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

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.