We review the observational properties and possible carriers of the 21 and 30 μm features seen in the spectra of evolved carbon-rich objects. The 30 μm feature is generally taken to be due to MgS, possibly in combination with other sulfides. It is commonly assumed that the sulfides coat grains of SiC or amorphous carbon, but it remains unclear if coated grains can produce the observed feature given the low relative abundance of sulfur and the optical constraints on the thickness of the coating and size of the grains. The lack of optical constants for MgS at wavelengths below 1 μm hampers attempts to model the dust shells of these objects. A small number of alternative carriers have been proposed, but these have not been investigated thoroughly. The situation for the 21 μm feature is more ambiguous. Many carriers have been proposed, but each has issues, and none have gained broad support. The feature only appears in carbon-rich objects which have evolved off of the asymptotic giant branch but have not yet become planetary nebulae. The spectral class of the central star is typically F or G. The transient nature of the 21 μm feature is an important clue to the nature of its carrier. It appears with a variety of other infrared emission features related to complex hydrocarbons, including aliphatic species and polycyclic aromatic hydrocarbons, but correlations with specific features have proven difficult to quantify.

中文翻译：

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富碳物体的 21 μm 和 30 μm 发射特征

我们回顾了在演化的富碳天体的光谱中看到的 21 和 30 微米特征的观测特性和可能的​​载体。30 μm 特征通常被认为是由于 MgS，可能与其他硫化物结合。通常假设硫化物覆盖 SiC 或无定形碳的晶粒，但鉴于硫的相对丰度较低以及涂层厚度和晶粒尺寸的光学限制，涂层晶粒是否能产生观察到的特征仍不清楚。波长低于 1 μm 的 MgS 缺乏光学常数阻碍了对这些物体的尘埃壳进行建模的尝试。已经提出了少量替代载体，但尚未对这些进行彻底研究。21 μm 特征的情况更加模糊。已经提出了许多载体，但每个人都有问题，而且没有一个得到广泛的支持。该特征仅出现在从渐近巨星分支演化而来但尚未成为行星状星云的富碳天体中。中央恒星的光谱类别通常是 F 或 G。21 μm 特征的瞬态性质是了解其载体性质的重要线索。它似乎具有与复杂碳氢化合物相关的各种其他红外发射特征，包括脂肪族物质和多环芳烃，但与特定特征的相关性已证明难以量化。21 μm 特征的瞬态特性是了解其载体性质的重要线索。它似乎具有与复杂碳氢化合物相关的各种其他红外发射特征，包括脂肪族物质和多环芳烃，但与特定特征的相关性已证明难以量化。21 μm 特征的瞬态特性是了解其载体性质的重要线索。它似乎具有与复杂碳氢化合物相关的各种其他红外发射特征，包括脂肪族物质和多环芳烃，但与特定特征的相关性已证明难以量化。