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arXiv 1102.3922astro-ph.SR
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棕矮星半径对大气金属丰度和云的依赖性：理论和观测的比较

The Dependence of Brown Dwarf Radii on Atmospheric Metallicity and Clouds: Theory and Comparison with Observations

Adam Burrows, Kevin Heng, Thane Nampaisarn

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Accepted to the Astrophysical Journal, May 3, 2011
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摘要：

采用现实和一致的大气边界条件，我们已经产生了不同的金属丰度（[Fe/H]），有云和无云的棕矮星和极低质量恒星（VLMs）的演化模型。我们发现，在一个给定的质量和年龄的半径的蔓延可以高达$\sim$ 10%至$\sim$ 25%，与较高的金属丰度，较高的云厚度的大气中产生的更大的半径相当自然。[Fe/H]每增加0.1 dex，半径增加$\sim$ 1%至$\sim$ 2.5%，这取决于年龄和质量。我们还发现，而较小的质量和年龄较大的褐矮星半径随氦分数（$Y$）的增加而减小（如预期的那样），对于更大质量的褐矮星和广泛的年龄范围内，他们增加与氦分数。从$Y=0.25$到$Y=0.28$的半径增加可高达$\sim$ 0.025 \rj \（$\sim$ 2.5%）。此外，我们发现，对于VLMs大气金属丰度从0.0增加到0.5 dex，半径增加$\sim$ 4%，从-0.5至0.5 dex增加$\sim$ 10%。因此，我们认为，由于较高的金属丰度的不透明度可能会自然地解释在一些食VLM系统的表观半径异常。半径的百分之十到二十五的变化超过了仅仅由状态方程的不确定性引起的误差。这有助于强调棕矮星半径的凌日和日食测量共同限制了许多效应，重要的是包括大气和冷凝云模型，而不仅仅是状态方程。在任何时候，我们都是在检验一个多参数理论，而不是一个普适的半径$-$质量关系。

英文摘要：

Employing realistic and consistent atmosphere boundary conditions, we have generated evolutionary models for brown dwarfs and very-low-mass stars (VLMs) for different metallicities ([Fe/H]), with and without clouds. We find that the spread in radius at a given mass and age can be as large as $\sim$10% to $\sim$25%, with higher-metallicity, higher-cloud-thickness atmospheres resulting quite naturally in larger radii. For each 0.1 dex increase in [Fe/H], radii increase by $\sim$1% to $\sim$2.5%, depending upon age and mass. We also find that, while for smaller masses and older ages brown dwarf radii decrease with increasing helium fraction ($Y$) (as expected), for more massive brown dwarfs and a wide range of ages they increase with helium fraction. The increase in radius in going from $Y=0.25$ to $Y=0.28$ can be as large as $\sim$0.025 \rj\ ($\sim$2.5%). Furthermore, we find that for VLMs an increase in atmospheric metallicity from 0.0 to 0.5 dex, increases radii by $\sim$4%, and from -0.5 to 0.5 dex by $\sim$10%. Therefore, we suggest that opacity due to higher metallicity might naturally account for the apparent radius anomalies in some eclipsing VLM systems. Ten to twenty-five percent variations in radius exceed errors stemming from uncertainities in the equation of state alone. This serves to emphasize that transit and eclipse measurements of brown dwarf radii constrain numerous effects collectively, importantly including the atmosphere and condensate cloud models, and not just the equation of state. At all times, one is testing a multi-parameter theory, and not a universal radius$-$mass relation.

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