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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.
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