Post Asymptotic Giant Branch (AGB) phase of evolution is one of the most
dynamically developing fields in astrophysics after the IRAS mission.
Objects in this phase of evolution, called proto-planetary nebulae (PPNe)
are formed during evolution along the AGB when mass of the H-rich envelope
drops to very small values mainly as a result of the large scale mass
loss process. PPNe are very intriguing objects because of rapid changes in
the stellar temperature and, in consequence, in the physical conditions
inside the ejected shell and the remnant stellar atmosphere. Their transitory
nature can be seen in dust properties which are manifested by peculiar bands
(e.g. the 21 micron band discovered by Kwok et al., 1989). One of the
intriguing aspect of post-AGB evolution is existence of the fast winds
detected for some of them. Such winds will interact with the envelope
ejected on the AGB producing specific excitation conditions for the molecules
present inside the shells.
The goal of our proposal is to detect CO far-IR lines and compare their
relative strengths in order to analyze excitation conditions inside the
envelopes.