Transitory O-rich chemistry in heavily obscured C-rich post-AGB stars
Transitory O-rich chemistry in heavily obscured C-rich post-AGB stars
复制标题
严重遮挡的富碳后 AGB 恒星中的瞬态富氧化学
DOI：
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
J. Perea
中科院分区：
文献类型：
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作者：
D. A. García;P. Garcı́a;J. Cernicharo;D. Engels;J. Perea
本文介绍了11颗被严重遮挡的富c源从渐近巨支（AGB）恒星迅速演化为行星状星云的Spitzer/IRS光谱。还报道了其中三个agb后的IRAM 30m观测结果。已知这些源中的少数(3)表现出强烈变化的含O分子（如OH和H2 O）的脉泽发射，这表明在这个短暂的进化阶段中，由于物理和化学条件的快速变化，存在短暂的富O化学。有趣的是，斯皮策/IRS光谱显示了丰富的星周碳化学，如C2H2， C4H2, C6H2， C6H6和HCN等小碳氢化合物分子的检测。苯被检测到两个源，使银河系中检测到这种分子的agb总数达到三个。此外，我们报告了在这些来源中可能检测到其他碳氢化合物分子如HC3N， CH3C2H和CH3的证据。可用的IRAM 30m数据证实，尽管存在富o脉射，但中心恒星是富c的，并且存在高速分子外流以及极端的AGB质量损失率（∼10-4 M突/年）。我们的观察证实了Cernicharo[1]的聚合模型，该模型预测这些物体的中性区域在比中心HII区域动态演化更短的时间尺度上发生丰富的光化学反应，导致富c小分子的形成和短暂的富o化学反应。
Spitzer/IRS spectra of eleven heavily obscured C-rich sources rapidly evolving from asymptotic giant branch (AGB) stars to Planetary Nebulae are presented. IRAM 30m observations for three of these post-AGBs are also reported. A few (3) of these sources are known to exhibit strongly variable maser emission of O-bearing molecules such as OH and H2 O, suggesting a transitory O-rich chemistry because of the quickly changing physical and chemical conditions in this short evolutionary phase. Interestingly, the Spitzer/IRS spectra show a rich circumstellar carbon chemistry, as revealed by the detection of small hydrocarbon molecules such as C2H2, C4H2, C6H2, C6H6, and HCN. Benzene is detected towards two sources, bringing up to three the total number of Galactic post-AGBs where this molecule has been detected. In addition, we report evidence for the possible detection of other hydrocarbon molecules like HC3N, CH3C2H, and CH3 in several of these sources. The available IRAM 30m data confirm that the central stars are C-rich - in despite of the presence of O-rich masers - and the presence of high velocity molecular outflows together with extreme AGB mass-loss rates (∼⃒10-4 Mʘ /yr). Our observations confirm the polymerization model of Cernicharo [1] that predicts a rich photochemistry in the neutral regions of these objects on timescales shorter than the dynamical evolution of the central HII region, leading to the formation of small C-rich molecules and a transitory O-rich chemistry.