On Polycyclic Aromatic Hydrocarbon Molecules in Comets

该项目包括对一类被称为多环芳烃（PAHs）的大型有机分子的激发、改变和破坏的理论研究。这些分子是在我们银河系和其他星系的星际介质（ISM）观测中通过它们的特征光谱检测到的。从逻辑上讲，它们应该存在于形成太阳系的原始物质中。但多环芳烃分子的脆弱性意味着它们不可能在太阳系的天体中存活，除非可能在彗星中。探测彗星中的多环芳烃将在ISM和太阳系之间建立重要的化学联系。在观测到的彗星红外光谱中，多环芳烃还没有被明确地探测到，部分原因是对彗星环境中多环芳烃的预期红外发射光谱和破坏率还没有充分了解。这就是提案要解决的问题。首席研究员和一名受资助的研究生将研究太阳照射下彗星中多环芳烃分子及其离子的光激发、光电离和光解离。他们将利用数值统计力学技术，计算23种具有代表性的特定多环芳烃分子及其离子的发射光谱和破坏速率，以及适用于富氮彗星的三种氮取代多环芳烃和近似星际物种的通用“天文多环芳烃”模型。目标是创建一个基于网络的发射光谱和多环芳烃分子的光解离和光电离率与太阳距离的函数“库”。通过比较观测到的彗星光谱与库中的模型光谱，研究人员将能够限制多环芳烃的产生和破坏速率，并解释观测到的检测和未检测到的数据。

This project is comprised of theoretical studies of the excitation, alteration, and destruction of the class of large organic molecules known as polycyclic aromatic hydrocarbons (PAHs). These molecules are detected by their characteristic spectra in observations of the interstellar medium (ISM) in our own Milky Way and other galaxies. Logically, they would have been present in the original material that formed our Solar System. But the fragility of PAH molecules implies that they would not have been expected to survive in Solar System bodies, except possibly in comets. Detecting PAHs in comets would establish an important chemical link between the ISM and the Solar System. PAHs have not been unambiguously detected in the observed infrared spectra of comets, in part because the expected infrared emission spectra and destruction rates of PAHs in cometary environments are not adequately understood. This is the problem that the proposal aims to address. The Principal Investigator and a supported graduate student will study the photoexcitation, photoionization, and photodissociation of PAH molecules and their ions in cometary comae illuminated by the Sun. They will calculate, using numerical statistical mechanics techniques, the emission spectra and destruction rates of 23 representative specific PAH molecules and their ions, as well as three nitrogen-substituted PAHs appropriate to nitrogen-rich comets and generic "astro-PAH" models approximating the interstellar species. The goal is to create a web-based "library" of the emission spectra and photodissociation and photoionization rates of PAH molecules as a function of distance from the Sun. By comparing observed cometary spectra the model spectra from the library, researchers will be able to constrain PAH production and destruction rates and interpret observational detections and non-detections.