The C${}^{14}$N/C${}^{15}$N Ratio in Diffuse Molecular Clouds

Abstract

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

13 pages, 3 figures, accepted for publication in ApJ Letters

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