MILLIMETER-WAVE OBSERVATIONS OF CN AND HNC AND THEIR 15N ISOTOPOLOGUES: A NEW EVALUATION OF THE 14N/15N RATIO ACROSS THE GALAXY

The N = 1 → 0 transitions of CN and C15N (X2Σ+), as well as the J = 1 → 0 lines of HN13C and H15NC, have been observed toward 11 molecular clouds using the new 3 mm ALMA-type receiver of the 12 m telescope of the Arizona Radio Observatory. These sources span a wide range of distances from the Galactic center and are all regions of star formation. From these observations, 14N/15N ratios have been determined using two independent methods. First, the measurements of C14N and C15N were directly compared to establish this ratio, correcting for high opacities when needed, as indicated by the nitrogen hyperfine intensities. Second, the ratio was calculated from the quantity [HN13C]/[H15NC], determined from the HNC data, and then scaled by 12C/13C ratios previously established, i.e., the so-called double isotope method. Values from both methods are in reasonable agreement, and fall in the range ∼120–400, somewhat lower than previous 14N/15N ratios derived from HCN. The ratios exhibit a distinct positive gradient with distance from the Galactic center, following the relationship14N/15N = 21.1 (5.2) kpc−1 DGC + 123.8 (37.1). This gradient is consistent with predictions of Galactic chemical evolution models in which 15N has a secondary origin in novae, while primary and secondary sources exist for 14N. The local interstellar medium value was found to be 4N/15N = 290 ± 40, in agreement with the ratio found in nearby diffuse clouds and close to the value of 272 found in Earth's atmosphere.