A new method for probing the atmospheres of transiting exoplanets
A new method for probing the atmospheres of transiting exoplanets
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探测凌日系外行星大气层的新方法
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发表时间：
2004
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通讯作者：
I. Snellen
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作者：
I. Snellen
虽然哈勃太空望远镜对HD 209458b的大气透射光谱已经非常成功，但到目前为止，还没有从地面探测到大气吸收特征。在这里，我们提出了一种利用罗西特效应探测凌日系外行星大气的新方法，这种方法可能更适合于地面观测。在凌日期间，一颗系外行星依次阻挡来自旋转恒星接近和后退部分的光线，造成人工径向速度摆动。该信号的振幅与过境物体的有效尺寸成正比，这种效应的波长依赖性可以揭示大气吸收特征，与透射光谱学类似。与传统的大气透射光谱学相比，这种方法的优势在于，它不依赖于凌日前后观测的精确光度比较，而是依赖于同一凌日光谱内单个恒星吸收线的相对速度变化。我们使用了超大望远镜上紫外-视觉梯队光谱仪的档案数据集，将该方法应用于HD 209458。在观测到的波长范围内，钠D谱线的罗西特效应幅度比所有其他吸收谱线的加权平均值高1.7 +1.1 -1.2 m s -1，相当于增加了4.3±3% （1.4σ）。这次测量的不确定度与传统大气透射光谱的光度精度5 × 10 -4相比，比以前使用地面望远镜的尝试高出一个数量级。为这种方法专门设计的观测结果可以进一步提高2-3倍的精度。
Although atmospheric transmission spectroscopy of HD 209458b with the Hubble Space Telescope has been very successful, so far no atmospheric absorption features have been detected from the ground. Here we present a new method for probing the atmospheres of transiting exoplanets which may be more suitable for ground-based observations, making use of the Rossiter effect. During a transit, an exoplanet sequentially blocks off light from the approaching and receding parts of the rotating star, causing an artificial radial velocity wobble. The amplitude of this signal is directly proportional to the effective size of the transiting object, and the wavelength dependence of this effect can reveal atmospheric absorption features, in a similar way as with transmission spectroscopy. The advantage of this method over conventional atmospheric transmission spectroscopy is that it does not rely on accurate photometric comparisons of observations on and off transit, but instead depends on the relative velocity shifts of individual stellar absorption lines within the same on-transit spectra. We used an archival data set from the UV-Visual Echelle Spectrograph on the Very Large Telescope to apply this method to HD 209458. The amplitude of the Rossiter effect is shown to be 1.7 +1.1 -1.2 m s -1 higher in the Sodium D lines than in the weighted average of all other absorption lines in the observed wavelength range, corresponding to an increment of 4.3 ± 3 per cent (1.4σ). The uncertainty in this measurement compares to a photometric accuracy of 5 x 10 -4 for conventional atmospheric transmission spectroscopy, more than an order of magnitude higher than previous attempts using ground-based telescopes. Observations specifically designed for this method could increase the accuracy further by a factor of 2-3.