Multi-dimensional radiative transfer in the expanding atmospheres of massive stars

大质量恒星是星系演化的关键因素，无论是在当今时代还是在早期宇宙中，它们都是解释我们宇宙的关键工具。它们的高亮度驱动着能量外流，即恒星风，这对邻近的ISM的条件、触发恒星形成和大质量恒星演化至关重要。近年来，热发光恒星的“基本”质量损失行为得到了理解，但是，例如，风的不均匀性的影响可能意味着比预测的质量损失率要低。然而，目前大气模式的局限性限制了我们检验更高级预测的诊断能力。这些特别指的是快速旋转、磁场和邻近Eddington-limit/super-Eddington条件的影响，需要多维度的处理，不仅涉及流体力学，还涉及辐射传递和职业数。尽管许多研究小组已经投入了相当多的精力来开发他们自己的针对特定目的设计的多维辐射传输代码，但这些代码都没有被常规地应用于考虑非lte影响的“现实”风条件。在这里，我们建议一个具有“最终”目标的项目（超越目前的提议），通过将3-D辐射传输与基于我们快速执行的大气代码FASTWIND的相应nlte计算相结合，来测试最近和即将到来的理论模型，这些模型预测（快速）旋转、磁场和不均匀性对恒星风和光球的影响。我们这个提议的“直接”目标是，在第一步，开发足够的工具来解决非对称、快速膨胀的大气中具有非单调速度场的三维辐射传输（连续体和线辐射）。我们计划使用笛卡尔网格，并测试两种方法的准确性和性能，即由Adam（1990）提出的空间隐式有限体积方法和（更传统的）短特征方法，两者都通过加速Lambda迭代进行修正。在比较两种方法之后，我们将决定我们的最终方法。由于并行化是最重要的，我们将考虑并测试适合各种体系结构的策略。作为第一个应用，我们将研究快速旋转和磁约束风模型中紫外共振线和Balmer_alpha（最重要的质量损失诊断）的形成，并与观测结果进行比较。这些调查已经允许对这些模型进行初步约束。在这个项目和未来的项目中开发的技术将使我们能够在大质量恒星大气中研究更多的多维问题。为了加快进度，我们计划将开发的工具提供给社区。

Massive stars are critical agents in galactic evolution, both in the present epoch as well as in the early cosmos, and they are key tools to interpret our Universe. Their high luminosities drive energetic outflows, the stellar winds, that are fundamentally important for the conditions in the neigboring ISM, for triggering star formation, and for massive star evolution. During recent years, the "basic" mass-loss behavior of hot luminous stars became understood, though, e.g., effects from wind-inhomogeneities potentially imply lower mass-loss rates than predicted. However, present limitations in our atmospheric models restrict our diagnostic capabilities to test more advanced predictions. These refer particularly to the effects of fast rotation, magnetic fields, and neighborhood to the Eddington-limit/super-Eddington conditions, requiring a multi-dimensional treatment, not only with respect to hydrodynamics, but also regarding radiative transfer and occupation numbers.Though many groups have invested considerable effort in developing their own multi-D radiative transfer codes designed for specific purposes, none of these codes has been routinely applied to "realistic" wind conditions accounting for non-LTE effects. Here, we suggest a project with the "ultimate" goal (beyond the present proposal) to test recent and upcoming theoretical models predicting the impact of (rapid) rotation, magnetic-fields, and inhomogeneities on the stellar winds and photospheres of massive stars, by coupling 3-D radiative transfer with corresponding NLTE-calculations based on our fast-performing atmosphere code FASTWIND.Our "immediate" objective of this proposal is to develop, in a first step, adequate tools for the solution of 3-D radiative transfer (continuum and line radiation) in asymmetric, rapidly expanding atmospheres with non-monotonic velocity fields. We plan to use a Cartesian grid, and to test the accuracy and performance of two methods, namely a spatially implicit finite volume method as suggested by Adam (1990), and a (more conventional) short characteristics method, both amended by an Accelerated Lambda Iteration. After comparing the two methods, we will decide on our final approach. Since parallelization is of prime importance, we will consider and test suitable strategies for various architectures. As first applications, we will study the formation of UV-resonance lines and Balmer_alpha (the most important mass-loss diagnostics) in models of rapidly rotating and magnetically confined winds, and compare with observations. Already these investigations will allow for preliminary constraints on such models.The techniques developed in this and future projects will enable to investigate many more multi-D problems in massive star atmospheres. To speed up progress, we plan to make the developed tools available to the community.