arXiv 1111.42182011-11astro-ph.GA

星前核动态演化的非平衡化学：II。电离与磁场

Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: II. Ionization and Magnetic Field

Konstantinos Tassis, Karen Willacy, Harold W. Yorke, Neal Turner

备注：: 7 pages, 5 figures, accepted for publication in ApJ
DOI：: 10.1088/0004-637X/754/1/6

摘要：

我们研究了在坍缩分子云核的动力学模型中的非平衡化学对测量这些核中的磁场、电离度和离子的平均分子量的影响。我们发现，OH和CN，通常用于塞曼观测的视线磁场，有一个丰富的减少对核心的中心比密度增加快得多。因此，塞曼观测倾向于对地核的外层进行采样，并始终低估了地核的磁场。电离度与数密度之间存在复杂的依赖关系，磁性模型的中心密度可达10^5 cm^{-3}，非磁性模型的中心密度可达10^6 cm^{-3}。在更高的中心密度下，标度接近于幂律，斜率为-0.6，归一化依赖于宇宙射线电离率τ和温度T，为（τ T）^1/2。离子的平均分子量系统地低于通常假设的值20 - 30，并且在高密度下，由于H3+离子的渐近优势，接近值3。该显著较低的值意味着双极扩散操作得更快。

英文摘要：

We study the effect that non-equilibrium chemistry in dynamical models of collapsing molecular cloud cores has on measurements of the magnetic field in these cores, the degree of ionization, and the mean molecular weight of ions. We find that OH and CN, usually used in Zeeman observations of the line-of-sight magnetic field, have an abundance that decreases toward the center of the core much faster than the density increases. As a result, Zeeman observations tend to sample the outer layers of the core and consistently underestimate the core magnetic field. The degree of ionization follows a complicated dependence on the number density at central densities up to 10^5 cm^{-3} for magnetic models and 10^6 cm^{-3} in non-magnetic models. At higher central densities the scaling approaches a power-law with a slope of -0.6 and a normalization which depends on the cosmic-ray ionization rate ζ and the temperature T as (ζT)^1/2. The mean molecular weight of ions is systematically lower than the usually assumed value of 20 - 30, and, at high densities, approaches a value of 3 due to the asymptotic dominance of the H3+ ion. This significantly lower value implies that ambipolar diffusion operates faster.

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英文摘要：

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