分子云的天體化學(xué)觀測和模擬研究

葛繼興

(中國科學(xué)院云南天文臺(tái)昆明650216)

分子云是恒星形成的主要場所.伴隨著望遠(yuǎn)鏡探測能力的提升,時(shí)至今日已經(jīng)有約200多種分子在星際云團(tuán)和星周環(huán)境中被探測到,為人們理解分子云內(nèi)物理化學(xué)過程提供了觀測窗口.天體化學(xué)就是通過理論、實(shí)驗(yàn)和模擬等手段,從物理化學(xué)角度理解分子云內(nèi)分子演化的化學(xué)過程,揭示分子在星際空間的演化途徑.本文首先簡要介紹了兩種分子頻譜數(shù)據(jù)的分析方法—轉(zhuǎn)動(dòng)圖和能級布局圖方法,然后總結(jié)了天體化學(xué)模擬方面的最新研究進(jìn)展,對一批大質(zhì)量恒星形成區(qū)中復(fù)雜有機(jī)分子的觀測進(jìn)行了豐度分析,對“氣體+塵埃”天體化學(xué)模型中微觀物理內(nèi)容做出了改進(jìn),對未來可能的研究做出了展望.論文具體研究內(nèi)容包括:

1.采用能級布局圖方法對北天區(qū)綠色延展天體(Extend Green Objects,縮寫為EGOs)中的復(fù)雜大分子頻譜數(shù)據(jù)進(jìn)行了分析,得到了CH3OH,CH3OCH3,HCOOCH3和CH3CH2CN的觀測豐度,導(dǎo)出了EGOs的物理和化學(xué)性質(zhì),推論出它們很可能處在大質(zhì)量恒星形成演化的早期階段.將EGOs中分子的觀測豐度與最新文獻(xiàn)中“氣體+塵埃”化學(xué)模型給出的分子豐度進(jìn)行比較發(fā)現(xiàn):現(xiàn)有的“氣體+塵?！被瘜W(xué)模型不能很好地解釋EGOs的分子豐度和豐度比,需要建立更加合理的化學(xué)模型.

2.編寫了Fortran90版本的“氣體+塵埃”化學(xué)模擬程序,并與文獻(xiàn)中5個(gè)典型物理模型中分子化學(xué)豐度演化進(jìn)行了比較和定標(biāo).

3.前人的“氣體+塵?！被瘜W(xué)模型簡單地假設(shè)了塵埃顆粒是靜止的.但星際分子云是湍動(dòng)的,塵埃顆粒相對氣體是運(yùn)動(dòng)的.為驗(yàn)證塵埃相對運(yùn)動(dòng)效應(yīng)對化學(xué)過程的影響,我們在“氣體+塵?！被瘜W(xué)模型中加入因?yàn)橥膭?dòng)引起的塵埃相對氣體運(yùn)動(dòng)的效應(yīng).我們發(fā)現(xiàn)塵埃運(yùn)動(dòng)對不同星際環(huán)境中的分子豐度的演化影響是不一樣的,效果跨越幾個(gè)量級.因此,塵埃相對運(yùn)動(dòng)效應(yīng)很可能會(huì)為更好地解釋分子豐度提供一個(gè)新的研究方向.

4.早先的“氣體+塵埃”化學(xué)模型簡單假設(shè)了塵埃具有單一的尺寸,然而觀測給出在星際云中塵埃的尺寸具有一定分布,甚至?xí)S塵埃溫度變化.為了驗(yàn)證塵埃尺寸對星際化學(xué)的影響,構(gòu)建新“氣體+塵?！蹦Ｐ蜁r(shí),我們考慮了塵埃尺寸分布、塵埃溫度分布和與塵埃尺寸密切相關(guān)的塵埃吸積離子的過程.模擬研究結(jié)果表明,離子吸積和尺寸分布對某些塵埃表面分子豐度的影響可達(dá)2–4個(gè)量級.因此,塵埃吸積離子和塵埃尺寸分布是天體化學(xué)模型中的兩個(gè)重要的影響因素,可能為解釋EGOs中的復(fù)雜有機(jī)分子的觀測豐度提供了新的視角.

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.