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