SPH modelling of companion-perturbed AGB outflows including a new morphology classification scheme
SPH modelling of companion-perturbed AGB outflows including a new morphology classification scheme
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伴随扰动 AGB 流出的 SPH 建模，包括新的形态分类方案
DOI：
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发表时间：
2021
期刊：
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通讯作者：
L. Decin
中科院分区：
文献类型：
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作者：
S. Maes;W. Homan;J. Malfait;L. Siess;J. Bolte;F. D. Ceuster;L. Decin
上下文已知渐近巨星分支（AGB）恒星会因恒星风而失去大量质量，而恒星风控制着它们剩余的恒星寿命。高角分辨率的观测显示，这些冷恒星的风通常表现出中等到小尺度的密度扰动，如螺旋和弧，据信是由与（亚）恒星伴星的引力相互作用引起的。目标。我们的目标是探索风-伴星相互作用对风的三维密度和速度分布的影响，作为三个关键参数的函数：风速，双星分离和伴星质量。这是第一次，我们比较了行星伴星和恒星伴星对外行星流的影响。我们打算设计一个基于奇异参数的形态分类方案。方法.我们运行了一个小网格的高分辨率多方模型与平滑粒子流体动力学（SPH）的数值代码P HANTOM检查的三维密度结构的AGB出流的轨道和赤道平面和周围的极点。通过建立伴星引力加速度的基本玩具模型，分析了模拟中外流所达到的终端速度。结果我们发现，具有恒星伴星、大双星分离和高风速的模型在轨道平面上获得了由单个螺旋结构组成的风形态，由于引力弹弓机制引起的速度色散，螺旋结构的两个边缘发散。在赤道平面上，螺旋线表现为同心弧，到达所有纬度。当降低风速和/或双星分离时，形态变得更加复杂：在轨道平面中出现双螺旋，这对于最近的系统来说是不规则的，并且风物质朝向轨道平面聚集，从而形成赤道密度增强（EDE）。将伴星质量从恒星质量降低到行星质量，可以显著减少密度扰动的形成。结论.有了这个网格模型，我们涵盖了显着的形态变化，在一个同伴扰动的AGB出风：缓慢的风与密切，大规模的双同伴显示出更复杂的形态。此外，我们证明了大质量行星能够显著影响AGB风的密度结构。我们发现与伴星的相互作用影响了风的终端速度，这可以用引力弹弓机制来解释。我们区分两种类型的风聚焦到轨道平面产生不同的机制：全球性的外对流的AGB星星的轨道运动的结果和形成的EDE作为一个后果的同伴的引力。我们研究了不同的形态分类方案，发现伴星的引力势能密度与AGB外流的动能密度之比为本文提出的模型提供了一个鲁棒的分类参数。
Context. Asymptotic giant branch (AGB) stars are known to lose a significant amount of mass by a stellar wind, which controls the remainder of their stellar lifetime. High angular-resolution observations show that the winds of these cool stars typically exhibit mid- to small-scale density perturbations such as spirals and arcs, believed to be caused by the gravitational interaction with a (sub-)stellar companion. Aims. We aim to explore the effects of the wind-companion interaction on the 3D density and velocity distribution of the wind, as a function of three key parameters: wind velocity, binary separation and companion mass. For the first time, we compare the impact on the outflow of a planetary companion to that of a stellar companion. We intend to devise a morphology classification scheme based on a singular parameter. Methods. We ran a small grid of high-resolution polytropic models with the smoothed particle hydrodynamics (SPH) numerical code P HANTOM to examine the 3D density structure of the AGB outflow in the orbital and meridional plane and around the poles. By constructing a basic toy model of the gravitational acceleration due to the companion, we analysed the terminal velocity reached by the outflow in the simulations. Results. We find that models with a stellar companion, large binary separation and high wind speed obtain a wind morphology in the orbital plane consisting of a single spiral structure, of which the two edges diverge due to a velocity dispersion caused by the gravitational slingshot mechanism. In the meridional plane the spiral manifests itself as concentric arcs, reaching all latitudes. When lowering the wind velocity and/or the binary separation, the morphology becomes more complex: in the orbital plane a double spiral arises, which is irregular for the closest systems, and the wind material gets focussed towards the orbital plane, with the formation of an equatorial density enhancement (EDE) as a consequence. Lowering the companion mass from a stellar to a planetary mass, reduces the formation of density perturbations significantly. Conclusions. With this grid of models we cover the prominent morphology changes in a companion-perturbed AGB outflow: slow winds with a close, massive binary companion show a more complex morphology. Additionally, we prove that massive planets are able to significantly impact the density structure of an AGB wind. We find that the interaction with a companion affects the terminal velocity of the wind, which can be explained by the gravitational slingshot mechanism. We distinguish between two types of wind focussing to the orbital plane resulting from distinct mechanisms: global flattening of the outflow as a result of the AGB star’s orbital motion and the formation of an EDE as a consequence of the companion’s gravitational pull. We investigate different morphology classification schemes and uncover that the ratio of the gravitational potential energy density of the companion to the kinetic energy density of the AGB outflow yields a robust classification parameter for the models presented in this paper.