獵戶座KL之複雜有機分子研究

Study of Complex Organic Molecules in Orion KL

許育森(Yu-Sen Hsu)

指導教授 ： 管一政

國立臺灣師範大學/理學院/地球科學系/碩士(2009年)

摘要

本研究使用高解析度的次毫米波陣列望遠鏡 (Submillimeter Array, SMA) 在 213 GHz 的頻段對獵戶座 KL 進行觀測。由於獵戶座 KL 擁有非常高的亮度 (~ 105 L⊙) 以及相對於我們非常近的距離 (~ 450 pc)，加上過去的觀測發現此區域非常豐富且複雜的化學性質，所以獵戶座 KL 提供了我們研究高質量恆星形成區的良好機會。在此研究中我們成功地觀測到 16 個分子 (其中包含 4 個同位素)，並且發現它們有顯著地空間分布上的差異。過去的研究認為在獵戶座 KL 的含氮分子主要集中在Hot Core 區域，而含氧分子則集中在 Compact Ridge 區域。但是我們發現這兩種族群的分子其實在這兩個區域之間有部分的重疊，顯示這個重疊的區域可能隱含了在這兩種族群的之間形成上的關聯性。此外，在本次觀測中偵測到的最大的分子丙酮 [(CH_3)_2CO]，只有在此重疊區域被觀測到，因此我們推論這個區域的環境可能較適合於更大的分子的形成。再者，我們推論乙基氰 (C_2H_5CN) 的分布可能與獵戶座 KL 的噴流結構有關，其空間分布及光譜特徵均顯示了複雜的結果，我們認為C2H5CN 能夠對於獵戶座 KL 的動力結構有進一步的研究。利用數個有機分子的空間分布，我們對目前在恆星形成區的有機分子演化路徑的化學模型做了初步的測試。若此演化途徑是有效率的，則在不同區域之間演化程度，以及這些分子周圍環境的能量機制的不同，可能是這些有機分子之間的空間上分布差異的一個解釋。本研究在獵戶座 KL 也觀測到可能來自嘧啶 (c-C_4H_4N_2) 的發射譜線，此分子是組成 DNA 以及 RNA 的重要元件，因此它是一個對於尋找太空中外星生命的重要元素。然而，我們需要其他更加確定的結果來佐證我們的發現。本研究有助對於瞭解在恆星形成區域的複雜有機分子的特性，並且對於尋找其他在太空中更複雜的有機分子與生物分子是非常重要的。

關鍵字

恆星形成區 ； 有機分子

並列摘要

We carried out high angular-resolution observations toward the massive star-forming region Orion KL at 213 GHz using the Submillimeter Array (SMA). Orion KL is one of the best sites for studying massive star formation because of its proximity ( 450 pc from the Sun), extraordinary luminosity
( 105 L) and rich chemistry. We have successfully imaged the spectral emission of 16 molecular species (including 4 isotopomers) in Orion KL and the distinct spatial distributions of the molecules observed are also presented.
Through our high resolution SMA data, the spatial distribution of oxygen-bearing molecules exhibit a notable, partial overlap with that of N-containing molecules in the region between the Hot Core and the Compact Ridge; hence
the well-perceived N/O chemical dierentiation in Orion KL between the Hot Core and the Compact Ridge may not be so well-dened as previously thought. In addition, we have identied a region southwest of the Hot Core
where (CH3)2CO only resides, thus may exist an environment favorable for building large organic molecules. We found the complicated, very broad yet almost identical line proles of various C2H5CN transitions illustrate the complexity of the kinematic structure and velocity eld in Orion KL.
Such intricate proles may be related to an expanding out
ow originated from a location near IRc2. A preliminary test of the formation route in chemical model is presented. Dierent evolution stage between the Hot Core and the Compact Ridge implies the variation of molecular composition on grain mantles that may be a possible explanation for the distinct spatial distribution among the molecules in Orion KL. We have tentatively detected the 212.8{GHz pyrimidine bandhead; however, denite detection of another isolated pyrimidine bandhead in Orion KL is urgently needed to settle the issue. In summary, our high-resolution spectral observations of Orion KL are essential for understanding the characteristics of complex organic molecules in hot molecular cores, and it is also important for searching for other complex organic molecules and biomolecules in the future.

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