The explosion mechanism of core-collapse supernovae (CCSNe) has not been fully understood yet, but multidimensional fluid instabilities such as standing accretion shock instability and convection are now believed to be crucial for shock revival. Another multidimensional effect that has been recently argued is the asymmetric structures in progenitors, which are induced by violent convections in silicon/oxygen layers that occur before the onset of collapse, as revealed by recent numerical simulations of the last stage of massive star evolutions. Furthermore, it has been also demonstrated numerically that accretions of such nonspherical envelopes could facilitate shock revival. These two multidimensional effects may hence hold a key to successful explosions. In this paper, we performed a linear stability analysis of the standing accretion shock in CCSNe, taking into account nonspherical, unsteady accretion flows onto the shock to clarify the possible links between the two effects. We found that such preshock perturbations can excite the fluid instabilities efficiently and hence help the shock revive in CCSNe.
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