  Low-mass condensations (LMCs) are observed inside the envelope of the
collapsing molecular cloud cores. In this research, we investigate the effects
of landing LMCs for occurrence of instability through the protostellar
accretion disks. We consider some regions of the disk where duration of
infalling and landing of the LMCs are shorter than the orbital period. In this
way, we can consider the landing LMCs as density bumps and grooves in the
azimuthal direction of an initial thin axisymmetric steady state
self-gravitating protostellar accretion disk (nearly Keplerian). Using the
linear effects of the bump quantities, we obtain a characteristic equation for
growth/decay rate of bumps; we numerically solve it to find occurrence of
instability. We also evaluate the minimum-growth-time-scale (MGTS) and the
enhanced mass accretion rate. The results show that infalling and landing of
the LMCs in the inner regions of the protostellar accretion disks can cause
faster unstable modes and less enhanced accretion rates relative to the outer
regions. Also, more fragmentation of landed LMCs in the azimuthal direction
have less chance for instability, and then can produce more values of enhanced
mass accretion rate.
