exocartographer: A Bayesian Framework for Mapping Exoplanets in Reflected Light
exocartographer: A Bayesian Framework for Mapping Exoplanets in Reflected Light
复制标题
exocartographer：反射光下绘制系外行星的贝叶斯框架
DOI：
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发表时间：
2018
影响因子：
5.3
通讯作者：
T. Robinson
中科院分区：
文献类型：
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作者：
B. Farr;W. Farr;N. Cowan;Hal M. Haggard;T. Robinson
未来的太空望远镜将在可见光波段直接拍摄太阳系外行星的图像。来自系外行星的时间分辨反射光编码了关于大气和表面不均匀性的信息。以前的研究表明，系外行星的光变曲线可以被反转，以获得行星的低分辨率地图，以及对它的自旋方向的限制。到目前为止，估计2D地图上的不确定性仍然是难以捉摸的。在这里，我们提出exocartographer，一个灵活的开源贝叶斯框架解决exocartography逆问题。该图使用等面积层次、等面积和等纬度像素（HEALPix）像素进行参数化。对于192像素的基准地图分辨率，描述了一个四参数高斯过程的空间尺度的行星自转变化，和两个未知的行星自转参数，exocartographer探索了一个198维的参数空间。为了测试代码，我们生成了一个无云地球在90°垂直轨道上的光变曲线。我们对这颗行星进行合成白光观测：在整个行星轨道上进行五个观测时期，每个时期包括24小时的观测，光度不确定性为1%（120个数据点）。我们检索到的一个Pendado地图和-第一次-它的不确定性，沿着自旋约束。这张地图可以识别出地球，典型的90%不确定性为0.14。反演的特征长度尺度为9800 km。在90%可信水平下，回收率>87.尽管检索到的Escodo地图的不确定性，我们鲁棒地确定了一个高Escodo区域（撒哈拉沙漠）和一个大的低Escodo区域（太平洋）。
Future space telescopes will directly image extrasolar planets at visible wavelengths. Time-resolved reflected light from an exoplanet encodes information about atmospheric and surface inhomogeneities. Previous research has shown that the light curve of an exoplanet can be inverted to obtain a low-resolution map of the planet, as well as constraints on its spin orientation. Estimating the uncertainty on 2D albedo maps has so far remained elusive. Here, we present exocartographer, a flexible open-source Bayesian framework for solving the exocartography inverse problem. The map is parameterized with equal-area Hierarchical, Equal Area, and isoLatitude Pixelation (HEALPix) pixels. For a fiducial map resolution of 192 pixels, a four-parameter Gaussian process describing the spatial scale of albedo variations, and two unknown planetary spin parameters, exocartographer explores a 198-dimensional parameter space. To test the code, we produce a light curve for a cloudless Earth in a face-on orbit with a 90° obliquity. We produce synthetic white-light observations of the planet: five epochs of observations throughout the planet’s orbit, each consisting of 24 hourly observations with a photometric uncertainty of 1% (120 data points). We retrieve an albedo map and—for the first time—its uncertainties, along with spin constraints. The albedo map is recognizably of Earth, with a typical 90% uncertainty of 0.14. The retrieved characteristic length scale is ∼9800 km. The obliquity is recovered to be >87.°9 at the 90% credible level. Despite the uncertainty in the retrieved albedo map, we robustly identify a high-albedo region (the Sahara desert) and a large low-albedo region (the Pacific Ocean).