驱动 EX Lupi 磁层吸积过程的数千高斯磁场
A multi-kiloGauss magnetic field driving the magnetospheric accretion process in EX Lupi
AI总结：
本研究通过分光偏振观测与 Zeeman-Doppler Imaging 等方法分析 EX Lupi 宁静期磁场，发现数千高斯强场截断星盘并支持磁层间歇性吸积机制。
摘要：
EX Lupi 是 EX Lup 型恒星的原型，这类恒星是指经典金牛 T 型星（cTTSs），会表现出持续数月至数年、幅度为 1 至 5 个星等的光度爆发和耀发。这些事件被归因于可重复发生的间歇性吸积，但其物理机制仍存在争议。在本工作中，我们旨在研究 EX Lup 在宁静期的磁驱动吸积，并首次同时考察小尺度和大尺度磁场。这使我们能够完整表征该系统的磁层吸积过程。我们使用了在 EX Lup 宁静阶段获得的分光偏振时间序列：2016 年和 2019 年利用 Echelle SpectroPolarimetric Device for the Observation of Stars 获取，以及 2019 年在 Canada-France-Hawaii 望远镜上利用 SpectroPolarimètre InfraRouge 获取。因此，我们能够沿不同历元和波长范围，对径向速度、发射线以及表面平均纵向磁场进行变率分析。我们还利用光球谱线的 Zeeman intensification 开展小尺度磁场分析，并利用 Zeeman-Doppler Imaging 重建大尺度磁场拓扑。研究表明，EX Lup 上正在进行典型的磁层吸积：一条主吸积漏斗流以稳定方式连接内盘与恒星，并在靠近磁偶极分量磁极的恒星表面产生吸积激波。我们还测得了 cTTSs 中迄今观测到的最强磁场之一。如此强的磁场表明，星盘在靠近但位于共转半径之外的位置被磁场截断；在共转半径处，星盘角速度等于恒星角速度。这种构型适合产生磁致星盘不稳定性，从而导致向恒星的间歇性吸积。
英文摘要：
EX Lupi is the prototype of EX Lup-type stars, meaning classical T Tauri stars (cTTSs) showing luminosity bursts and outbursts of 1 to 5 magnitudes lasting for a few months to a few years. These events are ascribed to an episodic accretion that can occur repeatedly but whose physical mechanism is still debated. In this work, we aim to investigate the magnetically-driven accretion of EX Lup in quiescence, including for the first time a study of the small and large-scale magnetic field. This allows us to provide a complete characterisation of the magnetospheric accretion process of the system. We use spectropolarimetric times series acquired in 2016 and 2019 with the Echelle SpectroPolarimetric Device for the Observation of Stars and in 2019 with the SpectroPolarimètre InfraRouge at the Canada-France-Hawaii telescope, during a quiescence phase of EX Lup. We were thus able to perform a variability analysis of the radial velocity, the emission lines and surface averaged longitudinal magnetic field along different epochs and wavelength domains. We also provide a small-scale magnetic field analysis using Zeeman intensification of photospheric lines and large-scale magnetic topology reconstruction using Zeeman-Doppler Imaging. Our study reveals a typical magnetospheric accretion ongoing on EX Lup, with a main accretion funnel flow connecting the inner disc to the star in a stable fashion and producing an accretion shock on the stellar surface close to the pole of the magnetic dipole component. We also measure one of the strongest fields ever observed on cTTSs. Such a strong field indicates that the disc is truncated by the magnetic field close but beyond the corotation radius, where the angular velocity of the disc equals the angular velocity of the star. Such a configuration is suitable for a magnetically-induced disc instability yielding episodic accretion onto the star.
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