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 ≳ 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 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.Based on observations made with the Very Large Telescope of the European Southern Observatory, Paranal, Chile, under programs 065.I-0526, 071.C-0367, 071.C-0513, 076.C-0431, and 092.C-0019.
The Astrophysical Journal
- Pub Date:
- May 2015
- ISM: lines and bands;
- ISM: molecules;
- Astrophysics - Astrophysics of Galaxies
- 13 pages, 3 figures, accepted for publication in ApJ Letters