离心驱动的质量损失所需的速度要慢。本文发展了一个想法（最初由Hiroyasu Ando和同事提出），即非径向恒星脉动向高层大气注入足够的角动量，使开普勒盘旋转。脉动本身在恒星光球层中是瞬逝的，但它们可能不稳定，从而在声学截止频率处产生共振振荡。从脉动到共振波的转换的详细理论还不存在于现实的热星大气中，因此目前的模型依赖于波激发效率的参数化近似。然而，一旦共振波形成，它们的振幅随着高度的增加而增大，变陡成激波，并对平均流体施加径向和方位雷诺应力。使用合理的恒星参数假设，这些过程被发现可以自然地产生致密开普勒盘所需的内边界条件，即使底层的B星光球旋转速度慢至其临界转速的60%。由于有证据表明Be星的脉动性质存在长期变化，这个模型也可以解释Be星的长期变化，包括正常相、Be相和壳相之间的转变。
英文摘要：
Classical Be stars are an enigmatic subclass of rapidly rotating hot stars characterized by dense equatorial disks of gas that have been inferred to orbit with Keplerian velocities. Although these disks seem to be ejected from the star and not accreted, there is substantial observational evidence to show that the stars rotate more slowly than required for centrifugally driven mass loss. This paper develops an idea (proposed originally by Hiroyasu Ando and colleagues) that nonradial stellar pulsations inject enough angular momentum into the upper atmosphere to spin up a Keplerian disk. The pulsations themselves are evanescent in the stellar photosphere, but they may be unstable to the generation of resonant oscillations at the acoustic cutoff frequency. A detailed theory of the conversion from pulsations to resonant waves does not yet exist for realistic hot-star atmospheres, so the current models depend on a parameterized approximation for the efficiency of wave excitation. Once resonant waves have been formed, however, they grow in amplitude with increasing height, steepen into shocks, and exert radial and azimuthal Reynolds stresses on the mean fluid. Using reasonable assumptions for the stellar parameters, these processes were found to naturally create the inner boundary conditions required for dense Keplerian disks, even when the underlying B-star photosphere is rotating as slowly as 60% of its critical rotation speed. Because there is evidence for long-term changes in Be-star pulsational properties, this model may also account for the long-term variability of Be stars, including transitions between normal, Be, and shell phases.
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