T Tauri星TW Hydrae的SPIRou光谱偏振观测：磁场、吸积与行星
SPIRou spectropolarimetry of the T Tauri star TW Hydrae: magnetic fields, accretion and planets
AI总结：
本文利用SPIRou对T Tauri星TW Hya进行四年光谱偏振观测，通过塞曼-多普勒成像揭示其大尺度偶极磁场与小尺度强磁场结构，并发现一个可能来自内盘结构或近距候选行星的8.3天径向速度信号。
摘要：
本文报告了2019、2020、2021和2022年利用加拿大-法国-夏威夷望远镜3.6米镜上的SPIRou高分辨率光谱偏振仪和测速仪对经典T Tauri星TW Hya进行的近红外观测。通过对圆偏振光谱应用最小二乘解卷积（LSD），我们推导出逐年变化的纵向磁场，范围从-200到+100 G，尽管该星几乎以极向视角被观测，仍在其3.6天自转周期上表现出低幅度调制。随后我们利用塞曼-多普勒成像将四个季节的未偏振和圆偏振LSD轮廓反演为TW Hya的亮度图和磁图，得到该T Tauri星的大尺度磁场主要由一个强度为1.0-1.2 kG、相对自转轴倾斜约20°的偶极子组成，而小尺度磁场强度可达3-4 kG。我们发现大尺度磁场足够强，使TW Hya能够以大致几何稳定的吸积模式从盘面吸积物质到恒星表面的极区，但不足以使恒星自转减速。我们还报告了在TW Hya光谱中发现的一个半振幅为$11.1^{+3.3}_{-2.6}$ m/s的径向速度信号（在8.3天周期上以4.3σ置信度检测到），其起源可能归因于内吸积盘中的非轴对称密度结构，或一个$0.55^{+0.17}_{-0.13}$木星质量的近距候选行星（若在盘面内运行），轨道距离为$0.075\pm0.001$ au。
英文摘要：
In this paper we report near-infrared observations of the classical T Tauri star TW Hya with the SPIRou high-resolution spectropolarimeter and velocimeter at the 3.6-m Canada-France-Hawaii Telescope in 2019, 2020, 2021 and 2022. By applying Least-Squares Deconvolution (LSD) to our circularly polarized spectra, we derived longitudinal fields that vary from year to year from -200 to +100 G, and exhibit low-level modulation on the 3.6 d rotation period of TW Hya, despite the star being viewed almost pole-on. We then used Zeeman-Doppler Imaging to invert our sets of unpolarized and circularly-polarized LSD profiles into brightness and magnetic maps of TW Hya in all 4 seasons, and obtain that the large-scale field of this T Tauri star mainly consists of a 1.0-1.2 kG dipole tilted at about 20° to the rotation axis, whereas the small-scale field reaches strengths of up to 3-4 kG. We find that the large-scale field is strong enough to allow TW Hya to accrete material from the disc on the polar regions at the stellar surface in a more or less geometrically stable accretion pattern, but not to succeed in spinning down the star. We also report the discovery of a radial velocity signal of semi-amplitude $11.1^{+3.3}_{-2.6}$ m/s (detected at 4.3$σ$ at a period of 8.3 d in the spectrum of TW Hya, whose origin may be attributed to either a non-axisymmetric density structure in the inner accretion disc, or to a $0.55^{+0.17}_{-0.13}$ Jupiter mass candidate close-in planet (if orbiting in the disc plane), at an orbital distance of $0.075\pm0.001$ au.
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