[论文解读] Two-component magnetohydrodynamical outflows around young stellar objects Interplay between stellar magnetospheric winds and disc-driven jets

Z. Méliani, Fabien Casse+1 more|arXiv (Cornell University)|Aug 28, 2006

Astrophysics and Star Formation Studies参考文献 50被引用 42

一句话总结

本研究首次实现了对年轻恒星物体（YSO）周围耦合恒星磁层风与盘驱动喷流的自洽2D非理想磁流体动力学（MHD）模拟。结果表明，恒星风中的热力与洛伦兹力加速，结合盘喷流中的磁-离心力加速，可增强质量喷射与准直化，其中致密的恒星风显著增加了盘驱动喷流的径向范围与质量流率，使模拟结果更符合观测。

ABSTRACT

We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar magnetospheric winds coupled with disc-driven jets where the resistive and viscous accretion disc is self-consistently described. These innovative MHD simulations are devoted to the study of the interplay between a stellar wind (having different ejection mass rates) and an MHD disc-driven jet embedding the stellar wind. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. We first investigate the various angular momentum transports occurring in the magneto-viscous accretion disc. We then analyze the modifications induced by the interaction between the two components of the outflow. Our simulations show that the inner outflow is accelerated from the central object's hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force.}{The simulations show that the MHD disc-driven outflow extracts angular momentum more efficiently than do viscous effects in near-equipartition, thin-magnetized discs where turbulence is fully developed. We also show that, when a dense inner stellar wind occurs, the resulting disc-driven jet has a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass-ejection rate in the disc-driven jet and a larger radial extension that is in better agreement with the observations, besides being more consistent.

研究动机与目标

• 使用非理想MHD方法，对年轻恒星物体（YSOs）中恒星磁层风与盘驱动喷流之间的相互作用进行建模。
• 研究吸积盘中的阻抗与黏性过程对角动量输运与喷流结构的影响。
• 确定恒星风质量损失率对盘驱动喷流启动的形态与效率的影响。
• 评估湍流加热与磁能耗散在加速恒星风及改变喷流结构中的作用。
• 实现两种喷流的自洽启动与准直化，包括恒星风从亚阿尔芬速度注入后的加速过程。

提出的方法

• 使用VAC代码求解2D轴对称阻抗MHD方程，采用有限体积法进行数值模拟。
• 对具有阻抗与黏性效应的磁-黏性吸积盘进行自洽建模，包含由磁能耗散引起的湍流加热。
• 恒星风以亚阿尔芬速度注入，并通过热压力与洛伦兹力加速，阻抗加热被建模为连续能量源。
• 盘驱动喷流通过磁-离心力机制启动，极向磁力线提供准直化作用。
• 动态模拟两种喷流之间的相互作用，允许反馈至磁场几何构型与喷流准直化。
• 模拟包含可变的恒星质量损失率（最高达10⁻⁷ M☉/yr），以评估其对喷流结构与效率的影响。

实验结果

研究问题

• RQ1 致密恒星风的存在如何影响盘驱动MHD喷流的结构与准直化？
• RQ2 在非理想MHD条件下，热压力与洛伦兹力在加速恒星风中的相对贡献如何？
• RQ3 恒星风中的湍流加热在多大程度上增强了其终端速度与质量喷射率？
• RQ4 恒星风如何改变磁力线构型与盘驱动喷流的径向延伸范围？
• RQ5 需要多高的恒星质量损失率才能显著改变盘驱动喷流的形态与效率？

主要发现

• 恒星风通过耗散磁能产生的阻抗加热实现热力加速，终端速度因湍流加热而增强。
• 盘驱动喷流主要通过磁-离心力加速，在薄且接近能量平衡的盘中，其角动量提取效率高于黏性过程。
• 当恒星风质量损失率达到约10⁻⁷ M☉/yr时，盘驱动喷流的径向延伸范围加倍，质量喷射率增至内盘吸积率的50%。
• 由于恒星风的压力，盘驱动喷流中的磁力线变得更加倾斜，导致更强的准直化与更接近圆柱形的喷流结构。
• 两种喷流的相互作用导致形成双层喷流结构，其中恒星风塑造了盘喷流的外层区域。
• 模拟表明，阻抗加热贡献了约35%的吸积能量，显著提升了恒星风的加速效率与整体性能。