獵戶座 KL 熱分子雲核中蟻酸分子之順反異構物研究

Abstract

蟻酸，最簡單的有機酸，具有兩種構形異構物:反式和 順式-HCOOH。星際反式構形異構之 HCOOH 是太空中構形 異構-I 的氨基乙酸之可能前體分子，順式 HCOOH 則是構形 異構-II 的氨基乙酸之可能先驅物。反式 HCOOH 在獵戶座 KL 熱分子核心中檢測到，但至今尚未測到順式 HCOOH。 獵戶座 KL 是個含有許不同有機分子及豐富的化學之高質量 恆星形成區，而氨基乙酸則是最簡單的氨基酸。因此，我們 使用阿塔卡瑪大型毫米及次毫米波陣列(ALMA)進行觀測 獵戶座 KL，以搜索順式 HCOOH。

透過角秒分辨率和高靈敏度的 ALMA 觀測，我們分別在 ALMA 第 6 頻段的 255 GHz 和 ALMA 第 7 頻段的 324 GHz 獲得了至今最佳的獵戶座 KL 連續譜影像。此外我們成功地在獵戶座 KL 的五個區域中明確地偵測出六個順式 HCOOH 的分子躍遷譜線。經由旋轉躍遷圖的分析，我們也得出了順式和反式-HCOOH 的旋轉激發溫度和柱密度，也獲得了在 Compact Ridge 中順式對反式-HCOOH 的相對豐度比值，而所獲豐度比支持塵粒化學對順式-HCOOH 形成路徑在冰粒表面上的預測，而非紫外光轉換。我們在獵戶座 KL 中偵測到 順式 HCOOH 不僅有助於擴展我們對星際立體異構研究的理 解，而且對天文生物學研究也至關重要。

Formic acid, the simplest organic acid, has two conformational isomers: trans- and cis-HCOOH. Interstellar trans-conformer of HCOOH is the likely precursor of conformer I glycine in space, and the cis-HCOOH, of conformer II glycine. trans-HCOOH was detected in the Orion KL hot molecular core whereas cis-HCOOH has not been detected. Orion KL is a massive star-forming region containing numerous organic molecular species with rich chemistry, and glycine is the simplest amino acid. Hence, we observed Orion KL using the Atacama Large Millimeter/submillimeter Array (ALMA) to search for cis-HCOOH.

With arcsecond-resolution and high-sensitivity of ALMA observations, we obtain the best continuum images of Orion KL to date in ALMA Bands 6& 7 at 255 GHz and 324 GHz, respectively. In addition, we successfully detected six transitions of cis- HCOOH unambiguously in five regions toward Orion KL. From rotation-diagram analysis, excitation temperatures and column densities of cis- and trans-HCOOH are derived. The abundance ratio of cis- to trans-HCOOH in the Compact Ridge is attained which supports the grain-chemistry prediction of the formation route of cis-HCOOH on icy-grain surface instead of photoswitching. Our detection of cis-HCOOH in Orion KL not only extends our comprehension of interstellar stereoisomerism but is also vital for astrobiology study.