Triggering mechanism of FU Orionis outbursts

Determine the physical mechanism that triggers FU Orionis-type accretion outbursts in pre-main-sequence stars, to clarify how these episodic accretion events are initiated and their role in stellar mass assembly and disk chemical evolution.

Background

FU Orionis objects undergo rare, large-amplitude accretion outbursts that are interpreted as episodes of dramatically enhanced disk accretion onto young stars. These events are thought to influence both the stellar mass assembly history and the chemical evolution of circumstellar disks.

Despite extensive observational and theoretical work, the specific physical mechanism that initiates FU Orionis outbursts has not been conclusively identified. This paper focuses on the unique pre-outburst characterization of HBC 722, but acknowledges that the broader question of what triggers FUor episodes remains unresolved.

References

These massive accretion events likely play a crucial role in stellar mass assembly and in the chemical evolution of disks and envelopes \citep[see review by][]{fischer23}, though the triggering mechanism remains uncertain (see for example discussions in \citealt{audard14} and \citealt{vorobyov21}).

— The Pre-Outburst Properties of the FU Ori Object HBC 722  (2511.04592 - Herczeg et al., 6 Nov 2025) in Section 1, Introduction