  Late asymptotic giant branch (AGB) and early post-AGB stars which are
progenitors of planetary nebulae lose mass at extremely high rate, in what is
termed a superwind. We show that the existence of this superwind during the
post-AGB phase cannot be explained with models where the stellar effective
temperature is the sole main physical parameter which determine the mass loss
rate. Instead, we argue that the envelope structure, in particular the entropy
and density gradients, should be among the main parameters which determine the
mass loss rate on the tip of the AGB and the early post-AGB evolutionary
phases. The entropy profile becomes steeper and the density profile becomes
shallower as the star becomes hotter on the early post-AGB phase, until the
star heats-up to about 8000 K. We do not propose a new mass loss mechanism, but
rather mention several mechanisms by which these profiles may influence the
mass loss rate within the popular mechanism for mass loss on the AGB, where
pulsations coupled with radiation pressure on dust cause the high mass loss
rate.
