  High spatial resolution images of PNe have shown their extremely complex
morphology. However, the circumstellar envelopes of their progenitors, the AGB
stars, are strikingly spherical. In order to understand the carving processes
leading to axisymmetric nebulae, we are carrying out a study of a large sample
of pre-PNe. Our emission model of the nebular molecular gas (12CO & 13CO) will
allow us to determine important physical parameters (mass, linear momentum,
kinetic energy) of the fast bipolar and slow spherical nebular components
separately. We will study in an innovative way the properties for each source
individually, and put our results in an evolutionary context with the help of
the data obtained by us and collected from the literature.
