The C14N/C15N Ratio in Diffuse Molecular Clouds
Abstract
We report the first detection of C15N in diffuse molecular gas from a detailed examination of CN absorption lines in archival VLT/UVES spectra of stars probing local diffuse clouds. Absorption from the C15N isotopologue is confidently detected (at 4σ) 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 27418. 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.