The large-scale magnetic field and poleward mass accretion of the classical T Tauri star TW Hya
The large-scale magnetic field and poleward mass accretion of the classical T Tauri star TW Hya
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DOI：
10.1111/j.1365-2966.2011.19288.x
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发表时间：
2011-06
影响因子：
4.8
通讯作者：
J. Donati;S. Gregory;S. Alencar;J. Bouvier;G. Hussain;M. Skelly;C. Dougados;M. Jardine;F. Ménard;M. Romanova;Y. Unruh;the MaPP collaboration
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文献类型：
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作者：
J. Donati;S. Gregory;S. Alencar;J. Bouvier;G. Hussain;M. Skelly;C. Dougados;M. Jardine;F. Ménard;M. Romanova;Y. Unruh;the MaPP collaboration
我们在这里报告的6800万年的经典金牛座T星星（cTTS）TW Hya的分光偏振观测结果进行了ESPaDOns在加拿大-法国-夏威夷望远镜的框架下的“磁性原恒星和行星”计划，并在两个不同的时代（2008年3月和2010年3月）。无论是在光球线还是在吸积动力发射线中，都能随时检测到明显的塞曼特征。在光球和吸积代理中观察到显著的固有变率和适度的旋转调制。使用层析成像，我们重建的大规模领域的光球亮度和吸积供电的发射TW Hya的表面在这两个时代的地图。我们发现，磁拓扑大多是极向的和轴对称的相对于旋转轴的星星和八极分量的大规模的字段（2.5-2.8千克在极点）在很大程度上占主导地位的偶极分量。这种大规模场拓扑结构是部分对流恒星的特征，支持TW Hya已经拥有辐射核心的结论（来自进化模型）。我们还表明，TW Hya具有高纬度的光球冷点重叠的主磁极（并产生所观察到的径向速度波动），这也是吸积集中的大部分时间，虽然吸积在低纬度发现发生偶发事件。我们认为TW Hya相对快速的旋转（相对于AA Tau类cTTSS）直接反映了大尺度偶极子的弱点，不再能够在共转半径（开普勒周期等于恒星旋转周期）内磁性破坏吸积盘。因此，我们得出结论，TW Hya是在一个快速自旋的阶段，其大规模的偶极子场逐渐消失。
We report here results of spectropolarimetric observations of the ≃8 Myr classical T Tauri star (cTTS) TW Hya carried out with ESPaDOnS at the Canada–France–Hawaii Telescope in the framework of the ‘Magnetic Protostars and Planets’ programme, and obtained at two different epochs (2008 March and 2010 March). Obvious Zeeman signatures are detected at all times, both in photospheric lines and in accretion-powered emission lines. Significant intrinsic variability and moderate rotational modulation are observed in both photospheric and accretion proxies. Using tomographic imaging, we reconstruct maps of the large-scale field of the photospheric brightness and the accretion-powered emission at the surface of TW Hya at both epochs. We find that the magnetic topology is mostly poloidal and axisymmetric with respect to the rotation axis of the star and that the octupolar component of the large-scale field (2.5–2.8 kG at the pole) largely dominates the dipolar component. This large-scale field topology is characteristic of partly convective stars, supporting the conclusion (from evolutionary models) that TW Hya already hosts a radiative core. We also show that TW Hya features a high-latitude photospheric cool spot overlapping with the main magnetic pole (and producing the observed radial velocity fluctuations); this is also where accretion concentrates most of the time, although accretion at lower latitudes is found to occur episodically. We propose that the relatively rapid rotation of TW Hya (with respect to AA Tau-like cTTSs) directly reflects the weakness of the large-scale dipole, no longer capable of magnetically disrupting the accretion disc up to the corotation radius (at which the Keplerian period equals the stellar rotation period). We therefore conclude that TW Hya is in a phase of rapid spin-up as its large-scale dipole field progressively vanishes.