Galactic¹²C/¹³C Ratios from Millimeter‐Wave Observations of Interstellar CN

The N = 1 → 0 transitions of 12 CN and 13 CN( X 2 Σ + ) at 113.5 and 108.8 GHz, respectively, have been observed in a sample of 13 Galactic molecular clouds using the Kitt Peak 12 m radio telescope. The objects studied include the Galactic center [SgrB2(OH)], sources in the solar neighborhood such as Orion A and NGC 2024, and various other clouds with and without star formation. Hyperfine structure, arising from the nitrogen nuclear spin, was resolved in the spectra of both species, enabling an accurate determination of the opacity in 12 CN. From these measurements, estimates of the 12 C/ 13 C isotope ratio were obtained. These values fall in the range 12 C/ 13 C ~ 20-70 and exhibit a noticeable gradient with distance from the Galactic center. In general, the ratios obtained from CN are very similar to those determined from millimeter observations of CO but are consistently lower than those derived from H 2 CO. If chemical fractionation is occurring in CN, it is comparable to that in CO. The highest 12 C/ 13 C ratios (65 ± 12 and 70 ± 11) were obtained toward two known photon-dominated regions (the Orion Bar and NGC 2024); CN in these two sources may be undergoing some isotope-selective photodissociation. The 12 C/ 13 C ratio of 43 ± 7, found in Orion A, is similar to that determined from optical observations of CN toward a nearby source, ζ Ophiuchi, but significantly lower than the average value found in comets (90 ± 10). This difference suggests that substantial enrichment of 13 C has occurred during the past 4.6 Gyr in the solar neighborhood.