Salama F, Bakes E L, Allamandola L J, Tielens A G
Space Science Division, NASA-Ames Research Center, Moffett Field, CA 94035-1000, USA.
Astrophys J. 1996 Feb 20;458(2 Pt 1):621-36. doi: 10.1086/176844.
The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high-energy range of the spectrum, and possesses a large oscillator strength), and seems to correlate with strong and broad DIBs. For noncompact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low-energy range of the spectrum, and possesses a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that (i) only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narow bands on the other can easily be interpreted in the context of the PAH proposal by a distribution of compact and noncompact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would reflect conditions within a cloud which locally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations in the ultraviolet range, is called for.
研究了中性和/或离子化多环芳烃（PAHs）与漫射星际带（DIB）载体之间的潜在联系。基于对PAHs一般物理和化学性质的研究，评估了它们对DIB载体的可能贡献。研究发现，在弥漫星际介质的主导条件下，PAHs可以以中性分子以及正离子和/或负离子的形式存在。然而，小PAHs的电荷分布一次由两种电荷状态主导，紧凑的PAHs仅以中性和阳离子形式存在。每个PAH根据其电荷状态具有独特的光谱特征。此外，离子化PAHs的光谱在DIB光谱范围内总是明显由单一谱带主导。对于紧凑的PAH离子，最强吸收带为A类（即谱带较宽，位于光谱的高能范围内，且具有较大的振子强度），似乎与强而宽的DIB相关。对于非紧凑的PAH离子，最强吸收带为I类（即谱带较窄，位于光谱的低能范围内，且具有较小的振子强度），似乎与弱而窄的DIB相关。通过将已知的DIB特征与PAHs的光谱性质进行比较，得出了星际PAHs的潜在分子大小和结构限制。研究发现：（i）如果存在，只有大于约30个碳原子的中性PAHs才可能对DIB有贡献。（ii）对于紧凑的PAHs，如果存在，只有碳原子数少于约250个的离子才可能对DIB有贡献。（iii）一方面，DIB在强/中等和宽带与另一方面的弱带和窄带之间的观测分布，通过分别分布紧凑和非紧凑的PAH离子，很容易在PAH假设的背景下得到解释。因此，PAH-DIB假设提供了PAH电荷状态与DIB“家族”之间合理的相关性。根据这一假设，DIB家族将反映云内的条件，这些条件局部决定了阳离子、阴离子和中性物种的相对重要性，而不是特定物种的示踪剂。给出了观测预测以确定PAH假设的可行性。结论是，小PAH离子作为DIB载体是非常有前途的候选者，前提是它们的种群由有限数量（100 - 200）的物种主导。需要进行一项关键测试，包括在紫外范围内的实验室和天文研究，以验证PAH假设。
