Neutral Carbon in Post–Asymptotic Giant Branch Stars and Planetary Nebulae

Abstract

The 492 GHz ( 3 P 1 ? 3 P 0 ) fine-structure line of neutral atomic carbon (C I ) has been observedin the planetary nebulae (PNe) NGC 6302, IRAS 21282+5050, and NGC 7027, and in the protoplanetarynebula (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-asymptoticgiant branch system from a PPN to a PN. Nondetections are also reported on 11 PPNe and extremecarbon stars. The upper limits placed on these objects suggest that they have C I abundances lowerthan the level found in the PPN AFGL 2688, again consistent with an increasing C I /CO ratio withevolution.