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渐近巨星支恒星风的三维模拟——II. 光线追踪器的实现及辐射对外流形态的影响

3D simulations of AGB stellar winds -- II. Ray-tracer implementation and impact of radiation on the outflow morphology

Mats Esseldeurs, Lionel Siess, Frederik De Ceuster, Ward Homan, Jolien Malfait, Silke Maes, Thomas Konings, Thomas Ceulemans, Leen Decin

发表信息：
A&A 674, A122 (2023)
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AI总结：

本研究针对受伴星扰动的AGB恒星外流，在SPH中实现了光线追踪辐射转移模块，对比四种辐射转移方法发现辐射力显著影响外流速度与形态，低质量损失率可用几何近似，高质量损失率下Lucy近似精度最优。

摘要：

初始质量低于约8倍太阳质量（Msun）的恒星会经历渐近巨星支（AGB）演化阶段，在此阶段它们会产生强烈的恒星风。近期观测发现这类恒星的外流存在显著的形态复杂性，这很可能是由伴星导致的。我们研究了辐射力对这类受伴星扰动的AGB外流的影响。我们提出了一种在光滑粒子流体动力学（SPH）中实现辐射转移的光线追踪器，并对比了四种不同的辐射转移描述方法：自由风近似、几何近似、Lucy近似和衰减近似。无论是低质量损失率还是高质量损失率情形，从自由风近似切换到几何近似时，外流的速度剖面都会发生改变，进而导致形态差异。在低质量损失率情况下，由于密度较低，Lucy近似和衰减近似的影响可以忽略不计，但在高质量损失率区间会出现形态差异。通过将辐射平衡温度和辐射力与全三维辐射转移的结果进行对比，我们发现Lucy近似的表现最佳。不过，在靠近伴星的区域会出现人为加热效应，且该方法无法模拟伴星投射的阴影。衰减近似得到的平衡温度更低、辐射力更弱，但它能够模拟出伴星投射的阴影。根据三维辐射转移的预测结果，我们还得出结论：辐射力沿径向指向是一个合理的假设。由此可见，辐射力在尘埃驱动的AGB风中发挥着关键作用，会影响速度剖面和形态结构。对于低质量损失率情形，几何近似就已足够；而高质量损失率情形则需要更严谨的方法，其中Lucy近似能提供最准确的结果，尽管它无法解释所有效应。

英文摘要：

Stars with an initial mass below ~ 8 Msun evolve through the asymptotic giant branch (AGB) phase, during which they develop strong stellar winds. Recent observations have revealed significant morphological complexities in their outflows, most likely caused by a companion. We study the impact of the radiation force on such companion-perturbed AGB outflows. We present the implementation of a ray tracer for radiative transfer in smoothed particle hydrodynamics (SPH) and compared four different descriptions of radiative transfer: the free-wind, the geometrical, the Lucy, and the attenuation approximation. For both low and high mass-loss rates, the velocity profile of the outflow is modified when going from the free-wind to the geometrical approximation, also resulting in a different morphology. In the case of a low mass-loss rate, the effect of the Lucy and attenuation approximation is negligible due to the low densities but morphological differences appear in the high mass-loss rate regime. By comparing the radiative equilibrium temperature and radiation force to full 3D radiative transfer, we show that the Lucy approximation works best. Although, close to the companion, artificial heating occurs and it fails to simulate the shadow cast by the companion. The attenuation approximation produces a lower equilibrium temperature and weaker radiation force, but it produces the shadow cast by the companion. From the predictions of the 3D radiative transfer, we also conclude that a radially directed radiation force is a reasonable assumption. The radiation force thus plays a critical role in dust-driven AGB winds, impacting the velocity profile and morphological structures. For low mass-loss rates, the geometrical approximation suffices, while high mass-loss rates require a more rigorous method, where the Lucy approximation provides the most accurate results although not accounting for all effects.

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