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arXiv 1901.07572astro-ph.HEastro-ph.SR
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双星在气体介质中的演化:Bondi-Hoyle-Lyttleton吸积的三维模拟

The Evolution of Binaries in a Gaseous Medium: Three-Dimensional Simulations of Binary Bondi-Hoyle-Lyttleton Accretion

Andrea Antoni, Morgan MacLeod, Enrico Ramirez-Ruiz

摘要:

双星是常见的。虽然只有那些间隔很小的双星可以相互交换气体,但即使是最宽的双星也会与周围的气体环境相互作用。拖曳力和吸积速率决定了这些系统如何被这些相互作用所改变。我们进行三维流体动力学模拟的Bondi-Hoyle-Lyttleton流,其中一个二进制超音速移动相对于均匀介质,使用自适应网格加密代码FLASH。我们模拟的初始半长轴的轨道相对于引力聚焦碰撞参量的对的值的范围。当双星分离度小于引力聚焦碰撞参量时,双星在一个共同的弓形激波内运行。当这一对较宽时,每个物体都有一个单独的弓形激波结构。双星的长期演化取决于吸积、质心减速和轨道内旋的时间尺度。我们发现这些时间尺度有一个清晰的层次结构,一个双星的质心运动在一个较短的时间尺度上比一对吸气或吸积慢。在以前的分析预测,假设一个未受干扰的背景介质,我们发现,轨道内旋的时间尺度是成比例的半长轴的$0.19 \pm 0.01$的权力。这种正的比例表明,气体阻力可以驱动双星合并或临界分离,在引力辐射主导其进一步的演变。我们讨论了嵌入在星际介质,活动星系核盘,和共同的信封阶段的双星我们的结果的影响。

英文摘要:

Binary stars are common. While only those with small separations may exchange gas with one another, even the widest binaries interact with their gaseous surroundings. Drag forces and accretion rates dictate how these systems are transformed by these interactions. We perform three-dimensional hydrodynamic simulations of Bondi-Hoyle-Lyttleton flows, in which a binary moves supersonically relative to a homogeneous medium, using the adaptive mesh refinement code FLASH. We simulate a range of values of the initial semi-major axis of the orbit relative to the gravitational focusing impact parameter of the pair. When the binary separation is less than the gravitational focusing impact parameter, the pair orbits within a shared bow shock. When the pair is wider, each object has an individual bow-shock structure. The long-term evolution of the binary is determined by the timescales for accretion, slowing of the center of mass, and orbital inspiral. We find a clear hierarchy of these timescales; a binary's center-of-mass motion is slowed over a shorter timescale than the pair inspirals or accretes. In contrast to previous analytic predictions, which assume an unperturbed background medium, we find that the timescale for orbital inspiral is proportional to the semi-major axis to the $0.19 \pm 0.01$ power. This positive scaling indicates that gaseous drag forces can drive binaries either to coalescence or to the critical separation at which gravitational radiation dominates their further evolution. We discuss the implications of our results for binaries embedded in the interstellar medium, active galactic nuclei disks, and common envelope phases.

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