太阳系地外天体有机物质的芳香（aromatic）性与脂肪（aliphatic）性

The infrared absorption spectra of some extraterrestrial objects (e.g., comets, asteroids, satellites, meteorites and interplanetary dust particles) in the solar system exhibit a broad absorption feature at 3.4μm as well as a weaker absorption feature at 3.3μm. The former is often attributed to the aliphatic C-H stretching vibrations of some sorts of organic hydrocarbon dust while the latter is ascribed to aromatic C-H stretches. Little is known about the nature of this organic hydrocarbon dust. We propose to study the structure and composition of this dust by determining the absorption strengths of the 3.3 and 3.4μm features and deriving its aromatic and aliphatic fractions (i.e., the fractional number of C atoms in aromatic rings and in aliphatic chains, respectively). The 3.3 and 3.4μm absorption features are also widely seen in the circumstellar envelopes around evolved stars and the interstellar medium of the Milky Way and external galaxies. We will also study the aromatic or aliphatic nature of the dust in these astrophysical regions and examine the origin and evolution of the solar system organic dust in the context of stellar evolution, interstellar processing and the formation of the solar system.

太阳系的一些地外（extraterrestrial)天体如彗星、小行星、卫星以及陨石和行星际尘埃的红外光谱既有一个波长为3.4μm的宽吸收特征，也有一个较弱的波长为3.3μm的吸收特征。通常认为,前者由某种有机碳氢尘埃（organic hydrocarbon dust）的长链状、脂肪性（chain-like aliphatic）C-H键所产生，而后者由该尘埃物质的苯环状、芳香性（benzene ring-like aromatic）C-H键所引起。人们对这种有机碳氢尘埃的具体成分和化学结构所知有限。我们拟通过计算太阳系各类天体的3.3μm和3.4μm的吸收特征的强度比，确定其有机物质中属于芳香苯环的C原子含量（芳香性）和属于脂肪链的C原子含量（脂肪性），从而对其化学结构和组分作出限止。另外，3.3μm和3.4μm这两个吸收特征也常见于演化晚期恒星的星周包层、银河系和河外星系的星际介质。

本项目选取太阳系的各类小天体，如彗星、小行星、行星卫星、行星际尘埃、陨石等作为研究对象，筛选了红外光谱中既包含3.3 um特征吸收（代表其成分中的苯环、芳香C-H键），又包含3.4 um特征吸收（代表其成分中的链状、脂肪C-H键）的天体，将其分类。其中最具代表性的天体有土卫七、土卫八、彗星尘埃81P/wild 2、星云NGC 1333、NGC2023、Orion Bar等。运用高斯程序计算以上天体在3.3 um和3.4 um处谱征的红外辐射面积，得到土卫七、土卫八等地外天体的两种C-H伸缩振动的红外吸收面积比值τint(3.3) /τint(3.4)。同理得到星云NGC 1333、星云NGC2023、Orion Bar、银心观测红外光谱中两种C-H键的红外辐射面积比值I3.3/I3.4 。根据结果是否大于1，将天体分为两大类。通过总结文献，系统探索了芳香C-H键和脂肪C-H键的强度A，得到两种键强比值的平均值A3.3/A3.4。运用以上结果，计算各类小天体有机尘埃中的芳香C原子与脂肪C原子数之比NC, arom/NC, aliph。并且，根据以上数据，结合各类天体所处的独特宇宙环境，寻找天体环境与其有机成分之间的关联性，为准确描绘星际介质的演化和太阳系的起源、形成与演化这些大背景图像增加数据与论据。