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arXiv 1201.2613astro-ph.GAastro-ph.EP
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原行星盘II中的化学过程。光化学与X射线电离的重要性

Chemical Processes in Protoplanetary Disks II. On the importance of photochemistry and X-ray ionization

Catherine Walsh, Hideko Nomura, T. J. Millar, Yuri Aikawa

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21 pages, 11 figures, 6 tables, accepted for publication in ApJ
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摘要：

我们研究了光化学和X射线电离对一个典型金牛座T星周围的原行星盘的分子组成和电离分数的影响。我们使用一个复杂的物理模型，其中包括一个强大的处理紫外线和X射线辐射的辐射转移，并计算与时间相关的化学结构，使用一个全面的化学网络。在以前的工作中，我们近似的光化学和X射线电离，在这里，我们重新计算的光反应速率显式紫外波长光谱和波长依赖的反应截面。我们重新计算的X射线电离率使用我们明确的元素组成和X射线能谱。我们发现光化学比X射线电离对分子组成有更大的影响。可观察到的对光酸盐敏感的分子包括OH、HCO+、N2 H+、H2O、CO2和CH 3OH。唯一受X射线电离显著影响的分子是N2 H+，这表明在典型的金牛座T星盘系统中采用现有的X射线电离率近似值是安全的。重新计算光电流增加了外盘中性分子的丰度，凸显了考虑原行星盘紫外光谱形状的重要性。由于H/H2和C+/C比率的调整，光反应速率的重新计算也影响气相化学。光化学和X射线电离的计算方法对盘电离率的影响不显著。我们确定有一个可能的“死区”的吸积被抑制，目前在一个层，Z/R <~ 0.1 - 0.2，在磁盘中平面，在R \approx 200 AU。

英文摘要：

We investigate the impact of photochemistry and X-ray ionization on the molecular composition of, and ionization fraction in, a protoplanetary disk surrounding a typical T Tauri star. We use a sophisticated physical model, which includes a robust treatment of the radiative transfer of UV and X-ray radiation, and calculate the time-dependent chemical structure using a comprehensive chemical network. In previous work, we approximated the photochemistry and X-ray ionization, here, we recalculate the photoreaction rates using the explicit UV wavelength spectrum and wavelength-dependent reaction cross sections. We recalculate the X-ray ionization rate using our explicit elemental composition and X-ray energy spectrum. We find photochemistry has a larger influence on the molecular composition than X-ray ionization. Observable molecules sensitive to the photorates include OH, HCO+, N2H+, H2O, CO2 and CH3OH. The only molecule significantly affected by the X-ray ionization is N2H+ indicating it is safe to adopt existing approximations of the X-ray ionization rate in typical T Tauri star-disk systems. The recalculation of the photorates increases the abundances of neutral molecules in the outer disk, highlighting the importance of taking into account the shape of the UV spectrum in protoplanetary disks. A recalculation of the photoreaction rates also affects the gas-phase chemistry due to the adjustment of the H/H2 and C+/C ratios. The disk ionization fraction is not significantly affected by the methods adopted to calculate the photochemistry and X-ray ionization. We determine there is a probable 'dead zone' where accretion is suppressed, present in a layer, Z/R <~ 0.1 - 0.2, in the disk midplane, within R \approx 200 AU.

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