[论文解读] Exploring the origin of clumpy dust clouds around cool giants. A global 3D RHD model of a dust-forming M-type AGB star

S. Höfner, B. Freytag|arXiv (Cornell University)|Feb 11, 2019

Astrophysics and Star Formation Studies参考文献 58被引用 4

一句话总结

本研究提出了一种M型渐近巨星支（AGB）星的三维辐射流体动力学模型，该模型自洽地模拟了尘埃形成与颗粒生长过程，揭示了由对流和脉动引起的非球形激波可自然产生内层引力束缚大气中的致密尘埃云团。该机制无需假设风驱动加速，即可解释观测到的中红外变异性与空间分辨尘埃形态，其中刚玉（Al₂O₃）在约2倍恒星半径处首先形成，而辉石类硅酸盐则标记着束缚层的外边缘。

ABSTRACT

Dust grains forming in the extended atmospheres of AGB stars are critical for the heavy mass loss of these cool luminous giants, as they provide radiative acceleration for the stellar winds. Characteristic mid-IR spectral features indicate that the grains consist mainly of silicates and corundum. The latter species seems to form in a narrow zone within about 2 stellar radii, preceding the condensation of silicate dust, which triggers the outflow. Recent high-angular-resolution observations show clumpy, variable dust clouds at these distances. We explore possible causes for the formation of inhomogeneous dust layers, using 3D dynamical simulations. We modeled the outer convective envelope and the dust-forming atmosphere of an M-type AGB star with the CO5BOLD radiation-hydrodynamics code. The simulations account for frequency-dependent gas opacities, and include a time-dependent description of grain growth and evaporation for corundum and olivine-type silicates. In the inner, gravitationally bound, and corundum-dominated layers of the circumstellar envelope, a patchy distribution of the dust emerges naturally, due to atmospheric shock waves that are generated by large-scale convective flows and pulsations. The formation of silicate dust at somewhat larger distances probably indicates the outer limit of the gravitationally bound layers. The current models do not describe wind acceleration, but the cloud formation mechanism should also work for stars with outflows. Timescales of atmospheric dynamics and grain growth are similar to observed values. In spherical averages of dust densities the variable 3D morphology manifests itself as cycle-to-cycle variations. Grain growth in the wake of large-scale non-spherical shock waves, generated by convection and pulsations, is a likely mechanism for producing the observed clumpy dust clouds, and for explaining their physical and dynamical properties.

研究动机与目标

• 理解在M型AGB星的高角分辨率图像中观测到的致密、变异性尘埃云团的起源。
• 研究对流与脉动等大气动力学是否能自然产生非均匀的尘埃分布，而无需假设预先存在的非均匀性。
• 确定颗粒在激波尾迹中生长在形成观测到的斑驳尘埃结构中的作用。
• 评估仅依靠三维动力学是否足以解释观测到的尘埃形态与变异性，而无需考虑风加速之前的过程。
• 通过分析尘埃密度及其他物理量的球对称平均值，将三维模型结果与未分辨观测及一维模型进行比较。

提出的方法

• 使用CO5BOLD辐射流体动力学代码，模拟M型AGB星的外对流包层与尘埃形成大气。
• 引入频率相关的气体消光系数，以准确模拟动态大气中的辐射转移与加热过程。
• 对刚玉（Al₂O₃）与橄榄石型硅酸盐（Mg₂SiO₄）实施颗粒成核、生长与蒸发的时间依赖处理。
• 追踪尘埃密度与温度在脉动与对流流作用下的演化，重点研究激波特性的动力学。
• 分析三维空间结构与尘埃密度的球对称平均值，以与未分辨观测及一维模型进行对比。
• 采用全局三维设置，覆盖恒星大气至约2–3倍恒星半径的区域，聚焦于尘埃形成但尚未经历风加速的区域。

实验结果

研究问题

• RQ1 由对流与脉动产生的大尺度激波是否能自然在M型AGB星的大气中形成致密尘埃分布？
• RQ2 非球形激波前缘尾迹中颗粒生长在塑造观测到的尘埃形态中起什么作用？
• RQ3 大气激波的动力学时标与颗粒生长速率与观测到的变异性时标相比如何？
• RQ4 刚玉与硅酸盐尘埃在星周包层的哪个区域形成？这对引力束缚层结构有何启示？
• RQ5 尘埃密度球对称平均值的周期间变化在多大程度上反映了其背后的三维、时间依赖的形态？

主要发现

• 致密尘埃分布可在三维模拟中自然形成，源于对流与脉动引起的非球形激波，无需引入外部非均匀性。
• 刚玉（Al₂O₃）在约2倍恒星半径以内的狭窄区域内形成，与观测结果一致，即其存在于最内层尘埃形成区。
• 硅酸盐尘埃（Mg₂SiO₄）在稍大半径处形成，标志着引力束缚层的外边缘，此处辐射压力可能在后期触发风加速。
• 颗粒生长主要发生在向外传播激波的尾迹中，从而增强局部尘埃密度，形成观测到的斑驳形态。
• 模型中大气动力学与颗粒生长的时标与观测到的变异性周期及周期间变化一致。
• 尘埃密度的球对称平均值表现出周期间变化，既反映了径向脉动，也体现了三维时间依赖形态，从而将未分辨观测与三维动力学联系起来。