The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets
The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets
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DOI：
10.3847/1538-4357/aaa08c
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发表时间：
2017-11
期刊：
影响因子：
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通讯作者：
B. Rackham;D. Apai;M. Giampapa
中科院分区：
文献类型：
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作者：
B. Rackham;D. Apai;M. Giampapa
透射光谱是利用恒星照明探测过境系外行星大气的差分测量。由于星斑和光斑而引起的照明光源和圆盘积分恒星光谱之间的任何光谱差异将被印刻在观测到的透射光谱中。然而，在M矮星的光球不均匀性的程度存在一些限制。在这里，我们的模型斑点和光斑覆盖率与观测到的光度变化的M矮星和相关的0.3 - 5.5 μ m恒星污染光谱一致。我们发现，大范围的斑点和光斑覆盖分数是一致的观测和校正假设的变化幅度之间的线性关系，覆盖分数一般低估了恒星污染。使用对斑点和光斑覆盖分数的现实估计，我们发现恒星污染可能比岩石系外行星大气特征预期的过境深度变化大10倍以上。我们还发现，恒星光谱污染会导致半径的系统误差，因此导出的小行星的密度。在TRAPPIST-1系统的情况下，我们表明，TRAPPIST-1的旋转变化是一致的斑点覆盖分数和光斑覆盖分数。相关的恒星污染信号改变了TRAPPIST-1行星在行星大气物种感兴趣的波长处的过境深度，大约是行星特征强度的1 - 15倍，这使得JWST对该系统的后续观测变得非常复杂。同样，我们发现，恒星污染可能导致低估的TRAPPIST-1行星的体积密度，从而导致高估其挥发性成分。
Transmission spectra are differential measurements that utilize stellar illumination to probe transiting exoplanet atmospheres. Any spectral difference between the illuminating light source and the disk-integrated stellar spectrum due to starspots and faculae will be imprinted in the observed transmission spectrum. However, few constraints exist for the extent of photospheric heterogeneities in M dwarfs. Here we model spot and faculae covering fractions consistent with observed photometric variabilities for M dwarfs and the associated 0.3–5.5 μm stellar contamination spectra. We find that large ranges of spot and faculae covering fractions are consistent with observations and corrections assuming a linear relation between variability amplitude, and covering fractions generally underestimate the stellar contamination. Using realistic estimates for spot and faculae covering fractions, we find that stellar contamination can be more than 10× larger than the transit depth changes expected for atmospheric features in rocky exoplanets. We also find that stellar spectral contamination can lead to systematic errors in radius and therefore the derived density of small planets. In the case of the TRAPPIST-1 system, we show that TRAPPIST-1's rotational variability is consistent with spot covering fractions and faculae covering fractions . The associated stellar contamination signals alter the transit depths of the TRAPPIST-1 planets at wavelengths of interest for planetary atmospheric species by roughly 1–15× the strength of planetary features, significantly complicating JWST follow-up observations of this system. Similarly, we find that stellar contamination can lead to underestimates of the bulk densities of the TRAPPIST-1 planets of , thus leading to overestimates of their volatile contents.