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

TL;DR

This study presents a radio line survey of evolved stars at 20-25 GHz, revealing chemical evolution in circumstellar envelopes during the transition from AGB stars to planetary nebulae.

Contribution

It provides the first unbiased spectral survey across this frequency range for multiple evolved star types, illustrating chemical changes during late stellar evolution.

Findings

Detection of cyanopolyynes and ammonia in AGB and PPN stages.

Recombination lines observed in planetary nebula NGC 7027.

Spectral behavior differences reflect chemical evolution.

Abstract

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.