摘要
在恆星形成的研究領域中，噴流現象的發現可以說是最有趣的進展之一，而透過分子旋轉能階的改變所產生的無線電或次毫米波段發射譜線而被觀測到的噴流又被稱做分子噴流。其中源自最年輕的原恆星系統(Class 0 Young Stellar Object) 中的分子噴流更在過去數十年中被深入研究，藉由不斷進步的儀器幫助，天文學家們得以用越來越高的解析度去觀測這些以高速、雙極的形式自原恆星系統中產生的噴流現象。典型的第零類分子噴流例子包括 L1448 和 HH211 ，相對於傳統低速且較分散的分子噴流，第零類分子噴流的外觀顯得比較集中準直(collimated)，其長寬比值通常大於10，此外它們也被觀測到具有極高速的噴流物質(> 40 km s-1)。如果將一氧化碳(CO)分子發射譜線的觀測區分為高速和低速兩部分來研究，我們會發現低速的物質發光分佈呈現一種圓錐狀殼層的外觀，相對的高速的部分則呈現非常集中準直的型態。另一方面，雖然一氧化矽(SiO) 分子發射譜線的觀測也在高速區段呈現出集中準直的發光分佈，但在低速部分則幾乎沒有偵測到任何發光。此外，高速的一氧化碳噴流外觀顯得較高速的一氧化碳噴流來得細一些。 為了研究這些第零類分子噴流的性質，我們選擇了一種能夠同時表現出高速噴流和低速錐狀殼層性質的噴流模型做為基礎(Shang et al. (2006))，並採用天文學家常用的臨界密度概念來模擬可能產生分子發射譜線的區域，一般對特定分子能階的發射譜線來說，當氣體的密度高過這個臨界密度時便會有足夠的分子透過碰撞激發的方式被激發到這個能階，進而產生被我們觀測到的發射譜線。我們分別採用1e6 cm-3 和 3e4 cm-3 這兩個密度來做為一氧化矽和一氧化碳的典型臨界密度進而篩選出噴流模型中具有較高密度的部分來做出模擬的影像和位置對速度圖(position-velocity diagram)。其結果成功的呈現出一氧化矽和一氧化碳分子間的不同觀測結果，顯示臨界密度的不同能夠部分解釋兩者觀測上的差異。 發射譜線的產生和天體的物理性質息息相關，但完整的模擬所有相關的物理過程並非易事，因此一些合理的簡化或假設往往是必要的。大速度梯度假設(Large Velocity Gradient approximation) 是其中一種常見簡化輻射傳遞問題的方式，目前也常被應用來和觀測結果比對以得到天體的物理性質。我們採用類似於Hiranoet al. (2006) 採用的方式，將大梯度假設方法應用於HH211 分子噴流上來試圖了解其物理性質。其中採用SiO J=5{4 和SiO J=1{0 觀測結果的比對能夠得到一組合理的結果，然而採用SiO J=8{7 和SiO J=5{4 觀測結果的比對則沒有明確的結論。可能的原因包含目前的觀測尚不足以的解析細微的噴流結構，以及在比較不同解析度觀測的資料時所帶來的不確定性。 最後，我們將噴流模型應用在IRAS 04166+2706分子噴流的研究上，這個第零類分子噴流的特色之一是其發射譜線的位置對速度圖上呈現出鋸齒狀的斜坡特徵。我們成功的利用控制噴流噴發速度的方式產生了和觀測果漂亮對應的位置對速度圖。在高速的噴流物質追上早先以較低的速度噴發出去的物質的過程中便會自然的產生這種特徵。
並列摘要
The outflow phenomena are among the most interesting findings in the field of star formation. High-velocity molecular outflows emerging bipolarly from the youngest Class 0 protostellar objects are intensively studied over the past few decades. Increasingly higher resolution interferometric observations at millimeter and submillimeter wavelengths of molecular transition lines such as CO and SiO reveal properties of the outflows in this earliest evolutionary stage. Typical Class 0 molecular outflows like L1448 and HH211 appear dominated by an extremely high velocity components (EHV, velocities > 40 km s-1), and generally appear highly collimated with collimation factors >10 relative the low velocity and weakly collimated classical molecular outflow. Two distinct components are observed with CO emission, where a shell-like structure is seen in the low-velocity channel, and a more collimated jet-like component is present in the high-velocity one. On the other hand, low-velocity shell components are almost not detected in SiO emission, while the high-velocity SiO jet appear thinner than that of the CO. Motivated by the observational results, we adopt the unified wind model of Shang et al. (2006), in which both shell and jet components are present, to study the properties of Class 0 molecular outflows. Critical densities above which level populations of the molecules are thermalized are calculated for CO and SiO, and are applied to the wind model as density criteria. Typical values of 1e6 cm^-3 and 3e4 cm^-3 for SiO and CO, respectively are adopted and the synthetic maps and position-velocity diagram capture the difference of CO and SiO emission. The result implies that the density effect could partly explain the different emission properties between CO and SiO. Emission property of an object is closely related to its physical conditions. Since it is usually too difficult to model the emission of a source in detail, simplification methods are often required. Large Velocity Gradient (LVG) approximation is one of the methods to simplify the radiative transfer problem, and the results are also used by several authors to constrain the physical properties of molecular outflows. We follow the method adopted by Hirano et al. (2006) and attempt to place constraints on physical conditions of HH211 molecular jet. The constrain with SiO J=5-4 and SiO J=1-0 yields reasonable results, while that with SiO J=8-7 and SiO J=5-4 is inconclusive. Possible interpretations are discussed. The insufficient resolution and the comparison between data from different observations may cause uncertainties. Finally, the emission pattern of Class 0 molecular outflow IRAS 04166+2706 is studied with the wind model. The puzzling slope feature observed in the position-velocity diagram is reproduced by adopting episodic ejection history of high and low velocity components in the simulations. The pattern is a natural consequence of the catching up process which occurs when high velocity material catches up and interact with material ejected with lower velocity in earlier episodes.
參考文獻
延伸閱讀
- Wu, Y. L. (2011). 旋渦星系NGC 6946中的巨分子雲的物理性質 [master's thesis, National Taiwan University]. Airiti Library. https://doi.org/10.6342/NTU.2011.10748
- 黃茂嘉（2007）。以耗散粒子動力學法研究星狀段鏈共聚合高分子溶液之微胞性質〔碩士論文，國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342/NTU.2007.00658
- 房哲名（2008）。利用耗散粒子動力學探討星形高分子溶液的滲透壓及維里係數〔碩士論文，國立中央大學〕。華藝線上圖書館。https://www.airitilibrary.com/Article/Detail?DocID=U0031-0207200917353190
- 趙奕姼（1994）。Molecular Mechanics。化學，52(2)，180-185。https://doi.org/10.6623/chem.1994024
- Lu, S. W. (2022). Property Prediction and Molecular Design Using Transformer [master's thesis, National Taiwan University]. Airiti Library. https://doi.org/10.6342/NTU202202303