ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N2H+ RING IN PROTOPLANETARY DISKS
ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N2H+ RING IN PROTOPLANETARY DISKS
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DOI：
10.1088/0004-637x/807/2/120
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发表时间：
2015-05
期刊：
影响因子：
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通讯作者：
Y. Aikawa;K. Furuya;H. Nomura;C. Qi
中科院分区：
文献类型：
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作者：
Y. Aikawa;K. Furuya;H. Nomura;C. Qi
我们调查的原行星盘的离子分子的化学，由TW Hya周围的N2 H+环检测的动机。虽然环的内半径与CO雪线重合，但尽管CO和N2的升华温度相似，但为什么N2 H+在CO雪线外丰富却不明显。利用全气体-颗粒网络模型，我们再现了毫米颗粒圆盘模型中的N_2H ~+环。CO和N2向低挥发性物种的化学转化（下文称为汇效应）被发现影响N2 H+分布。由于汇的效率取决于各种参数，如颗粒表面反应的活化势垒，这是没有很好的约束，我们还构建了无汇模型;总（气体和冰）CO和N2丰度设置为常数，和它们的气体丰度由吸附和解吸之间的平衡。通过对全网络模型的分析，推导出了无汇模型中分子离子丰度的解析计算公式。N2 H+环也由无汇模型再现。然而，N2 H+的二维（R-Z）分布在全网络模型和无汇模型中是不同的。在无汇模型中，N_2H ~+的柱密度敏感地依赖于CO和N_2的解吸速率以及宇宙线通量。我们还发现，即使CO和N2的脱附能相同，N2 H+丰度也可以在略低于CO升华的温度下达到峰值。
We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of the N2H+ ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N2H+ is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N2. Using the full gas-grain network model, we reproduced the N2H+ ring in a disk model with millimeter grains. The chemical conversion of CO and N2 to less volatile species (sink effect hereinafter) is found to affect the N2H+ distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain-surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N2 abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulae, which are derived by analyzing the full-network model. The N2H+ ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N2H+, however, is different among the full-network model and no-sink model. The column density of N2H+ in the no-sink model depends sensitively on the desorption rate of CO and N2 and the cosmic-ray flux. We also found that N2H+ abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N2 are the same.