On the ionisation fraction in protoplanetary disks. I. Comparing different reaction networks
On the ionisation fraction in protoplanetary disks. I. Comparing different reaction networks
复制标题
关于原行星盘中的电离分数。
DOI：
10.1051/0004-6361:20053678
复制
发表时间：
2005
影响因子：
6.5
通讯作者：
R. Nelson
中科院分区：
文献类型：
--
作者：
M. Ilgner;R. Nelson
我们使用许多不同的化学反应网络（包括气相和气粒反应方案）计算原恒星盘模型中的电离分数。我们考虑的盘模型是传统的α盘，包括粘性加热和辐射冷却。电离的主要来源被认为是来自中心恒星的 X 射线辐射。对于大多数计算，我们采用特定的盘模型（吸积率 ${\dot M}=10^{-7}$ $M_{\odot}$ yr -1 和 $\alpha=10_{}^{-2}$），并检查化学网络对电离分数、磁雷诺数和磁活性区域空间范围的预测。这是为了帮助比较不同的化学模型。我们考虑了许多气相化学网络。最简单的是 Oppenheimer 和 Dalgarno (1974) 提出的五种模型。我们通过从 UMIST 数据库中提取物种和反应来构建更复杂的模型。一般来说，我们发现简单的模型比更复杂的模型预测更高的分数电离水平和更广泛的活性区域。当包含重金属原子时，简单的模型预测圆盘自始至终都具有磁活性。复杂的模型预测，即使镁的丰度分数 $x_{\rm Mg}=10^{-8}$，圆盘的大部分区域仍保持磁不耦合（“死”）。这是因为形成了大量的分子离子，在镁存在的情况下，分子离子继续主导与自由电子的复合。添加浓度为 $x_{\rm gr}=10^{-12}$ 的亚微米尺寸颗粒会导致所有考虑的动力学模型的“死区”尺寸急剧增加，因为颗粒能够高效地清除自由电子。我们发现简单和复杂的气体-颗粒反应方案在所产生的“死区”的大小和结构上是一致的，因为颗粒在确定电离分数中起着主导作用。我们研究了消耗小颗粒浓度的影响，作为模拟行星形成过程中颗粒生长的粗略方法。我们发现，当不存在重金属时，10 -4 的消耗因子会导致气粒化学收敛到气相化学。当包含镁时，需要10 -8 的损耗因子来再现气相电离分数。这表明，在 1-10 个天文单位之间的行星形成区中的大部分盘质量变得具有磁活性和湍流之前，原行星盘需要有效的晶粒生长和沉降。只有在这种情况发生之后，气相化学模型才能用于可靠地预测原行星盘中的电离程度。
We calculate the ionisation fraction in protostellar disk models using a number of different chemical reaction networks, including gas-phase and gas-grain reaction schemes. The disk models we consider are conventional α -disks, which include viscous heating and radiative cooling. The primary source of ionisation is assumed to be X-ray irradiation from the central star. For most calculations we adopt a specific disk model (with accretion rate ${\dot M}=10^{-7}$ $M_{\odot}$ yr -1 and $\alpha=10_{}^{-2}$), and examine the predictions made by the chemical networks concerning the ionisation fraction, magnetic Reynolds number, and spatial extent of magnetically active regions. This is to aid comparison between the different chemical models. We consider a number of gas-phase chemical networks. The simplest is the five species model proposed by Oppenheimer & Dalgarno (1974). We construct more complex models by extracting species and reactions from the UMIST data base. In general we find that the simple models predict higher fractional ionisation levels and more extensive active zones than the more complex models. When heavy metal atoms are included the simple models predict that the disk is magnetically active throughout. The complex models predict that extensive regions of the disk remain magnetically uncoupled (“dead”) even when the fractional abundance of magnesium $x_{\rm Mg}=10^{-8}$. This is because of the large number of molecular ions that are formed, which continue to dominate the recombination with free electrons in the presence of magnesium. The addition of submicron sized grains with a concentration of $x_{\rm gr}=10^{-12}$ causes the size of the “dead zone” to increase dramatically for all kinetic models considered, as the grains are highly efficient at sweeping up the free electrons. We find that the simple and complex gas-grain reaction schemes agree on the size and structure of the resulting “dead zone”, as the grains play a dominant role in determining the ionisation fraction. We examine the effects of depleting the concentration of small grains as a crude means of modeling the growth of grains during planet formation. We find that a depletion factor of 10 -4 causes the gas-grain chemistry to converge to the gas-phase chemistry when heavy metals are absent. When magnesium is included a depletion factor of 10 -8 is required to reproduce the gas-phase ionisation fraction. This suggests that efficient grain growth and settling will be required in protoplanetary disks, before a substantial fraction of the disk mass in the planet forming zone between 1–10 AU becomes magnetically active and turbulent. Only after this has occurred can gas-phase chemical models be used to predict reliably the ionisation degree in protoplanetary disks.