KMS of Xinjiang Astronomical Observatory, CAS
| 生命相关的星际分子氧气与氰基甲亚胺的化学建模研究 | |
| Alternative Title | Chemical models of interstellar oxygen and prebiotic cyanomethanimine |
张霞
| |
| Subtype | 博士 |
| Thesis Advisor | 加尔肯·叶生别克 ; 全冬晖 |
| 2021-06-01 | |
| Degree Grantor | 中国科学院大学 |
| Place of Conferral | 北京 |
| Degree Name | 理学博士 |
| Degree Discipline | 天体物理 |
| Keyword | 天体化学 星际介质 分子 丰度 |
| Abstract | 随着天文技术的快速发展，越来越多的星际分子被天文学家观测到，其中，与生命相关的分子是天文学家们重点关注的研究对象。在这些分子中，一类是较简单分子，如氧气（O2）和水（H2O），它们与人类生存息息相关。氧气是地球上大多数生命生存必不可少的物质，虽然地球上氧气十分丰富，但在星际空间中人们却很少能探测到它的踪迹。目前科学家们仅在银河系内的蛇夫座星云? Oph A、猎户座星云Orion 及银河系外的类星体QSO Mrk 231 中观测到氧气。现有模拟冷核中氧气的理论模型结果同观测结果间存在的较大偏差，使得天体化学模型研究面临挑战。另一类是组成生命基元的复杂有机分子，如氰基甲亚胺（HNCHCN）。氰基甲亚胺作为星际介质中一种重要的生命前分子，有三种同分异构体，分别为Z，E 和N 构型，其可经过一系列化学反应形成构成遗传关键物质RNA 和DNA 核酸碱基的嘌呤——腺嘌呤。氰基甲亚胺的E 构型首次在大质量恒星形成区Sgr B2(N) 中探测到。之后，Z 和E 两种构型同时在静态分子云G+0.693中探测到。但是人们对其在星际空间中的形成和分解过程尚不清楚。 为了能更好的了解与生命相关的分子在星际介质中的演化机理，本论文使用了气相–尘埃两相模型 NAUTILUS 代码对氧气、氰基甲亚胺及其同分异构体进行了天体化学模拟研究。 针对氧气，本研究使用了两种典型初始丰度值下的恒温模型和变化物理条件下的加热模型进行模拟研究。模拟结果表明，在冷云核条件下主要的含氧分子O2、H2O 和CO 的丰度的峰值大小及其达到峰值的时间都与氢核密度的大小有关，其中氧气的丰度受其影响最大；在加热模型中，温度升高后氧气的模拟丰度与观测结果符合较好；当温度为30 K 时，氢核密度的大小对氧气的丰度影响最大；此外，在恒温模型和加热模型中，相对低的氢核密度更有利于O2 的生成。并由研究推测出分子云的年龄也许是能否探测到氧气的重要因素。 针对氰基甲亚胺，首先通过理论化学计算建立了氰基甲亚胺及其同分异构体的相关化学反应网。因氰基甲亚胺被观测到的复杂星际环境，本论文详细研究了氰基甲亚胺及其同分异构体在冷核、热核和激波三种星际物理环境下的化学演化过程。经模型计算结果分析，我们确定了此分子的主要生成及分解过程。通过模型结果与观测结果的比较发现，在热核模型与激波模型中Z、E 构型的丰度及这两种构型的丰度比都能很好地与观测结果符合，并由上述研究推测氰基甲亚胺及其同分异构体不仅可以在热核附近或热核中生成，也可以在激波存在的冷的环境中生成。此研究结果不仅为研究更大的复杂有机分子奠定了理论基础，为后期的观测提供了依据，同时也进一步揭示了生命前分子在严苛星际环境下的演化机理。 总之，本论文的研究对探索生命起源具有重要意义。但是，目前我们对星际空间中的化学物质认识的不完整，使得对与生命相关分子形成的物理化学过程理解地还远远不够。伴随着一些先进望远镜的建成，将会使人们对星际空间了解的更多，如ALMA望远镜，开启了复杂有机分子观测的新纪元，星际空间中的化学物质的清单也将越来越完善。未来我们将继续结合观测与实验结果，并应用理论计算化学对星际空间中的复杂有机分子进行模型研究。 |
| Other Abstract | More and more molecules have been detected in interstellar medium(ISM) with the rapid advances in astronomical technology. Molecules relating to life have caused special interests. These molecules include simple molecules such as oxygen (O2) and water (H2O), which are essential for human life, and the complex organic molecules that are precursors of biological molecules thus are called prebiotic molecules. Although molecular oxygen is rather abundant on earth, it’s rarely detected in the ISM and has only been detected in two galactic sources: 𝜌 Oph A, Orion and an extra-galactic source: QSO Mrk 231. The inconsistency between observations and theoretical studies for oxygen inthe cold core is a challenge for astrochemical models. Among the prebiotic molecules, cyanomethanimine is a chemical intermediate in the proposed synthetic routes of adenine, which is a purine of DNA and RNA nucleobases. It has three isomers, labeled Z, E and N, and may play an important role in forming genetically critical material in the ISM. While the E-isomer of cyanomethanimine has been first identified in the massive star forming region Sgr B2(N), both E- and Z-isomers were detected in the quiescent molecular cloud G+0.693. Their formation and destruction mechanism in interstellar space is not clear. In order to better understand the evolution of life-related molecules in the ISM, in this thesis, we simulated the chemical processes of molecular oxygen, as well as cyanomethanimine isomers, using the two phase gas–grain NAUTILUS code. We apply the isothermal cold dense models in the interstellar medium with two typical sets of initial elemental abundances, and the warm-up models with various physical conditions to simulate the abundance of molecular oxygen. Under cold core conditions, we find that the peak abundances for gas-phase O2, H2O and CO and the timescales to reach their peak values are dependent on the hydrogen density, the O2 abundance is more sensitive than the others to the variation of hydrogen density. In warm-up models, O2 abundances are in good agreement with observations at temperatures increasing. When the temperature is 30 K, the hydrogen density has a great effect on the abundance of molecular oxygen. In addition, low density is preferable for the formation of molecular oxygen, no matter it's under cold conditions or in the warm-up regions. The study suggests that the age of the molecular cloud may be an important factor in detecting molecular oxygen. We used quantum chemical calculations to establish the chemical reaction network of cyanomethanimine and its isomers. Toward the complicated interstellar environment in which cyanomethanimine was observed, we apply cold isothermal core, hot core, and C-type shock models to simulate the chemical evolution of cyanomethanimine isomers. The major formation and destruction routes of cyanomethanimine are identified by the analysis of the model simulated results. Compared with the observed results, we find that the calculated abundances of cyanomethanimine isomers and the abundance ratio between the Z-isomer and the E-isomer in gas phase are both in reasonable agreement with observations for selected environments. These isomers are most likely formed within or near the hot core without the impact of shocks, or in the cold regions with shocks. The study not only provides a theoretical basis for the study of large complex organic molecules and later observations, but also reveals the evolution mechanism of prebiotic molecules in the extreme interstellar environment. Overall, the research has great scientific significance for the exploration of the origin of life. However, our current understanding of the chemistry of interstellar space is still incomplete. This leaves many questions of the physicochemical processes of life-related molecules unanswered yet. With the construction of the advanced telescopes, such as the ALMA telescope, a new era of observing more types and more complicated organic molecules has started. The chemical inventory in interstellar space will be enriched and toward completion. In the future, we will continue to study observational and experimental results, and use theoretical computational chemistry to modelcomplex organic molecules in interstellar space. |
| Pages | 108 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.xao.ac.cn/handle/45760611-7/4740 |
| Collection | 研究生学位论文 |
| Affiliation | 中国科学院新疆天文台 |
| First Author Affilication | Xinjiang Astronomical Observatory, Chinese Academy of Sciences |
| Recommended Citation GB/T 7714 | 张霞. 生命相关的星际分子氧气与氰基甲亚胺的化学建模研究[D]. 北京. 中国科学院大学,2021. |
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