THE FU ORIONIS PHENOMENON¹

We summarize the properties of FU Orionis variables, and show how accretion disk models simply explain many peculiarities of these objects. FU Ori systems demonstrate that disk accretion in early stellar evolution is highly episodic, varying from ∼ 10⁻⁷ yr⁻¹ in the low (T Tauri) state to 10⁻⁴ yr⁻¹ in the high (FU Ori) state. This variability in mass accretion is matched by a corresponding variability in mass ejection, with mass loss rates reaching ∼ 10⁻¹ of the mass accretion rates in outburst. It appears that the FU Ori phenomenon is restricted to early phases of stellar evolution, probably with infall still occuring to the disk, which may help drive repetitive outbursts. Thermal instabilities are a promising way to produce FU Ori disk outbursts, although many uncertainties remain in the theory; triggering by interactions with companion stars on eccentric orbits may also play a role.