Self-Consistent Dynamic Models of Steady Ionization Fronts. I. Weak-D and Weak-R Fronts
Self-Consistent Dynamic Models of Steady Ionization Fronts. I. Weak-D and Weak-R Fronts
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稳定电离前沿的自洽动态模型。
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
G. Ferland
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作者：
W. Henney;S. J. Arthur;Robin J. R. Williams;G. Ferland
我们提出了一种方法，包括在等离子体物理代码多云，这是以前仅限于建模静态配置的稳态气体流动。详细描述的数值算法，连同一个例子应用到平面平行电离有界H II区域。除了为将来应用于更复杂的流动提供基础外，我们还发现了以下关于平流对H II区电离锋的影响的具体结果：（1）平流对全局发射特性的显著直接影响仅在电离参数低于H II区的典型参数时发生。对于较高的电离参数，平流效应是间接的，主要局限于电离锋附近。(2)部分电离气体在锋面的过热并不大，即使对于超音速（R型）锋面也是如此。对于亚音速（D型）锋，我们没有发现以前声称的温度峰值。(3)平流锋的最显著的形态特征是电子密度尖峰，每当电离气体和前沿之间的相对速度超过约一半的电离等温声速时，电子密度尖峰发生在电离前沿。在猎户座棒的[N II] λ6584图像中发现了这种尖峰的观测证据。(4)平面平行，弱D的前线被发现，在最好的平均速度和电离势的光发射线之间的浅相关性，即使当流速接近电离声速。速度与电离的陡峭梯度，如在猎户座星云中观察到的，似乎需要跨音速流动，这只有在包括辐射力的发散几何中才可能。
We present a method for including steady state gas flows in the plasma physics code Cloudy, which was previously restricted to modeling static configurations. The numerical algorithms are described in detail, together with an example application to plane-parallel ionization-bounded H II regions. As well as providing the foundation for future applications to more complex flows, we find the following specific results regarding the effect of advection on ionization fronts in H II regions: (1) Significant direct effects of advection on the global emission properties occur only when the ionization parameter is lower than is typical for H II regions. For higher ionization parameters, advective effects are indirect and largely confined to the immediate vicinity of the ionization front. (2) The overheating of partially ionized gas in the front is not large, even for supersonic (R-type) fronts. For subsonic (D-type) fronts we do not find the temperature spike that has been previously claimed. (3) The most significant morphological signature of advective fronts is an electron density spike that occurs at the ionization front whenever the relative velocity between the ionized gas and the front exceeds about one-half the ionized isothermal sound speed. Observational evidence for such a spike is found in [N II] λ6584 images of the Orion bar. (4) Plane-parallel, weak-D fronts are found to show at best a shallow correlation between mean velocity and ionization potential for optical emission lines even when the flow velocity closely approaches the ionized sound speed. Steep gradients in velocity versus ionization, such as those observed in the Orion Nebula, seem to require transonic flows, which are only possible in a diverging geometry when radiation forces are included.