为什么一维模型不能诊断来自非均匀恒星大气的平均光谱
Why one-dimensional models fail in the diagnosis of average spectra from inhomogeneous stellar atmospheres
摘要：
我们调查的可行性，表示一个结构化的多维恒星大气与一个单一的一维平均分层的目的，大气的平均光谱的光谱诊断。特别是，我们构建了四个不同的一维分层从一个单一的快照的磁流体动力学模拟太阳对流：一个通过平均其属性在表面上的恒定高度，和三个不同的平均在500 nm的恒定光学深度的表面。使用这些模型，我们计算连续，原子和分子线强度和他们的中心到肢体的变化。从新兴的光谱分析，我们确定了三个主要原因，为什么这些平均表示是不够的，通过光谱分析的恒星大气特性的准确测定。这些理由是：普朗克函数随温度的非线性，它使非均匀大气的平均出射强度高于平均性质大气的平均出射强度，即使它们的温度-光学深度分层是相同的;分子形成随温度和密度的非线性，它使非均匀大气的分子丰度高于具有相同平均性质的一维模型;对流运动的各向异性，强烈影响线芯强度的中心到边缘的变化。因此，我们认为，一个一维的大气模式，再现了平均光谱的非均匀大气不一定反映平均的物理性质的大气，因此本质上是不可靠的。
英文摘要：
We investigate the feasibility of representing a structured multi-dimensional stellar atmosphere with a single one-dimensional average stratification for the purpose of spectral diagnosis of the atmosphere's average spectrum. In particular we construct four different one-dimensional stratifications from a single snapshot of a magneto-hydrodynamic simulation of solar convection: one by averaging its properties over surfaces of constant height, and three different ones by averaging over surfaces of constant optical depth at 500 nm. Using these models we calculate continuum, and atomic and molecular line intensities and their center-to-limb variations. From analysis of the emerging spectra we identify three main reasons why these average representations are inadequate for accurate determination of stellar atmospheric properties through spectroscopic analysis. These reasons are: non-linearity in the Planck function with temperature, which raises the average emergent intensity of an inhomogeneous atmosphere above that of an average-property atmosphere, even if their temperature-optical depth stratification is identical; non-linearities in molecular formation with temperature and density, which raise the abundance of molecules of an inhomogeneous atmosphere over that in a one-dimensional model with the same average properties; the anisotropy of convective motions, which strongly affects the center-to-limb variation of line-core intensities. We argue therefore that a one-dimensional atmospheric model that reproduces the mean spectrum of an inhomogeneous atmosphere necessarily does not reflect the average physical properties of that atmosphere, and are therefore inherently unreliable.
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