2011ApJ...728...36R

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.