Neutral Carbon in the Protoplanetary Nebulae CRL 618 and CRL 2688

The 609 μm 3 P 1 → 3 P 0 line of neutral atomic carbon has been detected in the protoplanetary nebulae CRL 618 and CRL 2688. C I appears to be about 7% as abundant as CO in CRL 2688 and 70% as abundant as CO in CRL 618. C I emission arises primarily from the slow component of the wind from these objects. The data presented here indicate that C I gradually becomes more abundant as the star leaves the asymptotic giant branch and that C I is significantly enhanced relative to CO before the star develops a large H II region.