高質量恆星形成區中氫化二氮陽離子之化學演化

Abstract

根據觀測，大質量恆星形成區中N2H+之豐度隨演化增加。然而，這與我們目前認知的天文化學不同：隨著溫度上升，固態CO昇華、氣態CO成了N2H+的主要分解物。為了解決此相悖，我們去驗證一假設：核心區域有著更複雜的化學反應；若縮小觀測尺度、包含入化學反應相較單純之周圍物質，則N2H+豐度之演化趨近原本的天文化學預測。 我們使用李遠哲陣列觀測，其束尺度為5.5角分，大的束尺度能包含熱核心與周圍物質。我們觀測了六個目標、分屬演化的三個時期；然而，僅其中四個目標有觀測到訊號，此四者之豐度對演化時期的變化趨勢與我們的預測相同。 另一方面，我們發現使用的觀測策略「固定u-v位置」，造成了明顯的座標偏移與主光束衰減；我們用高斯模型來使束函數最適，並以此修正主光束衰減造成的通量損失，修正量最高達27.1%；此外，重力所造成的YTLA盤面變形，使不同基線有不同的相位誤差，估計基線相位誤差造成15.3%的損失。此外，李遠哲陣列尚有其他未被確認的系統不穩定，造成額外之22.7%誤差。因為系統不穩定與缺乏觀測體，我們在未來仍需更多觀測與目標來確認此趨勢。

Based on several high-resolution observations of high-mass star-forming regions, N2H+ abundance increases throughout the evolution process. The increment contrasts with our lack of understanding of astrochemistry: when the growing young star heats gas temperature, and the CO(solid) evaporates into CO(gas). CO(gas) is the primary destroyer to N2H+. Therefore, we expect the N2H+ abundance to decrease with evolution. To solve the inconsistency, we will test an assumption: the core has more complex reactions than the surrounding. In the core, not only CO destroys N2H+, but also other unknown reactions produce or destroy N2H+. If we zoom out the scale and include the surrounding, which has simpler reactions, we will find that the N2H+ abundance decreases in the early stage’s evolution. We used Yuan-Tseh Lee Array (YTLA), whose beam size was 5.5 arcmins, to observe the different early stages of high-mass star-forming regions. The larger beam probes the flux from both the warm core and the surrounding material. We observed six sources in three different evolution stages. Only four of them had detection, and their abundance’s variance in stages met our prediction, decreasing abundance with evolution. On the other hand, we found that our observing strategy, maintaining a fixed u-v pattern, caused the notable pointing offset and the primary beam attenuation. We used a Gaussian model to fit the beam function and corrected the primary beam attenuation. The improvement of our correction for the flux loss is up to 27.1%. Moreover, YTLA’s platform deformation by gravity causes the phase error in baselines. The estimation of the loss by the phase error is 15.3%. Besides, YTLA still had unknown system uncertainties in the measurement. The uncertainties caused an extra 22.7% error. With the system uncertainty and the lack of sources, we need more observations and sources to confirm the trend in the future.