SIMBAD references

The 492 GHz (³P₁⟶³P₀) fine-structure line of neutral atomic carbon (C I) has been observed in the planetary nebulae (PNe) NGC 6302, IRAS 21282+5050, and NGC 7027, and in the protoplanetary nebula (PPN) AFGL 2688. The estimated C I/CO abundance ratio is higher for a more evolved object, consistent with a trend that the C I/CO ratio increases with the evolution of a post-asymptotic giant branch system from a PPN to a PN. Nondetections are also reported on 11 PPNe and extreme carbon stars. The upper limits placed on these objects suggest that they have C I abundances lower than the level found in the PPN AFGL 2688, again consistent with an increasing C I/CO ratio with evolution.