The FU orionis outburst as a thermal accretion event: Theoretical and observational implications
Detailed calculations of viscous accretion disks suggest that FU Ori outbursts signify the existence of protostellar disks transporting mass at a time-averaged rate of (1–10)×10−6 M⊙/yr around some fraction (≈10%) of young stellar objects. Accretion through the inner parts of these disks is self regulated by the thermal ionization instability such that long periods (≈1000 yr) of low mass flux: (1–10)×10−8 M⊙/yr, are punctuated by short periods (≈100 yr) of high mass flux: (1–10)×10−5 M⊙/yr. The unstable region of the disk extends only to ≈ 1/4 AU. Beyond this region matter is transported stably through the disk at the infall rate, M in .
In systems for which M *=1 M⊙ with inner disk edges of 3 R⊙, the critical rate for outburst is 5×10−7 M⊙/yr independent of the magnitude of the viscosity. The magnitude of the alpha parameter may be constrained by global timescale fitting to be 10−4, where hydrogen is neutral, and 10−3 where ionized. This low viscosity has implications for self gravity as a possible mass transport mechanism through much of the disk. Light curves of V1515 Cyg, FU Ori, and V1057 Cyg are reproduced, and model results are compared to observations.
KeywordsMass Flux Light Curve Accretion Disk Stellar Mass Ionization Front
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