SIMBAD references

We present high quality data of HCN double isotopomer ratios observed in the J=1-0 line of 11 warm, dense molecular clouds in the galactic disk. After fitting ¹²C/¹³C ratios from published data as a function of galactocentric distance, we obtained the ¹⁴N/¹⁵N ratios. From these results and previously published data, we have determined the galactic disk gradient of the nitrogen isotope ratio using a linear regression. We have obtained a gradient of 19.7±8.9kpc^–1. We have compared our result with the theoretical model of Matteucci & D'Antona (1991A&A...247L..37M) and conclude that ¹⁵N is a purely secondary element; ¹⁴N must also be secondary, but with a significant primary contribution. We briefly discuss the production of nitrogen isotopes involving massive stars; however, it may be difficult to fit the ¹²C/¹³C gradient into this scenario.