C/O在PDR和行星形成盘中腈天体化学中的作用
The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
摘要：
复杂的腈，如HC 3 N和CH 3CN，在各种各样的天体物理环境中被观察到，包括在光子主导区（PDR）和行星形成盘的紫外线暴露大气中相对较高的丰度。后者已被推断为贫氧，这表明这些意见可能是由有机化学在富C环境中解释。在这项研究中，我们首先探讨，如果PDR复杂的腈观察可以解释气相PDR化学单独的元素C/O比升高。在马头PDR的情况下，我们发现，气相化学与C/O $\gtrsim$ 0.9确实可以解释所观察到的腈丰度，增加了几个数量级的预测丰度相比，标准的C/O假设。我们还发现，腈丰度是敏感的宇宙射线电离处理，并提供限制的分支比之间的CH 3 CN和CH 3 NC的产品。在一个基准盘模型中，升高的C/O比增加了CH 3CN和HC 3 N的产量超过一个数量级，使丰度预测在一个数量级内，从观测中推断出来。C/O比似乎是一个关键变量，在预测和解释复杂的有机分子丰度在光子主导的区域在一系列的尺度。
英文摘要：
Complex nitriles, such as HC3N, and CH3CN, are observed in a wide variety of astrophysical environments, including at relatively high abundances in photon-dominated regions (PDR) and the UV exposed atmospheres of planet-forming disks. The latter have been inferred to be oxygen-poor, suggesting that these observations may be explained by organic chemistry in C-rich environments. In this study we first explore if the PDR complex nitrile observations can be explained by gas-phase PDR chemistry alone if the elemental C/O ratio is elevated. In the case of the Horsehead PDR, we find that gas-phase chemistry with C/O $\gtrsim$ 0.9 can indeed explain the observed nitrile abundances, increasing predicted abundances by several orders of magnitude compared to standard C/O assumptions. We also find that the nitrile abundances are sensitive to the cosmic ray ionization treatment, and provide constraints on the branching ratios between CH3CN and CH3NC productions. In a fiducial disk model, an elevated C/O ratio increases the CH3CN and HC3N productions by more than an order of magnitude, bringing abundance predictions within an order of magnitude to what has been inferred from observations. The C/O ratio appears to be a key variable in predicting and interpreting complex organic molecule abundances in photon-dominated regions across a range of scales.
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