Characterising exoplanet atmospheres using high dispersion spectra

在过去的几年里，测量系外行星光谱的最常见方法是进行差分测量，在多个波段观察凌日（当行星从星星前面经过时）或日食（当行星从星星后面经过时），或者使用低色散光谱仪。另一种方法是使用高色散光谱，解析行星光谱中的各条谱线，并利用行星绕其轨道的轨道运动，使行星的光谱相对于星星和地球大气的透射光谱发生变化。这种方法可以应用于过境和非过境行星，可以允许测量行星的旋转速率，甚至其大气层中的风，并可能在未来十年内用于搜索围绕低质量恒星运行的可居住行星大气层中的生物特征分子（使用超大型地面望远镜）。然而，结果是非常敏感的恒星和大地信号的校正。残余光谱与理论模板交叉相关，这使得能够检测单个分子，但不能测量丰度。Aigrain教授最近开发了一种替代方法来分析这类数据集，该方法涉及直接对星星、大地和行星信号进行建模，使用高斯过程回归对（未知）行星光谱进行建模。学生将开发和应用这种方法，以现有的档案近红外光谱，并在博士学位的过程中，从新的数据（例如）该项目与STFC的关键挑战B保持一致：恒星和行星系统如何发展以及它们如何支持生命的存在。它将使用来自STFC支持的设施，特别是VLT/CRIRES的数据。这项工作将与沃里克大学的Matteo Brogi博士和阿姆斯特丹大学的Jayne Birkby教授合作完成。

For the last few years most common way to measure a spectrum of an exoplanets is to make a differential measurement, observing a transit (when the planet passes in front of a star) or eclipse (when the planet passes behind the star) in multiple bands, or with a low-dispersion spectrograph. An alternative approach is to use high dispersion spectroscopy, resolving individual lines in the planet's spectrum, and exploiting the orbital motion of the planet around its orbit, which shifts the planet's spectrum relative to that of the star and to the Earth's atmosphere's transmission spectrum. This method can be applied to transiting and non-transiting planets, can allow the measurement of the planet's rotation rate and even of winds in its atmosphere, and may be used in the next decade to search for biosignature molecules in the atmospheres of habitable planets orbiting low-mass stars (using extremely large, ground-based telescopes). However, the results are critically sensitive to the correction of the stellar and telluric signals. The residual spectrum is the cross-correlated with a theoretical template, which enable a detection of individual molecules but not abundance measurements. Prof Aigrain has recently developed an alternative methodology to analyse this kind of datasets, which involves modelling the star, telluric and planet signals directly, using Gaussian Process regression to model the (unknown) planet spectrum. The student will develop and apply this method to existing archival near-infrared spectra and, during the course of the PhD, to new data from (e.g.) the refurbished CRIRES+ instrument on the VLT.This project is aligned with STFC's key challenge B: How do stars and planetary systems develop and how do they support the existence of life. It will use data from STFC-supported facilities, in particular VLT/CRIRES. The work will be done in collaboration with Dr Matteo Brogi at the University of Warwick and Prof Jayne Birkby at the University of Amsterdam.