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.

中文翻译：

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20-25 GHz 演化恒星的光谱：在渐近巨星分支到行星状星云过渡期间追踪星周化学

我们报告了对 20 至 25 GHz 频率范围内演化恒星星周包络的无偏无线电线测量，旨在获得恒星演化最后阶段化学演化的更完整无偏图像。观测样本包括渐近巨星分支（AGB）星IRC+10216、原行星状星云（PPNs）CRL\,2688和CRL\,618、年轻行星状星云（PN）NGC\,7027，代表一个演化序列跨越约 10,000 年。在 AGB 星和 PPN 中检测到来自氰基多炔链的旋转转变和来自氨的反转线，而 PN 显示了几条重组线。这些演化恒星的不同光谱行为清楚地反映了 AGB-PPN-PN 跃迁期间星周化学的演化。