CHEMICAL PROCESSES IN PROTOPLANETARY DISKS. II. ON THE IMPORTANCE OF PHOTOCHEMISTRY AND X-RAY IONIZATION
CHEMICAL PROCESSES IN PROTOPLANETARY DISKS. II. ON THE IMPORTANCE OF PHOTOCHEMISTRY AND X-RAY IONIZATION
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DOI：
10.1088/0004-637x/747/2/114
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发表时间：
2012-01
期刊：
影响因子：
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通讯作者：
C. Walsh;H. Nomura;T. Millar;Yuri Aikawa Queen's University Belfast;K. University;Kobe University
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文献类型：
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作者：
C. Walsh;H. Nomura;T. Millar;Yuri Aikawa Queen's University Belfast;K. University;Kobe University
我们调查的影响，光化学和X射线电离的分子组成，电离分数，一个典型的金牛座T星星周围的原行星盘。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射线辐射的辐射传输，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使用明确的紫外波长光谱和波长依赖的反应截面。我们重新计算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。唯一受到X射线电离影响的分子是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。重新计算的光电流增加了中性分子在外盘的丰度，突出了考虑到紫外光谱的形状在原行星盘的重要性。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”，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 <$200 Au。
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 that 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 that 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 that 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 ≈ 200 AU.