SIMBAD references

We have carried out a systematic search for the molecular ion CO⁺ in a sample of eight protoplanetary and planetary nebulae in order to determine the origin of the unexpectedly strong HCO⁺ emission previously detected in these sources. An understanding of the HCO⁺ chemistry may provide direct clues for the physical and chemical evolution of planetary nebulae. We find that the integrated intensity of the CO⁺ line may be correlated with that of HCO⁺, suggesting that the reaction of CO⁺ with molecular hydrogen may be an important formation route for HCO⁺ in these planetary nebulae.