Origin of massive high-velocity stars in the halo

在2005年偶然发现超高速恒星12年后，超高速恒星(HSS)比以往任何时候都更是一个热门话题，人们对现在开始的盖亚时代的兴趣将会增强。这些天体是晚期的B型恒星，在我们的银河系中以非常高的速度运动，其中大多数甚至在引力作用下与我们的银河系无关。最初，与银河系中心(GC)超大质量黑洞的动力学相互作用是唯一被认为是它们的来源。然而，与此同时，越来越多的大质量高速恒星被发现，它们并非起源于GC，因此对这一图景提出了挑战。这些恒星是所谓的逃逸恒星的极端例子，逃逸恒星是一组众所周知的运动学上特殊的恒星，可以通过星团中的动力学相互作用或破坏双星系统的超新星爆炸被抛出环境。银河系外起源于大麦哲伦星云或瓦解的矮卫星星系也被认为是可能的。我们在过去几年的研究揭示了逃逸恒星和HSS之间的密切联系。因此，试图理解一类恒星需要理解另一类恒星。因此，我们已经将我们的研究项目扩展到了广泛的高速恒星。该项目的目标是了解大规模失控和高压气体的来源和性质。这需要结合定量光谱分析和天文/运动学分析。我们团队改进的模式大气和新的光谱诊断工具将被应用于高保真光学光谱，部分已经在手头。盖亚天体测量将是该项目的支柱。例如，这些数据可以用来改进银河系质量模型，并追溯恒星的轨迹到它们的发源地。此外，还可以建立高速恒星的完整样本来测试种群合成模型。原产地以及抛射机制将通过化学标记结合盖亚天体测量的信息来确定。

Twelve years after their serendipitous discovery in 2005, hyper-velocity stars (HVSs) are more than ever a hot topic and interest will be boosted in the Gaia era which started right now. These objects are late B-type stars that move with very high velocities through our Galaxy, and most of them are even gravitationally unbound to our Milky Way. Initially, the dynamical interaction with the supermassive black hole in the Galactic centre (GC) was their only suggested origin. In the meantime, however, more and more massive high-velocity stars were found that did not originate in the GC and thus challenged this picture. Those stars are extreme cases of so-called runaway stars, a well-known group of kinematically peculiar stars which can be ejected from their environment by dynamical interactions in star clusters or by a supernova explosion disrupting a binary system. An extragalactic origin from the Large Magellanic Cloud or disrupted dwarf satellite galaxies is also considered possible. Our work during the past couple of years has revealed strong links between runaway stars and HVSs. Consequently, trying to understand the one class of stars requires to understand the other class as well. Therefore, we had extended our research projects to a broad range of high-velocity stars. The goal of this project is to understand the origin and nature of massive runaway and HVSs. This requires a combination of quantitative spectroscopic and astrometric/kinematic analyses. Improved model atmospheres and novel spectral diagnostic tools derived in our team shall be applied to high fidelity optical spectra, partly already at hand. Gaia astrometry will be the backbone of the project. These data can be utilized, for example, to improve Galactic mass models and to trace back stellar trajectories to their places of origin. Furthermore a complete sample of high-velocity stars can be established to test population synthesis models. The places of origin as well as the ejection mechanisms will be identified from chemical tagging combined with the information from Gaia astrometry.