星际未证认红外谱带载体的化学结构研究

The "Unidentified Infrared Emission (UIE) Bands"are a distinctive set of emission features at 3.3, 6.2, 7.7, 8.6 and 11.3 micron. They are ubiquitously seen in a wide variety of astrophysical environments, including galaxies at redshifts z>4. They account for >20% of the total infrared power of the Milky Way and dominate the heating of interstellar gas. Since their first detection in 1973, the carriers of the UIE bands remain unidentified. A popular hypothesis is that the UIE bands arise from the vibrational modes of polycyclic aromatic hydrocarbon (PAH) molecules. More recently, it has also been suggested that the UIE bands are due to mixed aromatic/aliphatic organic nanoparticles ("MAON"). We propose to examine whether the UIE carriers are mostly aromatic (like PAHs) or largely aliphatic but with aromatic units (like "MAON"). As the aromatic C-H bond vibrates at 3.3 micron while the aliphatic C-H bond vibrates at 3.4 micron, the observed intensities of the 3.3 and 3.4 micron emission features could allow us to estimate the aliphatic fraction of the UIE carriers (i.e., the fraction of C atoms in aliphatic chains). To this end, we will first compile the UIE spectra of a wide variety of objects and determine the ratio of the power emitted from the 3.4 micron aliphatic C-H feature to that from the 3.3 micron aromatic C-H feature (I3.4/I3.3). We will use the DFT-based Gaussian09 software to calculate the band strengths of the 3.4 micron feature (A3.4) and the 3.3 micron feature (A3.3) for a large number of PAH molecules with methyl side-groups. Finally, we will estimate the aliphatic fraction of the UIE carriers from I3.4/I3.3 and A3.4/A3.3 and explore whether (and how) it varies with astrophysical environments (e.g., hardness of the exciting starlight photons, starlight intensity).

"未证认的红外谱带"(UIE)广泛分布于多种天体环境，占银河系红外辐射总量20％以上，并主导星际气体加热。UIE的载体目前尚无定论，流行的模型分为两类：①以芳香性苯环结构为主的PAH 分子；②芳香性苯环与脂肪性碳氢长链并重的纳米尘粒。本项目拟定量研究各种天体环境的UIE谱带载体的"脂肪度"。基于芳香C-H 键产生UIE的3.3微米振动谱征，而脂肪C-H键产生3.4微米振动谱征，（1）分析各种天体环境UIE光谱，得到3.3、3.4微米谱征辐射能量比值I3.3/I3.4；（2）通过密度泛函理论计算附着多种侧基（sidegroups）的多种PAH 分子的3.3 微米芳香C-H 键强度A3.3及3.4 微米脂肪C-H 键强度A3.4；（3）基于I3.3/I3.4 和A3.3/A3.4，算出各个天体环境的UIE 载体的脂肪度，并研究脂肪度与天体环境的关系。本项目将有助于确定UIE载体的化学结构。

本项目研究了各种天体环境下UIE谱带载体的化学结构：芳香性(aromatic) vs. 脂肪性(aliphatic)，及其与所处天体环境（星光“硬度”，星光强度）的关系，并计算了多种具有代表性的aromatic-aliphatic混合体的红外振动谱，确定芳香性和脂肪性振动谱征的本征振动强度。(1) 从第一性原理出发，采用基于密度泛函理论和分子轨道理论（多体微扰方法）量子化学软件包“Gaussian09”计算多种PAH分子附着多种侧基的红外振动光谱，获得了UIE光谱中几条重要的谱带（3.3、3.4、6.2、6.85微米）比较可信的振动本征强度值A3.3，A3.4，A6.2，A6.85。其中部分结果已经得到了实验的验证。(2) 将计算得到的本征强度值输入尘埃模型，模拟了不同天体环境下，不同大小、不同脂肪度的UIE载体分子、离子的理论光谱，给出了在不同天体环境下UIE载体脂肪度与谱征强度比之间的关系。(3) 整理了同时具有芳香性和脂肪性振动谱征的数十个天体的UIE光谱，对这些光谱进行拟合并对波长积分，给出了各个谱征辐射的总能量。结合理论结算结果，我们从多方面得出结论：UIE载体的脂肪度一般<10%，即UIE载体是芳香族主导的，脂肪性成分只占其中很小部分。同时，我们也系统研究了天体环境与UIE载体脂肪度的关系，发现星光越软、电离度越低，脂肪度越低。