獵戶座KL區域熱分子雲核之有機分子研究

Abstract

獵戶座KL熱分子雲核(Orion KL hot molecular core)是個已知富含有機分子的天體，它的亮度(~ 105 Lsun)以及相對於太陽如此近的距離(~ 450 pc)非常適合我們來尋找在太空中尚未被指認出來的重要分子以及做天文化學上的研究。本研究中，我們在此天體中探究組成DNA以及RNA的重要元件-嘧啶(c-C4H4N2)是否存在於太空中的可能性。我們利用了次毫米波陣列(the Submillimeter Array; SMA)觀測了在342 GHz 和212 GHz的嘧啶分子躍遷光譜並發現了在Orion Hot Core區域的光譜特徵疑似是來自於嘧啶，然而我們仍然需要更多的觀測來斷定嘧啶是否存在於太空中。此外，我們在一角秒的高解析度下同時利用SMA觀測到了許多存在於Orion KL的有機分子，例如：CH3CN, CH313CN, C2H3CN, C2H5CN, CH3OH, 13CH3OH, HCOOCH3以及(CH3)2O。我們利用了CH3CN的光譜特性以及rotation diagram的分析方法來測量此區域的溫度分布並發現在Orion Hot Core的激發溫度介於500-1400 K之間，而在Orion Compact Ridge的溫度則約210 K。同時，我們發現了在Hot Core和Compact Ridge有相當程度的分子種類空間分布差異；含氮的分子多半分布在Hot Core而含氧的分子多半在Compact Ridge。本研究可以用來檢驗不同的天文化學模型，且對於重要的前生物分子在太空中的形成方式可以有更進一步的了解，這對於未來尋找太空中的生物分子有很大的貢獻。

Orion KL hot molecular core is known to be rich in organic molecules; the brightness ( 105L ) and proximity ( 450 pc) of Orion KL make it a primary target for molecular line search and astrochemistry study. We have searched for the J 5 55 bandhead at 342.3 GHz and the J 5 34 bandhead at 212.8 GHz of pyrimidine (c-C4H4N2), which is the unsubstituted ring analogue for three of the DNA and RNA bases, toward Orion KL using the Submillimeter Array. Pyrimidine emission from the Hot Core region has been detected tentatively; however, more observations are needed before a firm detection can be established. In addition, we have imaged CH3CN, CH3 13CN, C2H3CN, C2H5CN, CH3OH, 13CH3OH, HCOOCH3 and (CH3)2O in Orion KL at arcsecond resolution. Based on the analysis of the symmetric rotor CH3CN, rotational temperatures of 500 1400 K and 200 Kwere derived, respectively, toward Orion Hot Core and Compact Ridge. A chemical differentiation of nitrogen and oxygen was found between Hot Core and Compact Ridge. Our study is important for testing different chemical models and is fundamental to our understanding of the formation of prebiotically important molecules, which will be crucial for our future searches for interstellar biomolecules.