绘制恒星表面III：一个有效的、可扩展的和开源的多普勒成像模型
Mapping stellar surfaces III: An Efficient, Scalable, and Open-Source Doppler Imaging Model
摘要：
对恒星表面的研究可以揭示恒星的化学组成、内部结构和磁性等信息。这对太阳系外行星的探测和定性也至关重要，特别是那些在极精密径向速度（EPRV）搜索中的目标，这必须与恒星的变化相抗衡，这种变化往往比行星信号强几个数量级。其中一个最成功的方法来映射的恒星表面是多普勒成像，其中存在的不均匀性是推断从微妙的线条形状的变化，在高分辨率恒星光谱。在本文中，我们提出了一种新的，有效的，和封闭形式的解决方案的问题，多普勒成像的恒星表面。我们的模型明确允许不完整的知识的本地（休息帧）恒星光谱，允许一个学习的光谱模板的差异，同时映射恒星表面。因此，它适用于混合谱线，谱线形成机制尚未得到很好理解的光谱区域，或者其斑点具有与光球其余部分本质不同的光谱的恒星。我们在开源星空框架内实现了该模型，使其快速，可区分，易于在优化和后验推理设置中使用。作为概念验证，我们使用我们的模型来推断棕矮星WISE 1049- 5319 B的表面地图，发现与Crossfield等人的解决方案非常一致。我们还讨论了多普勒成像的背景下，EPRV的研究和描述一个可解释的光谱时间高斯过程的恒星光谱的变化，我们预计将是重要的EPRV系外行星搜索。
英文摘要：
The study of stellar surfaces can reveal information about the chemical composition, interior structure, and magnetic properties of stars. It is also critical to the detection and characterization of extrasolar planets, in particular those targeted in extreme precision radial velocity (EPRV) searches, which must contend with stellar variability that is often orders of magnitude stronger than the planetary signal. One of the most successful methods to map the surfaces of stars is Doppler imaging, in which the presence of inhomogeneities is inferred from subtle line shape changes in high resolution stellar spectra. In this paper, we present a novel, efficient, and closed-form solution to the problem of Doppler imaging of stellar surfaces. Our model explicitly allows for incomplete knowledge of the local (rest frame) stellar spectrum, allowing one to learn differences from spectral templates while simultaneously mapping the stellar surface. It therefore works on blended lines, regions of the spectrum where line formation mechanisms are not well understood, or stars whose spots have intrinsically different spectra from the rest of the photosphere. We implement the model within the open source starry framework, making it fast, differentiable, and easy to use in both optimization and posterior inference settings. As a proof-of-concept, we use our model to infer the surface map of the brown dwarf WISE 1049-5319B, finding close agreement with the solution of Crossfield et al. (2014). We also discuss Doppler imaging in the context of EPRV studies and describe an interpretable spectral-temporal Gaussian process for stellar spectral variability that we expect will be important for EPRV exoplanet searches.
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