分子云的天体化学观测和模拟研究

葛继兴

(中国科学院云南天文台昆明650216)

分子云是恒星形成的主要场所.伴随着望远镜探测能力的提升,时至今日已经有约200多种分子在星际云团和星周环境中被探测到,为人们理解分子云内物理化学过程提供了观测窗口.天体化学就是通过理论、实验和模拟等手段,从物理化学角度理解分子云内分子演化的化学过程,揭示分子在星际空间的演化途径.本文首先简要介绍了两种分子频谱数据的分析方法—转动图和能级布局图方法,然后总结了天体化学模拟方面的最新研究进展,对一批大质量恒星形成区中复杂有机分子的观测进行了丰度分析,对“气体+尘埃”天体化学模型中微观物理内容做出了改进,对未来可能的研究做出了展望.论文具体研究内容包括:

1.采用能级布局图方法对北天区绿色延展天体(Extend Green Objects,缩写为EGOs)中的复杂大分子频谱数据进行了分析,得到了CH3OH,CH3OCH3,HCOOCH3和CH3CH2CN的观测丰度,导出了EGOs的物理和化学性质,推论出它们很可能处在大质量恒星形成演化的早期阶段.将EGOs中分子的观测丰度与最新文献中“气体+尘埃”化学模型给出的分子丰度进行比较发现:现有的“气体+尘埃”化学模型不能很好地解释EGOs的分子丰度和丰度比,需要建立更加合理的化学模型.

2.编写了Fortran90版本的“气体+尘埃”化学模拟程序,并与文献中5个典型物理模型中分子化学丰度演化进行了比较和定标.

3.前人的“气体+尘埃”化学模型简单地假设了尘埃颗粒是静止的.但星际分子云是湍动的,尘埃颗粒相对气体是运动的.为验证尘埃相对运动效应对化学过程的影响,我们在“气体+尘埃”化学模型中加入因为湍动引起的尘埃相对气体运动的效应.我们发现尘埃运动对不同星际环境中的分子丰度的演化影响是不一样的,效果跨越几个量级.因此,尘埃相对运动效应很可能会为更好地解释分子丰度提供一个新的研究方向.

4.早先的“气体+尘埃”化学模型简单假设了尘埃具有单一的尺寸,然而观测给出在星际云中尘埃的尺寸具有一定分布,甚至会随尘埃温度变化.为了验证尘埃尺寸对星际化学的影响,构建新“气体+尘埃”模型时,我们考虑了尘埃尺寸分布、尘埃温度分布和与尘埃尺寸密切相关的尘埃吸积离子的过程.模拟研究结果表明,离子吸积和尺寸分布对某些尘埃表面分子丰度的影响可达2–4个量级.因此,尘埃吸积离子和尘埃尺寸分布是天体化学模型中的两个重要的影响因素,可能为解释EGOs中的复杂有机分子的观测丰度提供了新的视角.

Observational and Simulation Studies of Molecular Cloud Chemistry

GE Ji-xing
(Yunnan Observatories,Chinese Academy of Sciences,Kunming 650216)

Molecular clouds are the main birth places of stars.More than 200 molecules have been detected in the interstellar clouds and circumstellar environments with more powerful telescopes,which serves as a window to understand the physical and chemical processes in molecular clouds.Astro-chemistry is revealing the mystery of history of interstellar molecules through theories,experiments,and modelings.In my PhD thesis I first briefly introduced two methods,that are often used for analyzing the observed spectral line data of complex organic molecules,i.e.the rotation diagram method and the population diagram analysis,and then I reviewed the recent progress in astro-chemical modeling.I also summarized the scientific results by analyzing the observed abundances of complex organic molecules in massive star-formation regions,and improved the microcosmic physical contents in the astro-chemical gas-grain models.An outlook of future works is given in the end.The main work I have done for this thesis includes the following aspects.

1.The abundances of CH3OH,CH3OCH3,HCOOCH3,and CH3CH2CN observed in Extend Green Objects(EGOs)in the northern sky are deduced by using the population diagram method.The physical and chemical properties of EGOs are then analyzed,which hints that they are in the early-stage of massive star-formation.We compare the observed abundances with the results from the gas-grain chemical models in literature,and find that previous models cannot explain the chemistry in EGOs.A more suitable gas-grain chemical model should be constructed.

2.We write a new gas-grain chemical code with Fortran90 programming language,and the comparisons and benchmarking of our code with the 5 typical models in literature have been carried out carefully.

3.The dust grains have been assumed in a quiescent status in previous gas-grain chemical models.However,the molecular clouds are turbulent which results in the velocity of dust grains relative to the gas components.In order to check the effect of the dust grain motion,we add it to our gas-grain model and test it in different interstellar environments.We find that the abundances of species can be affected differently up to several orders of magnitude,which may help us to explain the chemistry from a new view point.

4.In the previous gas-grain chemical model,the dust grains have also been assumed to have the same size.However,dust grains in reality should have various forms of grain size distributions according to observations,and they vary with the dust temperature fluctuations.In order to check this effect on chemistry,in our gas-grain model,we take into account the dust grain size distribution,the fluctuation of dust temperature,and the ion accretion.By simulations,we find that the abundances of some surface-species are enhanced by 2-4 orders of magnitude,therefore the ion accretion and the grain size distribution are important factors to interstellar chemistry which provides a new possible chance to interpret the observed abundances of molecules in EGOs.