行星状星云中的合并电离/离解锋
Merged ionization/dissociation fronts in planetary nebulae
摘要：
当电离光子通量与气体密度之比足够低且光谱足够硬时，紫外辐射源周围的氢电离和解离锋应该合并。这一机制特别适用于在已演化的行星状星云中常见的分子结，如赫布里底星云，在那里，传统的光解区模型已被证明无法解释H_2谱线中观测到的高亮度。在本文中，我们提出的结构和稳态动力学的这种对流为主的合并前，使用多云等离子体/分子物理代码计算的结果。我们发现H_2的主要破坏过程是极紫外光电离和与质子的电荷交换反应，这两个过程都形成H_2^+，H_2^+迅速与自由电子结合，进行解离复合。平流将离解前沿移动到比静态情况下更低的柱密度，这大大增加了部分分子气体中由于He^0、H_2和H^0的光电离而产生的加热。这导致相当一部分热辐射通量以热激发红外H_2线的形式再辐射，其中低激发纯转动线出现在1000 K气体中，高激发H_2线出现在2000 K气体中，这是解释赫氏彗星结H_2谱所需要的。
英文摘要：
The hydrogen ionization and dissociation front around an ultraviolet radiation source should merge when the ratio of ionizing photon flux to gas density is sufficiently low and the spectrum is sufficiently hard. This regime is particularly relevant to the molecular knots that are commonly found in evolved planetary nebulae, such as the Helix Nebula, where traditional models of photodissociation regions have proved unable to explain the high observed luminosity in H_2 lines. In this paper we present results for the structure and steady-state dynamics of such advection-dominated merged fronts, calculated using the Cloudy plasma/molecular physics code. We find that the principal destruction processes for H_2 are photoionization by extreme ultraviolet radiation and charge exchange reactions with protons, both of which form H_2^+, which rapidly combines with free electrons to undergo dissociative recombination. Advection moves the dissociation front to lower column densities than in the static case, which vastly increases the heating in the partially molecular gas due to photoionization of He^0, H_2, and H^0. This causes a significant fraction of the incident bolometric flux to be re-radiated as thermally excited infrared H_2 lines, with the lower excitation pure rotational lines arising in 1000 K gas and higher excitation H_2 lines arising in 2000 K gas, as is required to explain the H_2 spectrum of the Helix cometary knots.
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