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We report an unbiased radio line survey towards the circumstellar envelopesof evolved stars at the frequency range from 20 to 25 GHz, aiming to obtain amore complete unbiased picture of the chemical evolution in the final stages ofstellar evolution. The observation sample includes the asymptotic giant branch(AGB) star IRC+10216, the proto-planetary nebulae (PPNs) CRL\,2688 andCRL\,618, and the young planetary nebula (PN) NGC\,7027, representing anevolutionary sequence spanning about 10,000 years. Rotational transitions fromcyanopolyyne chains and inversion lines from ammonia are detected in the AGBstar and PPNs, while the PN displays several recombination lines. The differentspectral behaviors of these evolved stars clearly reflect the evolution ofcircumstellar chemistry during the AGB-PPN-PN transitions.

The spectra of evolved stars at 20--25\,GHz: tracing circumstellar chemistry during the asymptotic giant branch to planetary nebula transition