  The linear stability of stalled accretion shocks is investigated in the
context of core collapse of type II supernovae. We focus on a particular
instability mechanism based on the coupling of acoustic perturbations with
advected ones (vorticity, entropy). This advective-acoustic cycle takes place
between the shock and the nascent neutron star. Both adiabatic and
non-adiabatic processes may contribute to this coupling, but only adiabatic
ones are considered in this first approach. The growth time of the adiabatic
instability scales like the advection time, and is dominated by low degree
modes l=0,1,2. Non radial modes (l=1,2) found unstable by Blondin et al. (2003)
can be related to this mechanism.
