Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
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确定非均匀 AGB 流出中结块和孔隙率对化学的影响
DOI：
10.1051/0004-6361/201732276
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发表时间：
2018
影响因子：
6.5
通讯作者：
Van De Sande M
中科院分区：
文献类型：
--
作者：
Van De Sande M
在渐近巨枝(AGB)流出的内部区域，已经探测到几个分子的丰度比热力学平衡化学模型预测的要高得多。这些物种中的大多数的存在可以用激波诱导的非平衡化学模型来解释，在该模型中，脉动的恒星引起的激波使内部区域的化学失去平衡。此外，在几个AGB外流中发现了不均匀的密度结构。观察到了大尺度的结构，如螺旋和圆盘，以及小尺度的密度不均匀或块状。这些结构也可能对恒星周围的化学物质产生相当大的影响。到目前为止，还没有对非均匀流出的定量影响进行详细的参数研究。目的通过考虑随机的块状密度结构，我们检查了AGB流出中的非均匀密度分布对其化学的影响。方法我们实施疏松形式来处理与辐射通过块状多孔介质传输相关的光泄漏增加。然后，我们使用这种方法来研究改变的UV辐射场穿透对化学的影响，也解释了在过密集的团块中两体过程的反应速度增加的原因。比簇度由三个参数决定：恒星表面束的特征长度比例、束体积填充因子和束间密度对比度。在这篇文章中，假设团块在空间上具有恒定的体积填充因子，这意味着当它们在风中向外移动时，它们会膨胀。团块内较高的密度和增加的紫外线辐射场穿透率都对化学有重要影响，因为它们都改变了整个流出过程中的化学路径。内部区域紫外线辐射的增加导致母体物种的光解离，释放原本缺乏的元素。我们发现在假设热力学平衡化学的情况下，所有物种的内部区域都不会出现丰度的增加，例如富O流出的HCN，富C流出的H2O，以及两者中的NH3。结论不均匀的密度分布直接影响整个AGB流出的化学成分，既通过密度结构本身，也通过其对UV辐射场的影响。假设热力学平衡化学的假设，不会出现在流出内区的物种现在已经在该区域形成，包括那些没有通过激波诱导的非平衡化学模型形成的物种。对于所研究的大多数意想不到的物种，其聚集具有很大的超密度并且对星际紫外线辐射场非常疏松的流出产生的丰度与在富O和富C流出中观察到的丰度相当。然而，对富C流出的H2O、富氨、富O和富C流出的内风丰度预测不足。
ContextIn the inner regions of asymptotic giant branch (AGB) outflows, several molecules have been detected with abundances much higher than those predicted from thermodynamic equilibrium chemical models. The presence of the majority of these species can be explained by shock-induced non-equilibrium chemical models, where shocks caused by the pulsating star take the chemistry out of equilibrium in the inner region. Moreover, a non-uniform density structure has been detected in several AGB outflows. Both large-scale structures, such as spirals and disks, and small-scale density inhomogeneities or clumps have been observed. These structures may also have a considerable impact on the circumstellar chemistry. A detailed parameter study on the quantitative effects of a non-homogeneous outflow has so far not been performed.AimsWe examine the effects of a non-uniform density distribution within an AGB outflow on its chemistry by considering a stochastic, clumpy density structure.MethodsWe implement aporosity formalismfor treating the increased leakage of light associated with radiation transport through a clumpy, porous medium. We then use this method to examine the effects from the altered UV radiation field penetration on the chemistry, accounting also for the increased reaction rates of two-body processes in the overdense clumps. The specific clumpiness is determined by three parameters: the characteristic length scale of the clumps at the stellar surface, the clump volume filling factor, and the inter-clump density contrast. In this paper, the clumps are assumed to have a spatially constant volume filling factor, which implies that they expand as they move outward in the wind.ResultsWe present a parameter study of the effect of clumping and porosity on the chemistry throughout the outflow. Both the higher density within the clumps and the increased UV radiation field penetration have an important impact on the chemistry, as they both alter the chemical pathways throughout the outflow. The increased amount of UV radiation in the inner region leads to photodissociation of parent species, releasing the otherwise deficient elements. We find an increased abundance in the inner region of all species not expected to be present assuming thermodynamic equilibrium chemistry, such as HCN in O-rich outflows, H2O in C-rich outflows, and NH3in both.ConclusionsA non-uniform density distribution directly influences the chemistry throughout the AGB outflow, both through the density structure itself and through its effect on the UV radiation field. Species not expected to be present in the inner region of the outflow assuming thermodynamic equilibrium chemistry are now formed in this region, including species that are not formed in greater abundance by shock-induced non-equilibrium chemistry models. Outflows whose clumps have a large overdensity and that are very porous to the interstellar UV radiation field yield abundances comparable to those observed in O-rich and C-rich outflows for most of the unexpected species investigated. The inner wind abundances of H2O in C-rich outflows and of NH3in O-rich and C-rich outflows are however underpredicted.
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