扩散分子云中H3 ~+的邻对位比
On the Ortho:Para Ratio of H3+ in Diffuse Molecular Clouds
摘要：
由H_2的J = 0（对位）和J = 1（邻位）转动能级的相对强度导出的激发温度T_01被认为是星际环境中动力学温度的精确量度。在弥散分子云中，T01的平均值约为70 K.然而，在相同类型的环境中，由H3+的（J，K）=（1，1）（对位）和（1，0）（邻位）转动能级导出的激发温度T（H3+）被观察到约为30 K。在这项工作中，我们提出的观察H3+在三个额外的弥漫云视线的H2测量是可用的，表明在4的5例T_01和T（H3+）是不一致的。然后，我们研究的热化机制的邻：对位比的H3+和H2，得出的结论是，T_01确实是一个准确的测量云动力学温度，而邻：对位比的H3+不需要热。通过构建一个稳态的化学模型，考虑到核spindependence的反应涉及H3+，我们表明，邻：对位比的H3+在扩散的分子云可能是由解离重组与电子和热化之间的竞争通过与H2的反应碰撞。
英文摘要：
The excitation temperature T_01 derived from the relative intensities of the J = 0 (para) and J = 1 (ortho) rotational levels of H2 has been assumed to be an accurate measure of the kinetic temperature in interstellar environments. In diffuse molecular clouds, the average value of T_01 is ~70 K. However, the excitation temperature T(H3+) derived from the (J,K) = (1,1) (para) and (1,0) (ortho) rotational levels of H3+ has been observed to be ~30 K in the same types of environments. In this work, we present observations of H3+ in three additional diffuse cloud sight lines for which H2 measurements are available, showing that in 4 of 5 cases T_01 and T(H3+) are discrepant. We then examine the thermalization mechanisms for the ortho:para ratios of H3+ and H2, concluding that indeed T_01 is an accurate measure of the cloud kinetic temperature, while the ortho:para ratio of H3+ need not be thermal. By constructing a steady-state chemical model taking into account the nuclear-spindependence of reactions involving H3+, we show that the ortho:para ratio of H3+ in diffuse molecular clouds is likely governed by a competition between dissociative recombination with electrons and thermalization via reactive collisions with H2.
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