Tielens A G, Wooden D H, Allamandola L J, Bregman J, Witteborn F C
NASA Ames Research Center, Moffett Field, CA 94035, USA.
Astrophys J. 1996 Apr 10;461(1 Pt 1):210-22. doi: 10.1086/177049.
We have obtained 5-8 micrometers spectra of the Galactic center from the Kuiper Airborne Observatory at resolving powers of approximately 50, approximately 150, and approximately 300. These spectra show absorption features at 5.5, 5.8, 6.1, and 6.8 micrometers. Together with previously observed features in the 3 micrometers region, these features are compared with laboratory spectra of candidate materials. The 3.0 and 6.1 micrometers features are due to the OH stretching and bending variations of H2O and are well fitted by water of hydration in silicates (e.g., talc). The 3.0 micrometer band is equally well fitted by ice mixtures containing 30% H2O, but such mixtures do not provide a good fit to the observed 6.1 micrometer band. The 3.4 and 6.8 micrometers features are identified with the CH stretching and deformation modes in CH2 and CH3 groups in saturated aliphatic hydrocarbons. The 6.1 micrometer band shows a short wavelength shoulder centered on 5.8 micrometer, attributed to carbonyl (C double bond O) groups in this interstellar hydrocarbon dust component. Finally, the narrow 5.5 micrometer feature is also attributed to carbonyl groups, but in the form of metal carbonyls [e.g., Fe(CO)4]. We have derived column densities and abundances along the line of sight toward the Galactic center for the various identified dust components. This analysis shows that hydrocarbon grains contain only 0.08 of the elemental abundance of C and contribute only a relatively minor fraction (0.1) of the total dust volume. Most of the interstellar dust volume is made up of silicates (approximately 0.6). Small graphite grains, responsible for the 2200 angstroms bump, account for 0.07 of the total dust volume. The remaining one-quarter of the interstellar dust volume consists of a material(s) without strong IR absorption features. Likely candidates include large graphite grains, diamonds, or amorphous carbon grains, which all have weak or no IR active modes. Finally, various models for the origin of the hydrocarbon dust component of the interstellar dust are discussed. All of them face some problems in explaining the observations, in particular, the absence of the spectroscopic signature of hydrocrbon grains in sources associated with molecular clouds.
我们利用 Kuiper 机载天文台获得了银河系中心分辨率约为 50、约 150 和约 300 的 5 - 8 微米光谱。这些光谱在 5.5、5.8、6.1 和 6.8 微米处显示出吸收特征。连同之前在 3 微米区域观测到的特征，将这些特征与候选物质的实验室光谱进行了比较。3.0 和 6.1 微米处的特征归因于 H₂O 的 OH 伸缩和弯曲变化，并且由硅酸盐（如滑石）中的水合水很好地拟合。3.0 微米波段同样能被含 30% H₂O 的冰混合物很好地拟合，但这种混合物不能很好地拟合观测到的 6.1 微米波段。3.4 和 6.8 微米处的特征与饱和脂肪烃中 CH₂ 和 CH₃ 基团的 CH 伸缩和变形模式相关。6.1 微米波段在 5.8 微米处有一个以短波长为中心的肩峰，归因于这种星际烃类尘埃成分中的羰基（C = O）基团。最后，窄的 5.5 微米特征也归因于羰基，但以金属羰基化合物[如 Fe(CO)₄]的形式存在。我们已经推导了沿视线方向朝向银河系中心的各种已识别尘埃成分的柱密度和丰度。该分析表明，烃类颗粒仅含有 0.08 的碳元素丰度，并且仅占总尘埃体积的相对较小部分（0.1）。大部分星际尘埃体积由硅酸盐组成（约 0.6）。导致 2200 埃吸收峰的小石墨颗粒占总尘埃体积的 0.07。星际尘埃体积的其余四分之一由没有强红外吸收特征的物质组成。可能的候选物质包括大石墨颗粒、钻石或无定形碳颗粒，它们都具有微弱或没有红外活性模式。最后，讨论了星际尘埃中烃类尘埃成分的各种起源模型。所有这些模型在解释观测结果时都面临一些问题，特别是在与分子云相关的源中缺乏烃类颗粒的光谱特征。
