The C/c Isotope Gradient Derived from Millimeter Transitions of Cn: the Case for Galactic Chemical Evolution

New measurements of C/C ratios in Galactic molecular clouds have been conducted using the N 1⁄4 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 (DGC). The ratios derived from CN indicate a gradient with Galactic distance of C/C 1⁄4 6:01DGC þ 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 C/C 1⁄4 6:21DGC þ 18:71. CO, CN, and H2CO are synthesized from quite varied reactions, and any C fractionation must follow different pathways for these three species. The relatively good agreement between the C/C 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 C/C gradient found in the Galaxy is a true indicator of Galactic chemical evolution. The apparent discrepancy between the solar system (C/C 1⁄4 89) and local interstellar medium values (C/C 68) of this ratio may be a result of C enrichment since the formation of the solar system, as predicted by recent models. Subject headinggs: astrochemistry — Galaxy: evolution — ISM: abundances — ISM: clouds — radio lines: ISM — stars: AGB and post-AGB