The GTC exoplanet transit spectroscopy survey. X. Stellar spots versus Rayleigh scattering: the case of HAT-P-11b
The GTC exoplanet transit spectroscopy survey. X. Stellar spots versus Rayleigh scattering: the case of HAT-P-11b
复制标题
GTC 系外行星凌日光谱调查。
DOI：
10.1051/0004-6361/201834063
复制
发表时间：
2019
影响因子：
6.5
通讯作者：
Nowak G.
中科院分区：
文献类型：
--
作者：
Murgas F.;Chen G.;Palle E.;Nortmann L.;Nowak G.
上下文。在氢为主的系外行星大气中，瑞利散射可以通过地面或太空望远镜探测到，然而，在观测到的透射光谱中，以斑点形式出现的恒星活动可以模拟瑞利散射。量化这一现象是我们正确解释系外行星大气特性的关键。我们使用10米望远镜加那利大望远镜（GTC）对系外行星进行了地面透射光谱调查，以表征它们的大气特征。在本文中，我们研究了系外行星HAT-P-11b，这是一颗海王星大小的行星，围绕一颗活跃的k型恒星运行。2016年8月30日和2017年9月25日，利用低中分辨率集成光谱光学系统（OSIRIS）获得了HAT-P-11b两次过境的长缝光谱。我们在λ ~ 400-785 nm区域的几个光谱通道中整合了HAT-P-11和一颗参考星的光谱，创建了许多凌日的光曲线。考虑到系统效应和时间序列中存在的红噪声，我们拟合了分析凌日曲线，以测量行星与恒星半径比（Rp/Rs）在波长上的变化。通过将这两次凌日结合在一起，我们发现透射光谱中的斜率表明行星半径向蓝色波长方向增加。仔细观察单个数据集的透射谱，可以发现第一次凌日呈现这种斜率，而第二个数据集的透射谱是平坦的。此外，我们在第一个晚上检测到钠吸收的迹象，但在第二个晚上没有。我们得出结论，在第一次凌日观测中发现的透射光谱斜率和Na吸收过量是由未被遮蔽的恒星黑子引起的。通过模拟未观测光斑的贡献来重现第一夜的结果，我们发现光斑填充因子δ = 0.62 +0.20/-0.17，光斑与光球温差∆T = 429 +184/-299 K。
Context. Rayleigh scattering in a hydrogen-dominated exoplanet atmosphere can be detected from ground or space based telescopes, however, stellar activity in the form of spots can mimic Rayleigh scattering in the observed transmission spectrum. Quantifying this phenomena is key to our correct interpretation of exoplanet atmospheric properties..Aims. We use the 10 m telescope Gran Telescopio Canarias (GTC) to carry out a ground-based transmission spectra survey of extrasolar planets to characterize their atmospheres. In this paper we investigate the exoplanet HAT-P-11b, a Neptune-size planet orbiting an active K-type star..Methods. We obtained long-slit optical spectroscopy of two transits of HAT-P-11b with the Optical System for Imaging and low- Intermediate-Resolution Integrated Spectroscopy (OSIRIS) on August 30 2016 and September 25 2017. We integrated the spectrum of HAT-P-11 and one reference star in several spectroscopic channels across the λ ∼ 400-785 nm region, creating numerous light curves of the transits. We fit analytic transit curves to the data taking into account the systematic effects and red noise present in the time series in an effort to measure the change of the planet-to-star radius ratio (Rp/Rs) across wavelength..Results. By fitting both transits together, we find a slope in the transmission spectrum showing an increase of the planetary radius towards blue wavelengths. A closer inspection to the transmission spectrum of the individual data sets reveals that the first transit presents this slope while the transmission spectrum of the second data set is flat. Additionally we detect hints of Na absorption in the first night, but not in the second. We conclude that the transmission spectrum slope and Na absorption excess found in the first transit observation are caused by unocculted stellar spots. Modeling the contribution of unocculted spots to reproduce the results of the first night we find a spot filling factor of δ = 0.62 +0.20/-0.17 and a spot-to-photosphere temperature difference of ∆T = 429 +184/-299 K.