Molecular gas in 21 and 30 micron sources: the 2\,mm and 1.3\,mm spectra of IRAS\,21318+5631 and 22272+5435
arXiv:2006.07044 · doi:10.3847/1538-4357/ab9b80
Abstract
The carriers of the 21 and 30\,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\,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 and HCN lines and weaker SiS lines than IRAS\,21318+5631, presumably suggesting that this 21\,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\,m and 30\,m features.
arXiv comment43 pages, 6 figures, Accepted for publication in ApJ
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