THE NITROGEN ISOTOPE ABUNDANCE IN THE GALAXY .1. THE GALACTIC DISK GRADIENT

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 d isk. After fitting C-12/C-13 ratios from published data as a function of galactocentric distance, we obtained the N-14/N-15 ratios. From the se results and previously published data, we have determined the galac tic disk gradient of the nitrogen isotope ratio using a linear regress ion. We have obtained a gradient of 19.7 +/- 8.9 kpc(-1). We have comp ared our result with the theoretical model of Matteucci & D'Antona (19 91) and conclude that N-15 is a purely secondary element; N-14 must al so be secondary, but with a significant primary contribution. We brief ly discuss the production of nitrogen isotopes involving massive stars ; however, it may be difficult to fit the C-12/C-13 gradient into this scenario.