交错网格：三维恒星大气模型的网格- VI。恒星颗粒的表面形貌
The Stagger-grid: A grid of 3D stellar atmosphere models - VI. Surface appearance of stellar granulation
摘要：
在晚型恒星的表面层中，由于对流运动的存在，恒星对流表现出典型的粒化模式。由此产生的光球向上和向下流动在观测到的谱线轮廓中留下印记。我们对不同恒星参数下的恒星颗粒化及其性质进行了仔细的统计分析。我们采用现实的三维辐射流体动力学（RHD）模拟表面对流交错网格，一个全面的网格的大气模型，涵盖了一个大的参数空间Teff，logg，和[Fe/H]。单个颗粒检测（测辐射热）强度图在磁盘中心与一个有效的颗粒模式识别算法。从这些，我们得到它们各自的属性：直径，分形维数（面积周长关系），几何形状，拓扑结构，强度，温度，密度和速度随粒度的变化。此外，在光学表面的物理性质的相关性进行了研究。我们发现在我们所有的3D RHD模拟恒星颗粒图案印记，这是定性类似的太阳的情况下，尽管恒星参数的巨大差异。颗粒显示出大的尺寸范围，其可以通过平均颗粒尺寸分为两组-较小的颗粒和较大的颗粒。这些颗粒的性质不同：较小的颗粒形状规则且较暗，而较大的颗粒则越来越不规则且形状和强度对比度分布更复杂。这反映在它们的分形维数中，对于较小的颗粒，分形维数接近1，而对于较大的颗粒，分形维数接近2，这是由于颗粒的破碎。恒星表面对流似乎在很大的恒星参数范围内具有标度不变，这转化为自相似的恒星颗粒模式。
英文摘要：
In the surface layers of late-type stars, stellar convection is manifested with its typical granulation pattern due to the presence of convective motions. The resulting photospheric up- and downflows leave imprints in the observed spectral line profiles. We perform a careful statistical analysis of stellar granulation and its properties for different stellar parameters. We employ realistic 3D radiative hydrodynamic (RHD) simulations of surface convection from the Stagger-grid, a comprehensive grid of atmosphere models that covers a large parameter space in terms of Teff, logg, and [Fe/H]. Individual granules are detected from the (bolometric) intensity maps at disk center with an efficient granulation pattern recognition algorithm. From these we derive their respective properties: diameter, fractal dimension (area-perimeter relation), geometry, topology, variation of intensity, temperature, density and velocity with granule size. Also, the correlation of the physical properties at the optical surface are studied. We find in all of our 3D RHD simulations stellar granulation patterns imprinted, which are qualitatively similar to the solar case, despite the large differences in stellar parameters. The granules exhibit a large range in size, which can be divided into two groups - smaller and larger granules - by the mean granule size. These are distinct in their properties: smaller granules are regular shaped and dimmer, while the larger ones are increasingly irregular and more complex in their shapes and distribution in intensity contrast. This is reflected in their fractal dimensions, which is close to unity for the smaller granules, and close to two for larger granules, which is due to the fragmentation of granules. Stellar surface convection seems to operate scale-invariant over a large range in stellar parameters, which translates into a self-similar stellar granulation pattern.
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