The (Cn)-N-14/(Cn)-N-15 Ratio in Diffuse Molecular Clouds

We report the first detection of (CN)-N-15 in diffuse molecular gas from a detailed examination of CN absorption lines in archival spectra, obtained with the Ultraviolet and Visual Echelle Spectrograph of the Very Large Telescope of stars probing local diffuse clouds. Absorption from the (CN)-N-15 isotopologue is confidently detected ( at greater than or similar to 4 sigma) in three out of the four directions studied and appears as a very weak feature between the main (12) CN and (CN)-C-13 absorption components. Column densities for each CN isotopologue are determined through profile fitting, after accounting for weak additional line-of-sight components of (CN)-C-12, which are seen in the absorption profiles of CH and CH+ as well. The weighted mean value of N-14/(CN)-N-15 for the three sight lines with detections of (CN)-N-15 is 274 +/- 18. Since the diffuse molecular clouds toward our target stars have relatively high gas kinetic temperatures and relatively low visual extinctions, their N-14/(CN)-N-15 ratios should not be affected by chemical fractionation. The mean N-14/(CN)-N-15 ratio that we obtain should therefore be representative of the ambient N-14/N-15 ratio in the local interstellar medium. Indeed, our mean value agrees well with that derived from millimeter-wave observations of CN, HCN, and HNC in local molecular clouds.