After a star has been tidally disrupted by a black hole, the debris forms an elongated stream. We start by studying the evolution of this gas before its bound part returns to the original stellar pericenter. While the axial motion is entirely ballistic, the transverse directions of the stream are usually thinner due to the confining effects of self-gravity. This basic picture may also be influenced by additional physical effects such as clump formation, hydrogen recombination, magnetic fields and the interaction with the ambient medium. We then examine the fate of this stream when it comes back to the vicinity of the black hole to form an accretion flow. Despite recent progress, the hydrodynamics of this phase remains uncertain due to computational limitations that have so far prevented us from performing a fully self-consistent simulation. Most of the initial energy dissipation appears to be provided by a self-crossing shock that results from an intersection of the stream with itself. The debris evolution during this collision depends on relativistic apsidal precession, expansion of the stream from pericenter, and nodal precession induced by the black hole spin. Although the combined influence of these effects is not fully understood, current works suggest that this interaction is typically too weak to significantly circularize the trajectories, with its main consequence being an expansion of the shocked gas. Global simulations of disc formation performed for simplified initial conditions find that the debris experiences additional collisions that cause its orbits to become more circular until eventually settling into a thick and extended structure. These works suggest that this process completes faster for more relativistic encounters due to the stronger shocks involved. It is instead significantly delayed if weaker shocks take place, allowing the gas to retain large eccentricities during multiple orbits. Radiation produced as the matter gets heated by circularizing shocks may leave the system through photon diffusion and participate in the emerging luminosity. This current picture of accretion flow formation results from recent theoretical works synthesizing the interplay between different aspects of physics. In comparison, early analytical works correctly identified the essential processes involved in disc formation, but had difficulty developing analytic frameworks that accurately combined non-linear hydrodynamical processes with the underlying relativistic dynamics. However, important aspects still remain to be understood at the time of writing, due to numerical challenges and the complexity of this process.

中文翻译：

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吸积流的形成

在恒星被黑洞潮汐破坏后，碎片形成一条细长的流。我们首先研究这种气体在其束缚部分返回到原始恒星中心之前的演化。虽然轴向运动完全是弹道运动，但由于自重力的限制作用，流的横向方向通常更薄。这种基本情况还可能受到其他物理效应的影响，例如团块形成、氢复合、磁场以及与环境介质的相互作用。然后我们检查这条流回到黑洞附近形成吸积流时的命运。尽管最近取得了进展，但由于计算限制，迄今为止我们无法执行完全自洽的模拟，因此该阶段的流体动力学仍然不确定。大部分初始能量耗散似乎是由自交叉激波提供的，该激波是由流与其自身相交引起的。这次碰撞过程中的碎片演化取决于相对论的顶点进动、来自近中心的流的扩张以及由黑洞自旋引起的节点进动。尽管尚未完全了解这些效应的综合影响，但目前的研究表明，这种相互作用通常太弱，无法显着使轨迹循环，其主要后果是受冲击气体的膨胀。在简化的初始条件下对圆盘形成进行的全局模拟发现，碎片经历了额外的碰撞，导致其轨道变得更圆，直到最终形成一个厚实的延伸结构。这些工作表明，由于所涉及的冲击更强，这个过程在更多的相对论遭遇中完成得更快。如果发生较弱的冲击，它反而会显着延迟，从而使气体在多个轨道期间保持较大的偏心率。当物质被循环冲击加热时产生的辐射可能通过光子扩散离开系统并参与出现的光度。吸积流形成的当前图景来自最近的理论工作，这些工作综合了物理学不同方面之间的相互作用。相比之下，早期的分析工作正确地确定了圆盘形成所涉及的基本过程，但难以开发将非线性流体动力学过程与潜在的相对论动力学准确结合的分析框架。然而，