A generalized porosity formalism for isotropic and anisotropic effective opacity and its effects on X-ray line attenuation in clumped O star winds
A generalized porosity formalism for isotropic and anisotropic effective opacity and its effects on X-ray line attenuation in clumped O star winds
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各向同性和各向异性有效不透明度的广义孔隙度形式及其对聚集 O 星风中 X 射线线衰减的影响
DOI：
10.1111/j.1365-2966.2011.20141.x
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发表时间：
2011
影响因子：
4.8
通讯作者：
R. Townsend
中科院分区：
文献类型：
--
作者：
J. Sundqvist;S. Owocki;D. Cohen;M. Leutenegger;R. Townsend
我们提出了一个广义的形式主义治疗连续不透明度的孔隙度相关的减少时，发生在随机介质中的单个团块变得光学厚。在以前的工作中，我们专注于开发光学薄和厚团块的限制之间的桥接法律。我们考虑几何形状导致各向同性或各向异性的有效不透明度，并且，除了一个理想化的模型，其中所有的团块具有相同的局部密度和规模，我们还处理集的团块光学深度由马尔可夫统计。这种形式主义，然后适用于特定的情况下，在热星星风的X射线的束缚-自由吸收，一个过程中不直接受聚集在光学薄的限制。我们发现，马尔可夫模型给出了令人惊讶的相似的结果，以前发现的单丛模型，这表明多孔不透明度是不是很敏感的细节假设丛分布函数。此外，各向异性的有效不透明度有利于在切向方向上发射的X射线的逃逸（“百叶窗”效应），导致与从各向同性孔隙度模型计算的剖面相比，靠近线中心的较高通量的“凸起”。我们演示了如何可以使用这种特征线的形状来诊断团块的几何形状，我们确认以前的结果，光学厚结块显着影响X射线线剖面，非常大的孔隙度长度，定义为团块之间的平均自由程，是必需的。此外，我们提出的第一个X射线线轮廓计算直接从线驱动的不稳定性模拟使用3D补丁的方法，并发现从这样的模型的孔隙度的影响也是非常小的。这进一步支持了这样的观点，即孔隙度，最多，在O型星的X射线线诊断的边际效应，因此，这些诊断确实提供了一个很好的'clumping不敏感'的方法推导O星星质量损失率。
We present a generalized formalism for treating the porosity-associated reduction in continuum opacity that occurs when individual clumps in a stochastic medium become optically thick. As in previous work, we concentrate on developing bridging laws between the limits of optically thin and thick clumps. We consider geometries resulting in either isotropic or anisotropic effective opacity, and, in addition to an idealized model in which all clumps have the same local overdensity and scale, we also treat an ensemble of clumps with optical depths set by Markovian statistics. This formalism is then applied to the specific case of bound–free absorption of X-rays in hot star winds, a process not directly affected by clumping in the optically thin limit. We find that the Markov model gives surprisingly similar results to those found previously for the single-clump model, suggesting that porous opacity is not very sensitive to details of the assumed clump distribution function. Further, an anisotropic effective opacity favours escape of X-rays emitted in the tangential direction (the ‘venetian blind’ effect), resulting in a ‘bump’ of higher flux close to line centre as compared to profiles computed from isotropic porosity models. We demonstrate how this characteristic line shape may be used to diagnose the clump geometry, and we confirm previous results that for optically thick clumping to significantly influence X-ray line profiles, very large porosity lengths, defined as the mean free path between clumps, are required. Moreover, we present the first X-ray line profiles computed directly from line-driven instability simulations using a 3D patch method, and find that porosity effects from such models also are very small. This further supports the view that porosity has, at most, a marginal effect on X-ray line diagnostics in O stars, and therefore that these diagnostics do indeed provide a good ‘clumping insensitive’ method for deriving O star mass-loss rates.