arXiv 2007.077162020-07-15astro-ph.EPastro-ph.IMastro-ph.SR

碰撞参量在系外行星透射光谱学中的作用

Role of the impact parameter in exoplanet transmission spectroscopy

X. Alexoudi, M. Mallonn, E. Keles, K. Poppenhaeger, C. von Essen, K. G. Strassmeier

发表信息：: A&A 640, A134 (2020)
DOI：: 10.1051/0004-6361/202038080

摘要：

透射光谱学是一种很有前途的工具，用于过境系外行星的大气特性。由于行星信号很微弱，关于个别目标的报道存在差异。我们调查的依赖性的估计的透射光谱的恒星行星系统的轨道参数的偏差，是由于边缘变暗的影响的宿主恒星。我们描述了如何将轨道参数的不确定性转化为行星光谱斜率的不确定性。我们在七个不同的波长波段，从近紫外线到近红外线，创建合成的过境光曲线，并将它们与过境模型参数化的固定偏差值的碰撞参量$b$。我们的模拟显示了一个波长依赖的偏移，这是更明显的蓝色波长的边缘变暗的效果更强。该偏移在行星变速箱频谱中引入了一个斜率，该斜率随着$b$值的增加而变陡。$b$在其不确定性区间内的正值或负值的变化引入正斜率或负斜率，从而形成误差包络。对于$b$上的典型不确定性，从蓝光到近红外波长的振幅对应于一个大气压标度高度或更多。这碰撞参量简并确认为不同的主机类型，K星目前显着陡峭的斜率，而M星表示在蓝色波长的功能。我们证明，透射光谱可能很难解释，基本上是因为在定义一个精确的碰撞参量值过境系外行星的限制。因此，这限制了对其大气层的定性。

英文摘要：

Transmission spectroscopy is a promising tool for the atmospheric characterization of transiting exoplanets. Because the planetary signal is faint, discrepancies have been reported regarding individual targets. We investigate the dependence of the estimated transmission spectrum on deviations of the orbital parameters of the star-planet system that are due to the limb-darkening effects of the host star. We describe how the uncertainty on the orbital parameters translates into an uncertainty on the planetary spectral slope. We created synthetic transit light curves in seven different wavelength bands, from the near-ultraviolet to the near-infrared, and fit them with transit models parameterized by fixed deviating values of the impact parameter $b$. Our simulations show a wavelength-dependent offset that is more pronounced at the blue wavelengths where the limb-darkening effect is stronger. This offset introduces a slope in the planetary transmission spectrum that becomes steeper with increasing $b$ values. Variations of $b$ by positive or negative values within its uncertainty interval introduce positive or negative slopes, thus the formation of an error envelope. The amplitude from blue optical to near-infrared wavelength for a typical uncertainty on $b$ corresponds to one atmospheric pressure scale height and more. This impact parameter degeneracy is confirmed for different host types; K stars present prominently steeper slopes, while M stars indicate features at the blue wavelengths. We demonstrate that transmission spectra can be hard to interpret, basically because of the limitations in defining a precise impact parameter value for a transiting exoplanet. This consequently limits a characterization of its atmosphere.

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