Investigating fragmentation conditions in self-gravitating accretion discs
Investigating fragmentation conditions in self-gravitating accretion discs
复制标题
研究自引力吸积盘的破碎条件
DOI：
10.1111/j.1745-3933.2005.00105.x
复制
发表时间：
2005
影响因子：
--
通讯作者：
P. Armitage
中科院分区：
文献类型：
--
作者：
W. Rice;G. Lodato;P. Armitage
自引力气体吸积盘中的碎裂问题既对活动星系核中盘中恒星的形成，也对星盘中气态行星或棕矮星的形成都有意义。现在公认的是，如果在比局部动态时间尺度更小的时间尺度上冷却磁盘，则会发生碎裂，而对于更长的冷却时间，磁盘在热平衡中达到准稳态，冷却速度由由于重力应力而产生的加热来平衡。我们在这里研究碎裂边界如何依赖于假定的状态方程。我们发现，碎裂所需的冷却时间随着比热比γ的减小而增加，超过了γ=7/5时的局部动态时间标度。这一结果很容易解释为存在最大应力(以局部盘压力为单位)，这是准平衡状态下的自引力盘所能承受的最大应力的结果。如果冷却时间使达到热平衡所需的应力超过此值，则会发生碎裂，与γ无关。这一结果表明，准稳定的自引力圆盘永远不会产生导致粘性α参数超过∼0.06的应力。
The issue of fragmentation in self-gravitating gaseous accretion discs has implications both for the formation of stars in discs in the nuclei of active galaxies, and for the formation of gaseous planets or brown dwarfs in circumstellar discs. It is now well established that fragmentation occurs if the disc is cooled on a time-scale smaller than the local dynamical time-scale, while for longer cooling times the disc reaches a quasi-steady state in thermal equilibrium, with the cooling rate balanced by the heating due to gravitational stresses. We investigate here how the fragmentation boundary depends on the assumed equation of state. We find that the cooling time required for fragmentation increases as the specific heat ratio γ decreases, exceeding the local dynamical time-scale for γ = 7/5. This result can be easily interpreted as a consequence of there being a maximum stress (in units of the local disc pressure) that can be sustained by a self-gravitating disc in quasi-equilibrium. Fragmentation occurs if the cooling time is such that the stress required to reach thermal equilibrium exceeds this value, independent of γ . This result suggests that a quasi-steady, self-gravitating disc can never produce a stress that results in the viscous α parameter exceeding ∼0.06.