Atmosphere and spectrum modelling of cool stars

恒星最外层的半透明层构成了恒星的大气层，也是我们能最直接观察到的恒星部分。我们研究大气的主要观测数据是恒星发出的光谱。大气结构决定光谱的物理原理非常复杂，我们只能通过计算大气和光谱的模型来提取恒星的信息。由此可见，推断大气结构的准确性取决于计算模型的物理真实感。由于计算能力和数值方法的限制，迄今为止的模型对实际的恒星大气有些不切实际的近似。我建议对恒星大气和光谱建模中的许多现实性领域进行改进，包括大气的几何和水平结构，气体和辐射场的热力学状态的处理，流出物（风）的包含，以及原子吸收然后重新发射光子的现实性。这是一个雄心勃勃的计划，但该方法将强调在其中一些领域进行近似改进，同时在其他领域进行更实际的改进，以研究同时在几个领域进行改进的净综合效果。这些改进与研究相对较冷的恒星（如太阳和红巨星）尤其相关。这里将讨论两个广泛的科学问题：1)提高对非常古老的红巨星的化学成分的光谱测定的准确性，这些红巨星可以追踪我们银河系中遥远的全球结构，了解这些结构将有助于阐明宇宙中结构形成和化学元素合成的历史；2)在太阳和大多数冷恒星的外层大气中发现的神秘的戏剧性的逆温结构。对它的理解将揭示目前未知的在这样的恒星大气中发生的物理过程。

The outermost translucent layers of a star comprise the star's atmosphere, and is the part of the star thatwe can observe most directly.  The primary observational data that we have for studying the atmosphere isthe spectrum of the light emitted by the star.  The physics of how the spectrum is determined by thestructure of the atmosphere is sufficiently complex that we can only extract information about the star bycomputational modelling of the atmosphere and the spectrum.  It follows that the accuracy with which theatmospheric structure is inferred depends on the physical realism of the computational modelling.  Due tolimitations in computational power and numerical methodology, models to date have been somewhatunrealistic approximations to actual stellar atmospheres.  I propose to make improvements to a number ofareas of realism in stellar atmosphere and spectrum modelling, including the geometry and horizontalstructure of the atmosphere, the treatment of the thermodynamic state of the gas and radiation field, theinclusion of outflows (winds), and the realism with which atoms absorb, then re-emit, photons.  This is anambitious program, but the approach will emphasize making approximate improvements in some of theseareas while making more realistic improvements in others of them to study the net combined effect ofimprovement in several areas at once.  These improvements are especially relevant to the study ofrelatively cool stars such as the Sun and red giants.  There are two broad scientific problems that will beaddressed: 1) Increasing the accuracy with which the chemical composition is spectroscopically determinedfor very old red giant stars that trace remote global structures in our Galaxy, the understanding of whichwill help elucidate the history of structure formation and chemical element synthesis in the Universe, and2) The structure of the mysterious dramatic temperature inversion that is found in the outer atmosphere ofthe Sun and most cool stars, the understanding of which will reveal physical processes that occur in suchstellar atmospheres that are currently unknown.