来自年轻恒星和超新星的磁离心驱动流。VI。多极恒星场的吸积
Magnetocentrifugally Driven Flows from Young Stars and Disks. VI. Accretion with a Multipole Stellar Field
摘要：
以前的分析磁层吸积和流出的经典金牛座T星（CTTS），在X风模型和其他理论方案的背景下，假设一个偶极几何的恒星磁场，如果它不存在的吸积，导电盘的扰动。然而，CTTS的调查显示，吸积热点覆盖的恒星表面的一小部分，并在恒星表面的净场极化是小的。这两个事实都意味着恒星产生的磁场具有复杂的非偶极结构。为了解决理论和观测之间的这种差异，我们重新审视没有偶极子约束的X风理论。使用简单的物理参数的基础上的概念，被困通量，我们表明，偶极子配置实际上是不必要的。与恒星磁层的精确几何形状无关，对一定水平的捕获通量的要求预示着各种CTTS观测值之间的明确关系。此外，多极恒星场的叠加自然会产生小的热点覆盖率和小的净表面极化。广义的X-风的图片仍然是可行的，在这些条件下，从一个小环附近的内部磁盘边缘的流出几乎不受修改后的几何形状，但与流入高度依赖于如何紧急恒星通量的细节是连接和捕获的内部磁盘区域。我们的模型是一致的数据，包括最近的分光偏振测量的热点大小和场强在V2129 OPH和BP头。
英文摘要：
Previous analyses of magnetospheric accretion and outflow in classical T Tauri stars (CTTSs), within the context of both the X-wind model and other theoretical scenarios, have assumed a dipolar geometry for the stellar magnetic field if it were not perturbed by the presence of an accreting, electrically conducting disk. However, CTTS surveys reveal that accretion hot spots cover a small fraction of the stellar surface, and that the net field polarization on the stellar surface is small. Both facts imply that the magnetic field generated by the star has a complex non-dipolar structure. To address this discrepancy between theory and observations, we re-examine X-wind theory without the dipole constraint. Using simple physical arguments based on the concept of trapped flux, we show that a dipole configuration is in fact not essential. Independent of the precise geometry of the stellar magnetosphere, the requirement for a certain level of trapped flux predicts a definite relationship among various CTTS observables. Moreover, superposition of multipole stellar fields naturally yield small observed hot-spot covering fractions and small net surface polarizations. The generalized X-wind picture remains viable under these conditions, with the outflow from a small annulus near the inner disk edge little affected by the modified geometry, but with inflow highly dependent on the details of how the emergent stellar flux is linked and trapped by the inner disk regions. Our model is consistent with data, including recent spectropolarimetric measurements of the hot spot sizes and field strengths in V2129 Oph and BP Tau.
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