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arXiv 0810.2854astro-ph
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年轻星团中碰撞驱动的吸积--与FUors的联系？

Encounter-driven accretion in young stellar clusters - A connection to FUors?

S. Pfalzner

备注：
9 pages, 6 figures accepted by A&A
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摘要：

背景：FUors的亮度在一到几年内增加了几个数量级。目前对这种亮度提升的解释是，年轻恒星周围的盘状物质急剧增加。导致这种增长的机制是一个争论的焦点。目的：选择猎户座星云团作为代表，我们模拟吸积爆发驱动的遭遇在稠密的恒星环境。我们调查是否像上升和衰减时间，事件频率等属性。发言的遭遇作为一个可能的原因FUOR现象。方法：我们结合集群模拟Nbody 6 ++代码与粒子模拟，描述了一个年轻的恒星周围的光盘上的飞行的影响，以确定诱导的质量吸积。结果如下：诱导吸积率，整体的时间吸积轮廓，衰变时间，并可能的二进制率，我们得到的吸积器诱导吸积同意非常好的观察FUOR。然而，在一些FUOR中观察到的一年的上升时间在我们的模拟中很难实现，除非物质存储在靠近恒星的某个地方，然后在超过一定的质量限制后释放。反对FUOR现象是由相遇引起的最有力的论据是，大多数FUOR都是在低恒星密度的环境中发现的。我们将讨论扩展到偏心双星和引力不稳定盘，并发现这两种模型在实现必要的上升时间有类似的问题。结论：我们预测，在密集的年轻星团中，这些爆发应该主要发生在靠近星团中心的地方，并且所涉及的恒星之间的质量比很大。（节略）

英文摘要：

Context: The brightness of FUors increases by several magnitudes within one to several years. The currently favoured explanation for this brightness boost is that of dramatically rising accretion from the disc material around a young star. The mechanism leading to this accretion increase is a point of debate. Aims: Choosing the Orion Nebula Cluster as representative we simulate accretion bursts driven by encounters in dense stellar environments. We investigate whether properties like rise and decay times, event frequency etc. speak for encounters as a possible cause for FUor phenomena. Method: We combine cluster simulations performed with the Nbody6++ code with particle simulations that describe the effect of a fly-by on the disc around a young star to determine the induced mass accretion. Results: The induced accretion rates, the overall temporal accretion profile, decay time and possibly the binarity rate we obtain for encounter-induced accretion agree very well with observations of FUors. However, the rise-time of one year observed in some FUors is difficult to achieve in our simulations unless the matter is stored somewhere close to the star and then released after a certain mass limit is transgressed. The severest argument against the FUors phenomenon being caused by encounters is that most FUors are found in environments of low stellar density. We extend the discussion to eccentric binaries and gravitationally unstable discs and find that both models have similar problems in achieving the necessary rise-times. Conclusions: We predict that in dense young clusters these outbursts should predominantly happen close to the cluster center and with large mass ratios between the involved stars.(abridged)

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