Understanding Stellar Variability and Improving Exoplanet Characterisation

几乎我们所知道的关于系外行星的一切，那些在我们太阳系之外的行星，都来自于它们所围绕的恒星。这是因为，就像我们的太阳相对于地球一样，主恒星的亮度远远超过任何伴星。因此，宿主恒星表面的不均匀性和可变性会显著削弱探测和识别系外行星的能力;对于温带岩石行星来说尤其如此，因为它们的信号本来就很小。第一颗围绕类太阳星星运行的系外行星是通过径向速度技术发现的（Mayor & Queloz 1995），即测量行星围绕共同质心运行时在宿主星星中引起的多普勒反射运动。自从这一发现以来，我们已经走过了漫长的道路，探测了数千颗系外行星，并显着提高了我们仪器的精度。然而，我们还没有找到一个真正的类似于我们的地球，唯一已知的支持生命的行星。多亏了像VLT上的ESTERO摄谱仪这样的仪器，我们现在有技术能力探测类太阳恒星周围的岩石，温带行星，但这仍然是一个巨大的挑战。也许通往可居住的外星世界的道路上最大的绊脚石来自宿主恒星本身。系外行星宿主恒星表面的不均匀性可能会产生虚假的径向速度信号，这些信号可能会掩盖甚至模仿系外行星的多普勒摆动。这是因为恒星表面的等离子体正在移动，从而发出自己的多普勒频移，但也因为磁场集中可以抑制这些运动并改变这些区域表面的亮度。此外，来自太阳（和类太阳恒星）表面的变化比地球引起的~9 cm/s多普勒摆动大几个数量级。这个项目的重点是提高我们的恒星物理学知识，使我们可以解开恒星和行星的信号，使未来的确认和表征岩石，温带世界周围的太阳类恒星。几乎所有的恒星变化都是由对流和磁场之间的相互作用驱动的;因此，了解这种相互作用是揭示低质量，长周期行星的关键。在我们的太阳表面，热的气泡（称为颗粒）上升到表面，冷却，然后回落到星星中（在粒间通道内）;上升的运动引起多普勒频移，朝向光谱的蓝端，而下降的运动使光向光谱的红端移动。这将不对称性引入到用于测量多普勒频移的恒星反射线中，以及几十厘米/秒到几米/秒的净频移，这取决于与磁场的相互作用。该项目将使用最先进的3D磁流体动力学（MHD）模拟来创建迄今为止最真实的恒星模拟。创建一套恒星模型将使我们能够研究对流在不同磁场强度下是如何改变的，以及它如何在许多不同的（典型的）恒星吸收线中留下印记。目标是开发一个工具包，以识别和减轻系外行星数据中的恒星变化，从而进一步推动我们的探测水平，并帮助打开一条确认和描述类地外星世界的途径。

Almost everything we know about exoplanets, those planets outside of our solar system, is derived from the stars they orbit. This is because, like our Sun relative to the Earth, the host stars vastly outshine any companion planets. As a result, inhomogeneities and variability on the surface of the host stars can significantly impair the ability to detect and characterise exoplanets; this is especially true for temperate, rocky planets as their signals are inherently small. The first exoplanet around a Sun-like star was discovered through the radial velocity technique (Mayor & Queloz 1995), i.e. measuring the Doppler reflex motion induced in the host star by the planet as they orbit about their common centre of mass. Since this discovery we have come a long ways, detecting thousands of exoplanets and significantly improving the precision of our instruments. However, we are yet to find a true analogue to our Earth, the only known planet to support life. Thanks to instruments like the ESPRESSO spectrograph on the VLT, we now are within the technological capabilities to detect rocky, temperate planets around Sun-like stars, yet this remains an enormous challenge. Perhaps the biggest stumbling block on the pathway to habitable alien worlds originates from the host stars themselves.Inhomogeneities on the surfaces of the exoplanet host stars can produce spurious radial velocity signals that may mask or even mimic the Doppler wobble of an exoplanet. This happens both because the plasma on the stellar surface is moving, and thus emitting its own Doppler shifts, but also because magnetic field concentrations can suppress these motions and alter brightness of the surface in these regions. Moreover, the variability originating from the surface of the Sun (and Sun-like stars) is orders of magnitude larger than the ~9 cm/s Doppler wobble induced by the Earth. This project focuses on improving our knowledge of stellar physics so that we may disentangle stellar and planetary signals to enable the future confirmation and characterisation of rocky, temperate worlds around Sun-like stars. Almost all stellar variability is driven by the interplay between convection and magnetic fields; hence, understanding this interplay is key to unveiling low-mass, long-period planets. On the surface of our Sun, hot bubble of gas (known as granules) rise to the surface, cool, and fall back down into the star (within intergranular lanes); the uprising motions give rise to Doppler shifts towards the bluer end of the spectrum, while the falling motions shift the light towards the redder end of the spectrum. This introduces asymmetries into the stellar absorptions lines used to measure Doppler shifts, as well as net shifts of several 10s of cm/s to a several m/s, depending the interplay with the magnetic field. This project will use state-of-the-art 3D magnetohydrodynamical (MHD) simulations to create the most realistic stellar simulations to date. Creating a suite of stellar models will allow us to study how the convection is altered at various magnetic field strengths and how this imprints itself in a number of different (typical) stellar absorption lines. The goal is to develop a toolkit to identify and mitigate stellar variability in exoplanet data, thereby pushing our detections levels even further and helping to open a pathway to the confirmation and characterisation of Earth-like alien worlds.