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 (X ²Σ⁺), as well as the J= 1→ 0 lines of HN ¹³C 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 D _GC + 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.