含前AGB星双星系统的轨道偏心率
Orbital eccentricities of binary systems with a former AGB star
摘要：
许多双星系统中的主星是超越渐近巨星分支（AGB）的演化阶段表现出显着的轨道偏心率，而目前的二元相互作用模型预测他们的轨道是circularised。我们分析了如何在一个系统中的轨道参数修改下的质量损失和质量交换其二进制组件之间，并提出了一个模型，增强质量损失的AGB星由于潮汐相互作用与其同伴，这使得风和罗氏瓣溢出质量损失制度之间的平稳过渡。我们明确遵循它的影响，沿轨道的偏心率和轨道半长轴的变化，以及吸积的同伴的影响。我们计算这些轨道参数的变化的时间尺度，并将它们与潮汐循环时间尺度进行比较。我们发现，在许多情况下，由于增强的质量损失的AGB组件在轨道相位接近近星点，在一个轨道的净偏心率增长率是可比的潮汐循环率。我们表明，与这种偏心率增强机制，它是可能的重现天狼星系统的轨道周期和偏心率，这在标准的假设下的二进制相互作用，预计将circularised。我们还表明，这种机制可以提供一个解释的离心率的钡星系统，预计将循环由于潮汐耗散。通过提出一个潮汐增强模型的质量损失从AGB星，我们找到了一种机制，有效地工作对潮汐循环的轨道，这解释了显着的偏心率观测到的二元系统包含一个白矮星和一个不太发达的同伴，如天狼星和系统与钡星。
英文摘要：
Many binary stellar systems in which the primary star is beyond the asymptotic giant branch (AGB) evolutionary phase show significant orbital eccentricities whereas current binary interaction models predict their orbits to be circularised. We analyse how the orbital parameters in a system are modified under mass loss and mass exchange among its binary components and propose a model for enhanced mass-loss from the AGB star due to tidal interaction with its companion, which allows a smooth transition between the wind and Roche-lobe overflow mass-loss regimes. We explicitly follow its effect along the orbit on the change of eccentricity and orbital semi-major axis, as well as the effect of accretion by the companion. We calculate timescales for the variation of these orbital parameters and compare them to the tidal circularisation timescale. We find that in many cases, due to the enhanced mass loss of the AGB component at orbital phases closer to the periastron, the net eccentricity growth rate in one orbit is comparable to the rate of tidal circularisation. We show that with this eccentricity enhancing mechanism it is possible to reproduce the orbital period and eccentricity of the Sirius system, which under the standard assumptions of binary interaction is expected to be circularised. We also show that this mechanism may provide an explanation for the eccentricities of most barium star systems, which are expected to be circularised due to tidal dissipation. By proposing a tidally enhanced model of mass loss from AGB stars we find a mechanism which efficiently works against the tidal circularisation of the orbit, which explains the significant eccentricities observed in binary systems containing a white dwarf and a less evolved companion, such as Sirius and systems with barium stars.
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