Understanding signatures of magnetic activity in broadband stellar observations

具有外部对流区的冷恒星表现出各种各样的活动：色球和X射线发射、耀斑、光谱和亮度变化。所有这些现象基本上都是由恒星表面以下出现的磁场驱动的，这些磁场影响着恒星大气的结构。对恒星活动的兴趣不仅限于太阳和恒星天体物理学。例如，恒星亮度变化是用凌日测光法探测和表征系外行星的一个限制因素，其定量评估对即将进行的PLATO使命很重要。径向速度的磁抖动影响行星的光谱探测，而由磁活动引起的恒星摆动会妨碍行星的天体测量探测（例如盖亚空间天文台或预期的TOLIMAN使命）。最近的研究还表明，磁活动可能会干扰用透射光谱法确定系外行星大气的化学成分。在这种背景下，MAGicSTar的主要目标是理解和模拟宽带恒星观测中恒星磁活动的特征，即a）凌日测光，b）宽带凌日光谱，和c）天体测量。直到最近，这种建模的主要障碍是缺乏关于安静恒星区域磁性特征亮度对比的可靠信息。由于恒星大气的3D辐射MHD模拟的进展，这种情况现在已经改变了。特别是，利用申请人所在研究所开发的MURaM代码进行的模拟达到了高度的真实性，非常详细地再现了太阳观测。太阳MURaM模拟的成功启动了将其扩展到太阳以外的恒星的工作，申请人及其东道研究所模拟了具有各种有效温度和金属丰度的恒星的磁特征。这些模拟为MAGICSTar提供了在模拟恒星磁活动宽带特征方面迈出实质性一步所需的关键信息。它们将首先用于对凌日测光任务（如开普勒和TESS）观测到的恒星亮度变化进行建模。在下一步中，恒星亮度变化的模型将被扩展到计算透射光谱和天体测量中的内在恒星信号。MAGICSTar将有利于恒星和系外行星的研究，通过a）提高我们对恒星磁活动的理解; B）使我们更接近于有效地减轻恒星信号，以便更准确地描述系外行星的特征。

Cool stars with outer convection zones show various manifestations of activity: chromospheric and X-ray emission, flares, spectroscopic and brightness variations. All these phenomena are essentially driven by magnetic fields emerging from below the stellar surface and affecting the structure of the stellar atmosphere. The interest in stellar activity is not limited to solar and stellar astrophysics. For example, stellar brightness variability is a limiting factor for detecting and characterising exoplanets with transit photometry, and its quantitative assessment is important for the upcoming PLATO mission. The magnetic jitter in radial velocity affects the spectroscopic detection of planets, while stellar wobbles caused by magnetic activity can impede the astrometric detection of planets (e.g. with the Gaia space observatory or anticipated TOLIMAN mission). Recent studies have also shown that magnetic activity can interfere with identifying the chemical composition of exoplanetary atmospheres with transmission spectroscopy. Quantifying such magnetic contamination of transmission spectroscopy is urgently needed for the interpretation of data from the James Webb Space Telescope In this context, the main goal of MAGicSTar is to understand and model signatures of stellar magnetic activity in broadband stellar observations, namely a) transit photometry, b) broadband transit spectroscopy, and c) astrometry. Until recently, the main hurdle in such modelling was the absence of reliable information about the brightness contrasts of magnetic features with respect to the quiet stellar regions. The situation has now changed due to the progress in 3D radiative-MHD simulations of stellar atmospheres. In particular, simulations with the MURaM code developed in the host institute of the applicant have reached a high degree of realism reproducing solar observations in great detail. The success of solar MURaM simulations initiated their extension to stars other than the Sun with the applicant and the applicant’s host institute simulating magnetic features on stars with various effective temperatures and metallicities. These simulations provide MAGicSTar with a key information needed for making a substantial step forward in modelling broadband signatures of stellar magnetic activity. They will first be utilized for modelling of stellar brightness variations observed by transit photometry missions (such as Kepler and TESS). In the next step, the model of stellar brightness variability will be extended to also calculate intrinsic stellar signals in transmission spectra and astrometric measurements. MAGicSTar will benefit both stellar and exoplanetary research by a) boosting our understanding of stellar magnetic activity; b) bringing us closer to effectively mitigating stellar signal in observations needed for a more accurate characterisation of exoplanets.