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arXiv 1111.5978astro-ph.SR
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超紧X射线双星的演化

The evolution of ultracompact X-ray binaries

L. M. van Haaften, G. Nelemans, R. Voss, M. A. Wood, J. Kuijpers

备注：
18 pages, 13 figures. Accepted for publication in Astronomy and Astrophysics
发表信息：
A&A 537, A104 (2012)
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摘要：

上下文超紧凑X射线双星（UCXB）通常由白矮星施主和中子星或黑洞吸积体组成。UCXB和极低质量比双星的演化一般知之甚少。目标。我们调查UCXB的演化，以了解这些系统可以存在的质量比和吸积类型，如果它们存在，它们的轨道和中子星自旋周期，质量转移速率和演化时间尺度是什么。方法.对于不同的假设吸积盘的行为，我们计算系统参数动力学不稳定性，热粘性盘不稳定性或螺旋桨效应出现。结果在质量转移开始时，UCXB的存活取决于在超爱丁顿质量转移率的情况下，吸积体喷射物质的效率。在以后的时间里，系统的演化强烈地依赖于双星将角动量从盘返回到轨道的能力。我们发现，这种反馈机制很可能仍然有效。在稳定的质量转移的情况下，螺旋桨效应可以停止吸积到回收中子星完全在一个足够低的质量转移率，基于能量的考虑。然而，质量转移可能是不稳定的，因为盘的不稳定性允许一些转移物质的吸积。螺旋桨效应和盘的不稳定性共同导致低质量比的UCXB在最多的一小部分时间内可见，从而解释了缺乏对此类系统的观测。结论.最有可能的是，UCXB避免了后期动态不稳定的质量转移，并在宇宙年龄允许的情况下继续演化。这意味着存在大量的低质量比双星，轨道周期约为70 - 80分钟，除非有其他机制破坏了这些双星。

英文摘要：

Context. Ultracompact X-ray binaries (UCXBs) typically consist of a white dwarf donor and a neutron star or black hole accretor. The evolution of UCXBs and very low mass ratio binaries in general is poorly understood. Aims. We investigate the evolution of UCXBs in order to learn for which mass ratios and accretor types these systems can exist, and if they do, what are their orbital and neutron star spin periods, mass transfer rates and evolutionary timescales. Methods. For different assumptions concerning accretion disk behavior we calculate for which system parameters dynamical instability, thermal-viscous disk instability or the propeller effect emerge. Results. At the onset of mass transfer, the survival of the UCXB is determined by how efficiently the accretor can eject matter in the case of a super-Eddington mass transfer rate. At later times, the evolution of systems strongly depends on the binary's capacity to return angular momentum from the disk to the orbit. We find that this feedback mechanism most likely remains effective. In the case of steady mass transfer, the propeller effect can stop accretion onto recycled neutron stars completely at a sufficiently low mass transfer rate, based on energy considerations. However, mass transfer will likely be non-steady because disk instability allows for accretion of some of the transferred matter. Together, the propeller effect and disk instability cause the low mass ratio UCXBs to be visible a small fraction of the time at most, thereby explaining the lack of observations of such systems. Conclusions. Most likely UCXBs avoid late-time dynamically unstable mass transfer and continue to evolve as the age of the Universe allows. This implies the existence of a large population of low mass ratio binaries with orbital periods ~ 70 - 80 min, unless some other mechanism has destroyed these binaries.

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