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 (CN)-N-15 (X-2 Sigma(+)), as well as the J = 1 -> 0 lines of (HNC)-C-13 and (HNC)-N-15, 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, N-14/N-15 ratios have been determined using two independent methods. First, the measurements of (CN)-N-14 and (CN)-N-15 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 [(HNC)-C-13]/[(HNC)-N-15], determined from the HNC data, and then scaled by C-12/C-13 ratios previously established, i.e., the so-called double isotope method. Values from both methods are in reasonable agreement, and fall in the range similar to 120-400, somewhat lower than previous N-14/N-15 ratios derived from HCN. The ratios exhibit a distinct positive gradient with distance from the Galactic center, following the relationship N-14/N-15 = 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 N-15 has a secondary origin in novae, while primary and secondary sources exist for N-14. The local interstellar medium value was found to be N-4/N-15 = 290 +/- 40, in agreement with the ratio found in nearby diffuse clouds and close to the value of 272 found in Earth's atmosphere.