2008A&A...492..735P

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. 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. 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. The induced accretion rates, the overall temporal accretion profile, the 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. We find no conclusive answer as to whether the observed FUors are triggered by encounters. However, it seems an intense accretion burst phase should exist - possibly an FU phase - early on in the development of dense clusters. We predict that in dense young clusters these outbursts should happen predominantly close to the cluster centre and with high mass ratios between the involved stars.