Modelling the 3D high-resolution spectra of hot Jupiters

在过去的十年里，通过大型地面望远镜可以获得系外行星大气的高分辨率光谱。诸如夏威夷凯克望远镜上的NIRSPEC或智利超大望远镜上的CRIRES等仪器已经能够分辨出钠等原子和水、一氧化碳等分子的单独谱线。这些观测不仅可以检测和量化分子，而且还提供了对系外行星大气风速的第一次直接测量。随着十多台新仪器即将在世界范围内的大型望远镜中升空，其中包括甚大望远镜的ESPRESSO和Calar Alto的CARMENES已经开始观测，加拿大、法国、夏威夷望远镜的SPIRou也将很快开始观测，高分辨率的系外行星光谱将被高精度地常规测量。未来，欧洲超大望远镜上的METIS等高分辨率仪器将成为我们探测其他星球生物特征的最佳机会。尽管有这些令人兴奋的前景，但解释这些观察结果的工作还很少。我们从哈勃、斯皮策或詹姆斯·韦伯太空望远镜等太空望远镜的低分辨率光谱学中了解到，系外行星大气是复杂的物体，尤其是热木星，它是当前观测的主要目标。这些行星的大小和木星差不多，但它们的轨道比水星离恒星近得多。它们被潮汐锁定，白天的一面被加热到几千度，而夜晚的一面却看不到恒星。强大的通常是超音速的风将能量从白天的一面转移到夜晚的一面。太阳的白天面温度高到足以使分子分裂，而太阳的夜晚面则显示出由硅酸盐等岩石物质构成的云的迹象。目前使用一维大气模式解释高分辨率光谱观测结果。虽然这些可以用于大气风速和当前观测的丰度，但它们可能偏向于错误的结果。新的更精确的信息，比如最近由ESPRESSO仪器获得的信息，可以在凌日期间分辨出行星的边缘，显示出清晰的三维不对称迹象。在这个项目中，我们将使用最先进的热系外行星三维全球环流模型（SPARC/MITgcm）来研究三维热和风结构对高分辨率观测的影响。该动力学模型可以预测大范围系外行星复杂的热化学结构、云空间分布和风的模式。这些输出将被MCRT使用，这是一种最先进的3D蒙特卡罗辐射传输代码，用于高精度地计算高分辨率光谱学观察到的分子线的形状和位置。学生将确定不同行星参数的观测结果，如平衡温度、风阻、重力、云组成，并重新解释当前的观测结果。我们还将研究在从观测中获取风速和丰度时应使用的最佳三维线形参数化，这将大大提高这些行星大气物种的可探测性。与当地牛津天文台（如天文系的Suzanne Aigrain）合作的观测建议将在建模工作中获得见解后，在世界各地的主要天文台上获得开放时间

In the past ten years high resolution spectrum of exoplanet atmospheres became available from large ground based telescopes. Instruments such as NIRSPEC at the Keck telescope Hawaii or CRIRES at the Very Large Telescope Chile have been able to resolve individual spectral lines of both atoms such as sodium and molecules such as water and carbon monoxide. These observations not only allowed the detection and quantification of molecules but have provided the first direct measurement of an exoplanet atmospheric wind speed. With more than ten new instruments about to come to the sky in large telescope worldwide among them ESPRESSO at the Very Large Telescope and CARMENES at the Calar Alto have begun observing SPIRou at the Canada France Hawaii Telescope should start soon, high resolution spectra of exoplanet will be measured routinely with a high accuracy. Further ahead high resolution instruments such as METIS on the European Extremely Large Telescope will become our best chance to detect biosignatures in other worlds. Despite these exciting prospects, very small work has been done to interpret these observations. We learned from low resolution spectroscopywith space based telescopes such as Hubble or Spitzer or soon the James Webb Space Telescope that exoplanetary atmospheres are complex objects, particularly Hot Jupiters, the main targets of current observations. These planets have the size of Jupiter but orbit much close to their star than mercury. They are tidally locked, with a dayside heated to several thousands of degrees while their nightside never sees the star. Strong often supersonic winds transfer energy from the dayside to the nightside. Whereas the dayside can be hot enough to break molecules apart, the nightside shows signs of clouds made of rocky material such as silicates. High resolution spectroscopy observations are currently interpreted using one-dimensional atmospheric models. Whereas these can be used to atmospheric wind speeds and abundances from current observations they can be biased towards the wrong results. New more precise information such as recent ones by the ESPRESSO instrument can resolve the planetary limb during a transit, showing clear signs of 3D asymetries. For this project we will use a state of the art 3 dimensional global circulation model of hot exoplanets (the SPARC/MITgcm) to investigate the effects of a 3D thermal and wind structure on high-resolution observations. The dynamical model can predict complex thermal and chemical structure, cloud spatial distribution and wind patterns for a wide range of exoplanets. These outputs will be used by MCRT, a state of the art 3D monte carol radiative transfer code to calculate with a high precision the shape and position of the molecular lines observed with high resolution spectrocscopy. The student will determine the observational consequences of varying planetary parameters, such as equilibrium temperature, wind drag, gravity, cloud composition and re interpret current observations. We will also investigate what best 3D line shapes parametrization should be used when retrieving wind speed and abundances from the observations which should greatly enhanced the detectability of atmospheric species in these planets. Observation proposals in collaboration with local Oxford observes (such as Suzanne Aigrain in the astronomy departement) will be written to obtain open time on major observatories around the world following the insights gained from the modelling work