  Observations indicate that mass accretion rates onto low-mass protostars are
generally lower than the rates of infall to their disks; this suggests that
much of the protostellar mass must be accreted during rare, short outbursts of
rapid accretion. We explore when protostellar disk accretion is likely to be
highly variable. While constant $\alpha$ disks can in principle adjust their
accretion rates to match infall rates, protostellar disks are unlikely to have
constant $\alpha$. In particular we show that neither models with angular
momentum ransport due solely to the magnetorotational instability (MRI) nor
ravitational instability (GI) are likely to transport disk mass at rotostellar
infall rates over the large range of radii needed to move infalling envelope
material down to the central protostar. We show that the MRI and GI are likely
to combine to produce outbursts of rapid accretion starting at a few AU. Our
analysis is consistent with the time-dependent models of Armitage, Livio, &
Pringle (2001) and agrees with our observational study of the outbursting
object FU Ori.
