X-Ray Ionization of the Disks of Young Stellar Objects
X-Ray Ionization of the Disks of Young Stellar Objects
复制标题
年轻恒星体盘的 X 射线电离
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
A. Glassgold
中科院分区：
文献类型：
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作者：
J. Igea;A. Glassgold
我们已经开发了一个蒙特卡罗程序的恒星X射线在轴对称磁盘的传输。该代码处理康普顿散射和光电吸收，并遵循X射线，直到它们被完全吸收。我们确认，硬X射线从一个低质量的年轻恒星物体（YSO）穿透相关的吸积盘。即使没有被YSO风强烈衰减的低能光子，吸积盘内部区域（<1 Au）的电离率也比银河宇宙线的标准电离率大许多个数量级。在离源固定的半径处，X射线电离率是垂直柱密度的普适函数，与盘的结构细节无关。电离率与X射线光度成比例，并且仅轻微地取决于X射线温度，至少对于与低质量YSO相关的温度。因此，来自YSO的X射线可以探测到吸积盘中排除低能宇宙射线的区域，例如，是由恒星风的作用造成的使用一个简单的理论的电子分数，我们估计，对于最小的太阳星云，X射线的盘超过5 Au的水平足以耦合磁场和中性盘材料。在这个半径内，X射线对垂直柱密度远大于~1025 cm-2的情况是无效的，因此磁盘的内部区域将与磁场解耦。如果盘吸积是介导的MHD湍流，由Balbus和Hawley提出的，那么我们的结果表明，分层吸积发生在内部区域的磁盘电离的X射线，在雅阁Gammie的建议的基础上宇宙射线电离。
We have developed a Monte Carlo code for the transport of stellar X-rays in an axially symmetric disk. The code treats Compton scattering and photoelectric absorption and follows the X-rays until they are completely absorbed. We confirm that hard X-rays from a low-mass young stellar object (YSO) penetrate the associated accretion disk. Even without the low-energy photons that are strongly attenuated by the YSO wind, the ionization rate in the inner region of the accretion disk (<1 AU) is many orders of magnitude larger than the standard ionization rate due to Galactic cosmic rays. At a fixed radius from the source, the X-ray ionization rate is a universal function of the vertical column density, independent of the structural details of the disk. The ionization rate scales with the X-ray luminosity and depends only mildly on the X-ray temperature, at least for the temperatures relevant for low-mass YSOs. Thus X-rays from a YSO can ionize regions of an accretion disk from which low-energy cosmic rays are excluded, e.g., by the action of stellar winds. Using a simple theory for the electron fraction we estimate that, for a minimum solar nebula, X-rays ionize the disk beyond 5 AU at a level sufficient to couple magnetic fields and neutral disk material. Inside this radius, the X-rays are ineffective for vertical column densities much larger than ~1025 cm-2, and thus an interior region of the disk will be uncoupled from magnetic fields. If disk accretion is mediated by MHD turbulence, as proposed by Balbus & Hawley, then our results suggest that layered accretion occurs in the inner regions of a disk ionized by X-rays, in accord with Gammie's suggestion based on cosmic-ray ionization.