化学在中国III。--DM Tau、LkCa 15和MWC 480中乙炔基（CCH）探测的光化学和X射线驱动化学
Chemistry in Disks. III. -- Photochemistry and X-ray driven chemistry probed by the ethynyl radical (CCH) in DM Tau, LkCa 15, and MWC 480
摘要：
我们研究了几个有代表性的拱盘周围的赫比格Ae星MWC 480和金牛T星LkCa 15和DM Tau在（亚）毫米波长的CCH线。我们的目标是在严重的紫外线照射MWC 480磁盘的光化学特性和比较结果的磁盘周围较冷的金牛座T星。我们用IRAM Plateau de Bure干涉仪在这些盘的C-和D-构型的（1-0）和（2-1）跃迁中检测和映射了CCH。使用迭代最小化技术，CCH柱密度和激发条件被约束。非常低的激发温度的金牛T星。这些值进行了比较与先进的化学建模，这是基于一个稳态的喇叭形的磁盘结构与垂直的温度梯度，和气体-晶粒化学网络与表面反应的结果。模型和观测结果都表明，CCH是X射线和紫外辐射的敏感示踪剂。预测的径向依赖性和源到源的CCH柱密度的变化定性地同意与观测值，但预测的柱密度太低的几个因素。化学模型不能再现高浓度的CCH在非常冷的磁盘中平面，从观察到的低激发条件的（1-0）和（2-1）的转变。
英文摘要：
We studied several representative circumstellar disks surrounding the Herbig Ae star MWC 480 and the T Tauri stars LkCa 15 and DM Tau at (sub-)millimeter wavelengths in lines of CCH. Our aim is to characterize photochemistry in the heavily UV-irradiated MWC 480 disk and compare the results to the disks around cooler T Tauri stars. We detected and mapped CCH in these disks with the IRAM Plateau de Bure Interferome- ter in the C- and D-configurations in the (1-0) and (2-1) transitions. Using an iterative minimization technique, the CCH column densities and excitation conditions are con- strained. Very low excitation temperatures are derived for the T Tauri stars. These values are compared with the results of advanced chemical modeling, which is based on a steady-state flared disk structure with a vertical temperature gradient, and a gas- grain chemical network with surface reactions. Both model and observations suggest that CCH is a sensitive tracer of the X-ray and UV irradiation. The predicted radial dependency and source to source variations of CCH column densities qualitatively agree with the observed values, but the predicted column densities are too low by a factor of several. The chemical model fails to reproduce high concentrations of CCH in very cold disk midplane as derived from the observed low excitation condition for both the (1-0) and (2-1) transitions.
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