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arXiv 1709.07385astro-ph.EP
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太阳系外行星Hubble/WFC3 Transit光谱的统计分析

Statistical Analysis of Hubble/WFC3 Transit Spectroscopy of Extrasolar Planets

Guangwei Fu, Drake Deming, Heather Knutson, Nikku Madhusudhan, Avi Mandell, Jonathan Fraine

摘要:

透射光谱学为研究系外行星大气提供了一个窗口,但这个窗口被云层和薄雾所笼罩。云和霾在气体吸收特征的强度和行星的物理参数(如丰度)之间引入了简并。打破这种简并性的一种方法是通过统计研究。收集了HST/WFC 3在1.1-1.65 $μ$ m水蒸气吸收的所有凌日光谱,并对水吸收特征与行星参数之间的潜在相关性进行了统计研究。我们用Exo-Transmit代码为每个行星计算的模板拟合观测到的光谱。我们表示的规模高度的吸水量,从而消除了已知的依赖于温度,表面重力,和平均分子量。我们发现,在尺度高度的吸收有一个积极的基线与行星平衡温度的相关性,我们的假设是,随着温度的升高,云冷凝减少是负责这个基线斜率。然而,所观察到的样品也本质上是简并的,因为平衡温度与行星质量相关。我们编译的吸收分布的规模高度,我们发现,这个分布是更接近对数正态比高斯。然而,我们也发现,所观察到的行星的平衡温度的分布是类似的对数正态分布。这表明吸收值受到观测偏差的影响,观测者尚未瞄准足够的最热行星样本。

英文摘要:

Transmission spectroscopy provides a window to study exoplanetary atmospheres, but that window is fogged by clouds and hazes. Clouds and haze introduce a degeneracy between the strength of gaseous absorption features and planetary physical parameters such as abundances. One way to break that degeneracy is via statistical studies. We collect all published HST/WFC3 transit spectra for 1.1-1.65 $μ$m water vapor absorption, and perform a statistical study on potential correlations between the water absorption feature and planetary parameters. We fit the observed spectra with a template calculated for each planet using the Exo-Transmit code. We express the magnitude of the water absorption in scale heights, thereby removing the known dependence on temperature, surface gravity, and mean molecular weight. We find that the absorption in scale heights has a positive baseline correlation with planetary equilibrium temperature; our hypothesis is that decreasing cloud condensation with increasing temperature is responsible for this baseline slope. However, the observed sample is also intrinsically degenerate in the sense that equilibrium temperature correlates with planetary mass. We compile the distribution of absorption in scale heights, and we find that this distribution is closer to log-normal than Gaussian. However, we also find that the distribution of equilibrium temperatures for the observed planets is similarly log-normal. This indicates that the absorption values are affected by observational bias, whereby observers have not yet targeted a sufficient sample of the hottest planets.

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