The 12C/13C Isotope Gradient Derived from Millimeter Transitions of CN: The Case for Galactic Chemical Evolution

New measurements of ¹²C/¹³C ratios in Galactic molecular clouds have been conducted using the N=1-->0 transition of the CN radical. This species is unique in that it has extensive hyperfine structure that can be accurately used to correct for line saturation effects. Combined with the past observations of Savage and coworkers, the ratios derived from CN are the most extensive data set to date for molecular clouds, and they include sources that lie in the range of 0.09-16.41 kpc in distance from the Galactic center (D_GC). The ratios derived from CN indicate a gradient with Galactic distance of ¹²C/¹³C=6.01D_GC+12.28. This gradient agrees rather closely with those derived from measurements of CO and H₂CO. The least-squares fit to all data points for the three molecules is ¹²C/¹³C=6.21D_GC+18.71. CO, CN, and H₂CO are synthesized from quite varied reactions, and any ¹³C fractionation must follow different pathways for these three species. The relatively good agreement between the ¹²C/¹³C ratios of the three molecules, as well as their lack of correlation with gas kinetic temperature, suggests that chemical fractionation and isotope-selective photodissociation both do not play a substantial role in influencing such ratios. Therefore, the ¹²C/¹³C gradient found in the Galaxy is a true indicator of Galactic chemical evolution. The apparent discrepancy between the solar system (¹²C/¹³C=89) and local interstellar medium values (¹²C/¹³C~68) of this ratio may be a result of ¹³C enrichment since the formation of the solar system, as predicted by recent models.