2015ApJ...804L...3R

We report the first detection of C¹⁵N 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 C¹⁵N isotopologue is confidently detected (at ≳ 4 σ) in three out of the four directions studied and appears as a very weak feature between the main ¹²CN and ¹³CN absorption components. Column densities for each CN isotopologue are determined through profile fitting, after accounting for weak additional line-of-sight components of ¹²CN, which are seen in the absorption profiles of CH and CH⁺ as well. The weighted mean value of C¹⁴N/C¹⁵N for the three sight lines with detections of C¹⁵N is 274±18. Since the diffuse molecular clouds toward our target stars have relatively high gas kinetic temperatures and relatively low visual extinctions, their C¹⁴N/C¹⁵N ratios should not be affected by chemical fractionation. The mean C¹⁴N/C¹⁵N ratio that we obtain should therefore be representative of the ambient ¹⁴N/¹⁵N 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.