, T. Forveille 
, P. Huggins 
, & P. Cox 
Observatorio Astronómico Nacional (IGN), Apartado 1143,
E-28800 Alcalá de Henares, Spain Observatoire de Grenoble, B. P. 53X,
F-38041 Grenoble Cedex, France Physics Department, New York University,
4 Washington Place, New York NY 10003, USA
Institut d'Astrophysique Spatiale, Bât. 121,
Université de Paris XI, F-91405 Orsay Cedex, France
Institut d'Astrophysique de Paris, 92b
bd. Arago, F-75014 Paris, France 
, SiO, SiC 
,
CN, HCN, HNC, 
, CS, and HC 
N to study the chemistry in
planetary nebulae (PNe) with massive envelopes of molecular gas. The
sample observed consists of representative objects at different stages
of development in order to investigate evolutionary effects: the
proto-PNe CRL 2688 and CRL 618, the young PN NGC 7027, and the evolved
PNe NGC 6720 (the Ring), M4-9, NGC 6781, and NGC 7293 (the Helix).
The observations confirm that the chemical composition of the
molecular gas in PNe is radically different from that in interstellar
clouds and the circumstellar envelopes of
Asymptotic Giant Branch (AGB) stars. There are also
clear trends in the chemical evolution of the envelopes. As a star
evolves beyond the AGB, through the proto-PN and PN phases, the
abundances of SiO, SiC , CS, and HC N decrease,
and they are not detected in the PNe, while the abundances of CN, HNC,
and increase dramatically. Once a PN has formed, the observed
abundances in the molecular clumps of the envelope remain relatively
constant, although HNC is anomalously underabundant in NGC 7027. In
the evolved PNe, CN is about an order of magnitude more abundant than
HCN, HNC, and , and the average abundance ratios are CN/HCN =
9, HNC/HCN = 0.5, and /HCN = 0.5. These ratios are,
respectively, one, two, and three orders of magnitude higher than in
the prototypical AGB envelope IRC+10216.
The 
ratios are 
, within
the large range found in AGB envelopes.
The chemical evolution of the
envelopes likely occurs through the development of photon-dominated
regions produced by the ultraviolet radiation field of the central
star.
The observations also provide important information on the physical
conditions in the molecular gas. Multi-line observations of CN, CO,
and show that the clumps which form the envelopes of the
evolved PNe maintain remarkably high gas densities ( 
few 
cm 
) and low temperatures ( 
K).
These values are consistent with the idea that the clumps are in rough
pressure equilibrium with the more diffuse, ionized gas and can last for a
significant part of the nebular lifetime, providing the environment
needed for the survival of the molecules. Thus the clumping of the gas
in these PNe is an essential aspect of both their physical and
chemical evolution.
Accepted by Astron. Astrophys.
Preprints can be obtained by contacting
bachiller@oan.es