Molecular gas in 21 and 30 micron sources: the 2\,mm and 1.3\,mm spectra of IRAS\,21318+5631 and 22272+5435

TL;DR

This study investigates the molecular composition of two circumstellar envelopes with 21 and 30 micron features, revealing typical chemistry but subtle differences that suggest variations in carbon richness and dust formation processes.

Contribution

It provides detailed molecular line observations and analysis of two 21/30 micron sources, exploring their chemistry and potential dust feature carriers for the first time.

Findings

Detection of 13 molecular species in each object

IRAS 22272+5435 is more carbon-rich and dust-formed

No unexpected molecules found in normal C-rich stars

Abstract

The carriers of the 21 and 30\,$\mu$m emission features in infrared spectra of circumstellar envelopes are a long-standing enigma. In this paper, we present the results of molecular line observations toward two circumstellar envelopes exhibiting the 21 and/or 30\,$\mu$m features, IRAS\,21318+5631 and 22272+5435, aiming at investigating whether they have unusual gas-phase chemistry and searching for possible gas-phase precursor of the carriers of the two dust features. The spectra cover several discrete frequency ranges of 130--164\,GHz and 216.5--273\,GHz, resulting in a detection of 13 molecular species and isotopologues in each object. Rotation-diagram analysis is carried out to determine the molecular abundances, column densities, and excitation temperatures. We did not discover any molecular species that is unexpected in a normal C-rich star. Nevertheless, there exists subtle difference between their molecular abundances. IRAS\,22272+5435 shows stronger SiC$_2$ and HC$_3$N lines and weaker SiS lines than IRAS\,21318+5631, presumably suggesting that this 21\,$\mu$m source is more carbon rich and has experienced a more efficient dust formation. We discuss the potential implications of the results for the carriers of the 21\,$\mu$m and 30\,$\mu$m features.