In simulations of viscously evolving accretion disks, the inner boundary condition is particularly important. If treated incorrectly, it induces incorrect behavior very quickly, because the viscous time is shortest near the inner boundary. Recent work has determined the correct inner boundary in Eulerian simulations. But in quasi-Lagrangian simulations (e.g., SPH, moving mesh, and mesh-less), where the inner boundary is modeled by removing mass within a finite zone, the inner density profile typically becomes anomalously depleted. Here we show how the boundary condition should be applied in such codes, via a simple modification of the usual approach: when one removes mass, one must speed up the remaining material so that the disk's angular momentum is unchanged. We show with both 1D and 2D moving-mesh (AREPO) simulations that this scheme works as desired in viscously evolving disks. It produces no spurious density depletions and is independent of the mass removal rate, provided that the disk is adequately resolved and that the mass removal rate is not so extreme as to trigger instabilities. This "torque-free" mass removal technique permits the use of quasi-Lagrangian codes to simulate viscously evolving disks, while including a variety of additional effects. As an example, we apply our scheme to a 2D simulation of an accretion disk perturbed by a very massive planet, in which the disk is evolved to viscous steady state.
Planet formation at the inner edge of the dead zone I. the interplay between accretion outbursts and dust growth
Ludwig-Maximilians-Universität München（慕尼黑大学）；Max Planck Institute for Astronomy（马克斯·普朗克天文学研究所）；Center for Computational Astrophysics, Flatiron Institute（平顿研究所计算天体物理中心）
Correlation between accretion rate and free-free emission in protoplanetary disks -- A multi-wavelength analysis of central mm/cm emission in transition disks
Radial Stresses and Energy Transport in Accretion Disks
在研究粘性吸积盘的早期，人们认识到来自遥远来源的能量转移一定很重要，尤其是因为在盘的大部分中，盘中平面处的流动很可能是向外的，离开引力势阱。如果粘性的来源是由吸积提供动力的，例如在磁旋转不稳定性的情况下，那么这种遥远的能源必须位于盘的最内部区域，在那里吸积甚至发生在中平面。我们在这里认为，调制在这个能源供应可以改变吸积率的动态，而不是更长的粘性，时间尺度。这意味着内盘吸积率的稳态值和波动（取决于内边界条件的细节并发生在内盘的快速演化时间）都会影响外盘。这是特别有趣的，因为观测表明，磁盘吸积是不稳定的（例如~ EX Lupi类型对象）。我们还注意到，提供给剪切盒的功率是由剪切盒本身设定的，而不是由全球盘中的物理能量通量设定的。也就是说，它们的饱和磁场不受实际磁盘中存在的全套能量约束的影响。我们的分析表明，大尺度径向能量输运对吸积盘的演化和变化有着重要的影响。
