INTERSTELLAR CN AND CH+ IN DIFFUSE MOLECULAR CLOUDS: 12C/13C RATIOS AND CN EXCITATION
INTERSTELLAR CN AND CH+ IN DIFFUSE MOLECULAR CLOUDS: 12C/13C RATIOS AND CN EXCITATION
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弥散分子云中的星际 CN 和 CH：12C/13C 比率和 CN 激发
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
D. Lambert
中科院分区：
文献类型：
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作者：
A. Ritchey;S. Federman;D. Lambert
我们提出了非常高的信噪比的CN和CH+吸收线观测沿着13条视线通过弥漫分子云。检查数据，以提取12 CN/13 CN和12 CH +/13 CH+的精确同位素比，以评估扩散云化学的预测。我们对12 CH +/13 CH+的研究结果证实，如果CH+的形成涉及非热过程，则该比率不会偏离局部星际云中的环境12 C/13 C比率。我们发现，12 CN/13 CN，然而，可以显着分馏远离环境值。在我们的样品中的12 CN/13 CN比率的分散性与最近的12 CO/13 CO调查中发现的相似。对于已经确定两个比率的视线，12 CN/13 CN比率通常以与12 CO/13 CO相反的意义进行分馏。CO中的化学分馏是选择性光解和同位素电荷交换（ICE）之间竞争的结果。12 CN/13 CN和12 CO/13 CO之间的反比关系来自于扩散云核中CN和CO的共存。然而，与CN的ICE反应可以减轻对于具有低12 CO/13 CO比率的视线的12 CN/13 CN的增强。对于两个视线与高值的12 CO/13 CO，我们的研究结果表明，约50%的碳被锁定在CO，这是一致的概念，这些视线探测分子云信封的过渡从C+到CO预计发生。CN转动激发的分析得出T01（12 CN）的加权平均值为2.754 ± 0.002 K，这意味着超过宇宙微波背景（CMB）的温度仅为29 ± 3 mK。这种适度的过量消除了对电子和中性碰撞之外的局部激发机制的需要。13 CN的转动激发温度没有超过CMB的温度。
We present very high signal-to-noise ratio absorption-line observations of CN and CH+ along 13 lines of sight through diffuse molecular clouds. The data are examined to extract precise isotopologic ratios of 12CN/13CN and 12CH+/13CH+ in order to assess predictions of diffuse cloud chemistry. Our results on 12CH+/13CH+ confirm that this ratio does not deviate from the ambient 12C/13C ratio in local interstellar clouds, as expected if the formation of CH+ involves nonthermal processes. We find that 12CN/13CN, however, can be significantly fractionated away from the ambient value. The dispersion in our sample of 12CN/13CN ratios is similar to that found in recent surveys of 12CO/13CO. For sight lines where both ratios have been determined, the 12CN/13CN ratios are generally fractionated in the opposite sense compared to 12CO/13CO. Chemical fractionation in CO results from competition between selective photodissociation and isotopic charge exchange (ICE). An inverse relationship between 12CN/13CN and 12CO/13CO follows from the coexistence of CN and CO in diffuse cloud cores. However, an ICE reaction with CN may mitigate the enhancements in 12CN/13CN for lines of sight with low 12CO/13CO ratios. For two sight lines with high values of 12CO/13CO, our results indicate that about 50% of the carbon is locked up in CO, which is consistent with the notion that these sight lines probe molecular cloud envelopes where the transition from C+ to CO is expected to occur. An analysis of CN rotational excitation yields a weighted mean value for T01(12CN) of 2.754 ± 0.002 K, which implies an excess over the temperature of the cosmic microwave background (CMB) of only 29 ± 3 mK. This modest excess eliminates the need for a local excitation mechanism beyond electron and neutral collisions. The rotational excitation temperatures in 13CN show no excess over the temperature of the CMB.