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arXiv 1809.01979astro-ph.HE
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透镜-Thirring力矩作用下吸积盘对准的声速依赖性

Sound Speed Dependence of Alignment in Accretion Disks Subjected to Lense-Thirring Torques

John F. Hawley, Julian H. Krolik

摘要:

我们提出了一系列的模拟,在纯流体力学(HD)和磁流体力学(MHD)探索的程度,对准磁盘受到外部进动扭矩(例如,如在“Bardeen-Petterson”效应中)取决于盘声速c_s。在检查的声速范围内,我们发现声速的影响可以封装在Sorathia等人提出的简单的“集总参数”模型中。在该模型中,对准前沿以约0.2 rOmega_precess(r)的速度向外传播,其中Omega_precess是局部测试粒子进动频率。同时,跨音速径向运动以一定的速率向内和向外传递角动量,对于局部轨道频率ω,该速率可以用扩散系数约为2c_s^2/ω的定向扩散模型来粗略描述。在等温圆盘中,这两种竞争导致在与c_s^(-4/5)成比例的半径处的对准前沿的固定位置。为了使对准发生,磁盘必须是湍流的,这是由于磁流体力学中的磁旋转不稳定性,或者,在HD中,它必须足够冷,以便由磁盘翘曲驱动的弯曲波在其发射点处是非线性的。与长期以来的预测相反,磁流体磁盘中的翘曲传播是扩散的,与参数c_s/(α v_orb)无关,对于轨道速度v_orb和应力与压力比α。在纯HD盘中,即,对于那些没有内部应力而不是体积粘度的盘,较热的盘排列微弱或根本不排列;较冷的盘与MHD盘在性质上排列相似。

英文摘要:

We present a series of simulations in both pure hydrodynamics (HD) and magnetohydrodynamics (MHD) exploring the degree to which alignment of disks subjected to external precessional torques (e.g., as in the `Bardeen-Petterson' effect) is dependent upon the disk sound speed c_s. Across the range of sound speeds examined, we find that the influence of the sound speed can be encapsulated in a simple "lumped-parameter" model proposed by Sorathia et al. (2013a). In this model, alignment fronts propagate outward at a speed ~0.2 rOmega_precess(r), where Omega_precess is the local test-particle precession frequency. Meanwhile, transonic radial motions transport angular momentum both inward and outward at a rate that may be described roughly in terms of an orientation diffusion model with diffusion coefficient ~2c_s^2/Omega, for local orbital frequency Omega. The competition between the two leads, in isothermal disks, to a stationary position for the alignment front at a radius proportional to c_s^(-4/5). For alignment to happen at all, the disk must either be turbulent due to the magnetorotational instability in MHD, or, in HD, it must be cool enough for the bending waves driven by disk warp to be nonlinear at their launch point. Contrary to long-standing predictions, warp propagation in MHD disks is diffusive independent of the parameter c_s/(alpha v_orb$, for orbital speed v_orb and ratio of stress to pressure of alpha. In purely HD disks, i.e., those with no internal stresses other than bulk viscosity, warmer disks align weakly or not at all; cooler disks align qualitatively similarly to MHD disks.

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