The Burst Mode of Protostellar Accretion

We present new numerical simulations in the thin-disk approximation whichcharacterize the burst mode of protostellar accretion. The burst mode beginsupon the formation of a centrifugally balanced disk around a newly formedprotostar. It is comprised of prolonged quiescent periods of low accretion rate(typically $\la 10^{-7} \Msun$ yr$^{-1}$) which are punctuated by intensebursts of accretion (typically $\ga 10^{-4} \Msun$ yr$^{-1}$, with duration$\la 100$ yr) during which most of the protostellar mass is accumulated. Theaccretion bursts are associated with the formation of denseprotostellar/protoplanetary embryos, which are later driven onto the protostarby the gravitational torques that develop in the disk. Gravitationalinstability in the disk, driven by continuing infall from the envelope, isshown to be an effective means of transporting angular momentum outward, andmass inward to the protostar. We show that the disk mass always remainssignificantly less than the central protostar mass throughout this process. Theburst phenomenon is robust enough to occur for a variety of initial values ofrotation rate, frozen-in (supercritical) magnetic field, anddensity-temperature relations. Even in cases where the bursts are nearlyentirely suppressed, a moderate increase in cloud size or rotation rate canlead to vigorous burst activity. We conclude that most (if not all) protostarsundergo a burst mode of evolution during their early accretion history, asinferred empirically from observations of FU Orionis variables.