On the origin of accretion bursts in FU Ori

Accretion luminosity of young star FU Ori (FUOR) increased from undetectable levels to hundreds of L☉ in 1937 and remains nearly as high at the present time. In a recent paper, we showed how Extreme Evaporation (EE) of a young gas giant planet that migrated to a ∼10 d orbit around the star may power FUOR. However, our model assumed a power-law mass–radius relation for the evaporating planet. Here, we employ a stellar evolution code to model mass losing planets. We find that adiabatic planets expand rapidly, which results in runaway FUOR bursts. Superadiabatic planets contract while losing mass; their outbursts are dimming with time. Long steadily declining bursts such as FUOR require relatively fine-tuned internal planetary structure, which may be rare. More commonly we find that superadiabatic planets contract too rapidly and their EE falters, leading to FUOR burst stutter. This stutter allows a single planet to produce many short repeating bursts, which may be relevant to bursts observed in V346 Nor, V899, and V1647. We compute broad-band spectra of our best-fitting scenario for FUOR. Since the outburst is triggered behind the planet location, the mid-infrared (mid-IR) emission rises many months before the optical, similar to bursts in Gaia-17bpi and Gaia-18dvy. We show that in outbursts powered by the classic thermal instability, mid-IR lags the optical, whereas the dead zone activation models predict mid-IR light precede the optical burst by many years to decades. We comment on the stellar flyby scenario for FUOR.