2008AJ....136.2350P

A spectral line survey of the proto-planetary nebula CRL 2688 was carried out in the frequency range of 85-116 GHz with the 14 m radio telescope at the Taeduk Radio Astronomy Observatory. Eight detected molecules are all carbon bearing molecules: HCN, HNC, C₂H, HC₃N, CS, CCS, CN, and CO, and their isotopic species H¹³CN and ¹³CO. A CCS line was newly detected in CRL 2688. The molecular lines detected in CRL 2688 were also compared with those of the proto-typical AGB star IRC +10216. The SiS (J = 5-4, v = 0) molecular line detected in IRC +10216 was not detected in CRL 2688, indicating a lower abundance of the SiS molecule in CRL 2688 than in IRC +10216. Excitation temperatures and column densities of HC₃ N in both CRL 2688 and IRC +10216 were derived using standard LTE rotational diagram analysis. The HC₃ N column density of CRL 2688 was found to be lower than that of IRC +10216. This fact, together with the lower abundance of the SiS molecule in CRL 2688, may be related to the chemical evolution of the circumstellar envelope of CRL 2688.