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We present numerical simulations of the newly discovered burst mode
of protostellar accretion.
The burst mode begins upon the formation of a centrifugally balanced
disk around a newly formed protostar. It is comprised of prolonged
quiescent periods of low accretion rate (typically 10^{-7} Msun/yr)
which are punctuated by intense bursts of accretion (typically 10^{-
4} Msun/yr, with duration <100 yr) during which most of the
protostellar mass is accumulated. The accretion bursts are associated
with the formation of dense protostellar/protoplanetary embryos,
which are later driven onto the protostar by the gravitational
torques that develop in the disk. Like the process of throwing logs
into a fireplace, these episodes of embryo infall produce excess
energy which cause the protostar to temporarily brighten by a factor
of hundreds to thousands. The burst phenomenon is robust enough to
occur for a variety of initial values of rotation rate, frozen-in
(supercritical) magnetic field, and density-temperature relations. We
conclude that most (if not all) protostars undergo a burst mode of
evolution during their early accretion history, as also inferred from
observations of FU Orionis variables.
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