  We have studied the effect of the mass of the central star (CS) on the gas
evolution during the planetary nebula (PN) phase. We have performed numerical
simulations of PN formation using CS tracks for six stellar core masses
corresponding to initial masses from 1 to 5 Msun. The gas structure resulting
from the previous asymptotic giant branch (AGB) evolution is used as the
starting configuration. The formation of multiple shells is discussed in the
light of our models, and the density, velocity and halpha emission brightness
profiles are shown for each stellar mass considered. We have computed the
evolution of the different shells in terms of radius, expansion velocity, and
halpha peak emissivity. We find that the evolution of the main shell is
controlled by the ionization front rather than by the thermal pressure provided
by the hot bubble during the early PN stages. This effect explains why the
kinematical ages overestimate the age in young CSs. At later stages in the
evolution and for low mass progenitors the kinematical ages severely
underestimate the CS age. Large (up to 2.3 pc), low surface brightness shells
(less than 2000 times the brightness of the main shell) are formed in all of
our models (with the exception of the 5 Msun model). These PN halos contain
most of the ionized mass in PNe, which we find is greatly underestimated by the
observations because of the low surface brightness of the halos.
