We present new numerical simulations in the thin-disk approximation which characterize the 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 \lesssim 10 ^ { -7 } M _ { \odot } yr ^ { -1 } ) which are punctuated by intense bursts of accretion ( typically \gtrsim 10 ^ { -4 } M _ { \odot } yr ^ { -1 } , with duration \lesssim 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 .
Gravitational instability in the disk , driven by continuing infall from the envelope , is shown to be an effective means of transporting angular momentum outward , and mass inward to the protostar .
We show that the disk mass always remains significantly less than the central protostar mass throughout this process .
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 .
Even in cases where the bursts are nearly entirely suppressed , a moderate increase in cloud size or rotation rate can lead to vigorous burst activity .
We conclude that most ( if not all ) protostars undergo a burst mode of evolution during their early accretion history , as inferred empirically from observations of FU Orionis variables .