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.

中文翻译：

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21 和 30 μm 源中的分子气体：IRAS 21318+5631 和 22272+5435 的 2 mm 和 1.3 mm 光谱

星周包络红外光谱中 21 和 30\,$\mu$m 发射特征的载流子是一个长期存在的谜。在本文中，我们展示了对两个表现出 21 和/或 30\,$\mu$m 特征的星周包络，IRAS\,21318+5631 和 22272+5435 的分子线观测结果，旨在研究它们是否有异常气相化学和寻找可能的两个尘埃特征载体的气相前体。光谱覆盖了 130--164\,GHz 和 216.5--273\,GHz 的几个离散频率范围，从而在每个物体中检测到 13 种分子种类和同位素体。进行旋转图分析以确定分子丰度、柱密度和激发温度。我们没有在普通富含 C 的恒星中发现任何出乎意料的分子种类。然而，它们的分子丰度之间存在细微的差异。IRAS\,22272+5435 显示出比 IRAS\,21318+5631 更强的 SiC$_2$ 和 HC$_3$N 谱线以及更弱的 SiS 谱线，大概表明这个 21\,$\mu$m 源更富碳，并且具有经历了更有效的粉尘形成。我们讨论了结果对 21\,$\mu$m 和 30\,$\mu$m 特征的载体的潜在影响。