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arXiv 1202.0548astro-ph.SRastro-ph.EP
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从三维恒星模型大气导出的两颗系外行星宿主星的临边暗化规律

Limb darkening laws for two exoplanet host stars derived from 3D stellar model atmospheres

W. Hayek, D. Sing, F. Pont, M. Asplund

备注：
To be published in Astronomy & Astrophysics (article accepted)
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摘要：

我们比较了两个过境系外行星系统HD 209458和HD 189733的主恒星的3D流体动力学模型大气和1D流体静力学MARCS模型的临边暗化规律。使用3D LTE光谱形成和不透明度采样计算恒星盘的表面亮度分布。我们测试我们的预测使用最小二乘拟合模型的光变曲线与观测到的哈勃太空望远镜（HST）的主要日食。从HD 209458在2900 A和5700 A之间的3D模型导出的临边暗化定律产生了与HST数据的显著更好的拟合，去除了先前基于1D预测的模型光变曲线观察到的系统残差。这种差异主要来自3D模型的较浅的平均温度结构，这是表面颗粒化的显式模拟的结果。在HD 189733的情况下，模型大气在2900 A和5700 A之间产生实际上等效的临边暗化曲线，部分原因是谱线的阻挡，并且数据不足以区分光变曲线。我们还分析了5350 A和10500 A之间的HST观测这颗星的3D模型导致更好的拟合相比，1D预测。HD 209458系统拟合质量的显著改善表明了3D模型的更高真实度以及表面颗粒化对晚型恒星大气辐射场形成的重要性。这一结果与最近对太阳连续谱临边暗化的研究和其他观测试验相吻合。HD 189733的情况并不矛盾，因为模型的光变曲线对大气的温度分层不太敏感，并且在2900 A - 5700 A区域的观测数据不足以更清楚地区分3D和1D临边暗化。

英文摘要：

We compare limb darkening laws derived from 3D hydrodynamical model atmospheres and 1D hydrostatic MARCS models for the host stars of the two transiting exoplanet systems HD 209458 and HD 189733. The surface brightness distribution of the stellar disks is calculated using 3D LTE spectrum formation and opacity sampling. We test our predictions using least-squares fits of model light curves to primary eclipses that were observed with the Hubble Space Telescope (HST). The limb darkening law derived from the 3D model of HD 209458 between 2900 A and 5700 A produces significantly better fits to the HST data, removing systematic residuals that were previously observed for model light curves based on 1D predictions. This difference arises mainly from the shallower mean temperature structure of the 3D model, which is a consequence of the explicit simulation of surface granulation. In the case of HD 189733, the model atmospheres produce practically equivalent limb darkening curves between 2900 A and 5700 A, partly due to obstruction by spectral lines, and the data are not sufficient to distinguish between the light curves. We also analyze HST observations between 5350 A and 10500 A for this star; the 3D model leads to a better fit compared to 1D predictions. The significant improvement of fit quality for the HD 209458 system demonstrates the higher degree of realism of 3D models and the importance of surface granulation for the formation of the atmospheric radiation field of late-type stars. This result agrees well with recent investigations of limb darkening in the solar continuum and other observational tests. The case of HD 189733 is no contradiction as the model light curves are less sensitive to the temperature stratification of the atmosphere and the observed data in the 2900 A - 5700 A region are not sufficient to distinguish more clearly between 3D and 1D limb darkening.

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