On the origin of accretion bursts in FUORs

TL;DR

This paper models accretion bursts in FU Orionis objects as resulting from extreme evaporation of migrating gas giant planets, revealing different burst behaviors depending on planetary internal structure and providing spectral predictions.

Contribution

It employs stellar evolution modeling to analyze mass-losing planets, offering new insights into the mechanisms behind FU Ori outbursts and their observational signatures.

Findings

Adiabatic planets cause runaway bursts; super-adiabatic planets produce dimming outbursts.

Stutter in bursts can lead to multiple short bursts from a single planet.

Spectral analysis shows outbursts triggered behind the planet, with specific IR-optical lag patterns.

Abstract

Accretion luminosity of young star FU Ori increased from undetectable levels to hundreds of Solar luminosities 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 day orbit around the star may power FU Ori. 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. Super-adiabatic planets contract while losing mass; their outbursts are dimming with time. Long steadily declining bursts such as FU Ori require relatively fine tuned internal planetary structure, which may be rare. More commonly we find that super-adiabatic 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, V1647. We compute broad band spectra of our best fitting scenario for FU Ori. Since the outburst is triggered behind the planet location, the 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 FU Ori.