探讨密近双星演化中稳定质量转移与并合的边界
Exploring the borderline between stable mass transfer and mergers in close binary evolution
AI总结：
本研究通过计算恒星吸积模型网格，推导了稳定质量转移与双星并合边界的解析公式，并应用于双星演化模型，发现质量转移效率对结果影响更大，且稳定转移参数空间小于文献方案。
摘要：
大多数大质量恒星存在于双星系统中，预计在其演化过程中会经历质量转移。然而，到目前为止，质量转移在何种条件下会导致共同包络，从而可能导致两颗恒星并合，尚不清楚。主要的不确定性来自于非守恒质量转移过程中质量接收者的可能膨胀，以及双星系统的角动量损失。我们计算了一个以恒定速率吸积质量的恒星的详细模型密集网格，以确定它们由于热不平衡而导致的半径增加。虽然我们发现快于热时标的吸积模型通常会膨胀，但在中等质量范围内，即使吸积率超过热时标吸积率100倍，这种膨胀仍然相当有限。在这些条件下，我们的大质量吸积星模型膨胀到极端半径。当吸积率超过Eddington吸积率时，我们的模型发生动力学膨胀。我们推导了模型半径演化的解析拟合，以及任意吸积效率下稳定质量转移与并合之间边界的公式。然后，我们将我们的结果应用于采用各种恒定质量转移效率和角动量预算的双星模型网格。我们发现前一个参数对Roche lobe overflow的结果有更强的影响。我们的结果与详细的双星演化模型一致，并且通常导致比文献中其他方案更小的稳定质量转移初始参数空间。我们使用这种方法研究了Small Magellanic Cloud中具有O星伴星的Wolf-Rayet星的起源，并发现导致Wolf-Rayet星形成的质量转移过程的效率可能低于50%。
英文摘要：
The majority of massive stars reside in binary systems, which are expected to experience mass transfer during their evolution. However, so far the conditions under which mass transfer leads to a common envelope, and thus possibly to a merging of both stars, are not well understood. Main uncertainties arise from the possible swelling of the mass gainer, and from angular momentum loss from the binary system, during non-conservative mass transfer. We have computed a dense grid of detailed models of stars accreting mass at constant rates, to determine their radius increase due to their thermal disequilibrium. While we find that models with faster than thermal timescale accretion generally expand, this expansion remains quite limited in the intermediate mass regime even for accretion rates which exceed the thermal timescale accretion rate by a factor of 100. Our models of massive accretion stars expand to extreme radii under those conditions. When the accretion rate exceed the Eddington accretion rate, our models expand dynamically. We have derived analytical fits to the radius evolution of our models and a prescription for the borderline between stable mass transfer and mergers for arbitrary accretion efficiencies. We then apply our results to grids of binary models adopting various constant mass transfer efficiencies and angular momentum budgets. We find that the former parameter has the stronger effect on the outcome of the Roche lobe overflow. Our results are consistent with detailed binary evolution models, and often lead to a smaller initial parameter space for stable mass transfer than other recipes in the literature. We use this method to investigate the origin of the Wolf-Rayet stars with O star companions in the Small Magellanic Cloud, and find that the efficiency of the mass transfer process which lead to the formation of the Wolf-Rayet star was likely below 50%.
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