标题： 星际空间中的CN和CH+在弥漫分子云中：12C/13C比率与CN激发 显示英文标题

标题： Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation

摘要： 我们展示了沿13条视线穿过弥散分子云的高信噪比的CN和CH+吸收线观测数据。 这些数据被分析以提取精确的同位素比值12CN/13CN和12CH+/13CH+，以便评估弥散云化学的预测结果。 我们的12CH+/13CH+的结果证实，该比值不会偏离本地星际云中的环境12C/13C比值，正如预期的那样，如果CH+的形成涉及非热过程的话。 然而，我们发现12CN/13CN可以显著偏离环境值。 我们样本中的12CN/13CN比值的离散程度与最近的12CO/13CO调查中发现的相似。 对于那些两种比值都已确定的视线，12CN/13CN比值通常与12CO/13CO比值反向分馏。 CO中的同位素分馏源于选择性光解离和同位素电荷交换之间的竞争。 由于CN和CO在弥散云核中共存，因此12CN/13CN与12CO/13CO之间存在反比关系。 然而，与CN的同位素电荷交换反应可能会缓解低12CO/13CO比值视线上的12CN/13CN增强现象。 对于两条具有高12CO/13CO比值的视线，我们的结果显示约50%的碳锁定在CO中，这与这些视线探测到分子云包层的观点一致，在这些包层中预计会发生从C+到CO的转变。 CN旋转激发的分析给出了T_01(12CN)的加权平均值为2.754 ± 0.002 K，这意味着仅比宇宙微波背景温度高出29 ± 3毫开尔文。 这种适度的过剩排除了需要局部激发机制（除了电子和中性碰撞）的可能性。 13CN的旋转激发温度没有超过CMB的温度。 显示英文摘要

摘要： 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. An inverse relationship between 12CN/13CN and 12CO/13CO follows from the coexistence of CN and CO in diffuse cloud cores. However, an isotopic charge exchange 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 percent 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 T_01(12CN) of 2.754 +/- 0.002 K, which implies an excess over the temperature of the cosmic microwave background 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.