We report high-sensitivity millimetre observations of several molecular species ( ^ { 13 } CO , HCN , HNC , CN , HCO ^ { + } and N _ { 2 } H ^ { + } ) in a sample of compact planetary nebulae .
Some species such as HCO ^ { + } and CN are particularly abundant compared to envelopes around AGB stars or even interstellar clouds .
We have estimated the following average values for the column densities ratios : CN/HCN \sim 2.6 , HCO ^ { + } /HCN \sim 0.5 , and HNC/HCN \sim 0.4 .
Thus , the chemical composition of the molecular envelopes in these compact PNe appears somewhat intermediate between the composition of proto-PNe ( such as CRL 2688 or CRL 618 ) and well evolved PNe ( such as the Ring , M4–9 , or the Helix ) .
From observations of the CO isotopomers , we have estimated that the ^ { 12 } C/ ^ { 13 } C ratio is in the range 10 \raise 2.58 pt \hbox { $ < $ } \kern - 7.5 pt \lower 2.021 pt \hbox { $ \sim$ } ^ { 12 } C / ^ { 13 % } C \raise 2.58 pt \hbox { $ < $ } \kern - 7.5 pt \lower 2.021 pt \hbox { $ \sim$ } 40 .
These values are below those expected from standard asymptotic giant branch models and suggest non-standard mixing processes .
The observed molecular abundances are compared to very recent modelling work , and we conclude that the observations are well explained , in general terms , by time-dependent gas-phase chemical models in which the ionization rate is enhanced by several orders of magnitude with respect to the average interstellar value .
Thus , our observations confirm that the chemistry in the neutral shells of PNe is essentially governed by the high energy radiation from the hot central stars .
The complexity of the chemical processes is increased by numerous factors linked to the properties of the central star and the geometry and degree of clumpiness of the envelope .
Several aspects of the PN chemistry that remains to be understood are discussed within the frame of the available chemical models .