薄吸积盘中翘曲的扩散传播
On the diffusive propagation of warps in thin accretion discs
摘要：
在本文中，我们重新讨论的问题，传播的翘曲薄和粘性吸积盘。在这种情况下，翘曲是扩散传播的，扩散系数近似与盘的粘度成反比。以前对这个问题的数值研究（Lodato & Pringle 2007）没有发现数值结果与翘曲传播的分析理论的预测之间的良好一致性，无论是在线性还是非线性情况下。在这里，我们利用一个新的，低内存和高效的SPH代码运行一个很大的一组非常高分辨率的模拟（高达2000万SPH粒子）的翘曲传播，以不同的方式实现各向同性光盘粘度，调查理论和数值结果之间的差异的起源。我们的新的和改进的分析，现在显示出显着的协议与分析理论无论是在线性和非线性政权，在翘曲扩散系数和进动率。值得注意的是，仅对于小幅度翘曲和小的盘$α$系数（$α< 0.1$）值，所得扩散系数与盘粘度成反比。对于非线性翘曲，扩散系数是半径和时间的函数，并且明显小于标准值。翘曲的吸积盘存在于许多环境中，从原恒星盘到超大质量黑洞周围的吸积盘。在所有这些情况下，翘曲扩散系数的精确值可能会强烈影响系统的演化，因此为了正确估计系统动力学，对其进行仔细评估是至关重要的。
英文摘要：
In this paper we revisit the issue of the propagation of warps in thin and viscous accretion discs. In this regime warps are know to propagate diffusively, with a diffusion coefficient approximately inversely proportional to the disc viscosity. Previous numerical investigations of this problem (Lodato & Pringle 2007) did not find a good agreement between the numerical results and the predictions of the analytic theories of warp propagation, both in the linear and in the non-linear case. Here, we take advantage of a new, low-memory and highly efficient SPH code to run a large set of very high resolution simulations (up to 20 million SPH particles) of warp propagation, implementing an isotropic disc viscosity in different ways, to investigate the origin of the discrepancy between the theory and the numerical results. Our new and improved analysis now shows a remarkable agreement with the analytic theory both in the linear and in the non-linear regime, in terms of warp diffusion coefficient and precession rate. It is worth noting that the resulting diffusion coefficient is inversely proportional to the disc viscosity only for small amplitude warps and small values of the disc $α$ coefficient ($α< 0.1$). For non-linear warps, the diffusion coefficient is a function of both radius and time, and is significantly smaller than the standard value. Warped accretion discs are present in many contexts, from protostellar discs to accretion discs around supermassive black holes. In all such cases, the exact value of the warp diffusion coefficient may strongly affect the evolution of the system and therefore its careful evaluation is critical in order to correctly estimate the system dynamics (abridged).
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