2021ApJS..257...39C

Our aim is to measure the interstellar ¹⁴N/¹⁵N ratio across the Galaxy, to establish a standard data set on interstellar ammonia isotope ratios, and to provide new constraints on the Galactic chemical evolution. The (J, K) = (1, 1), (2, 2), and (3, 3) lines of ¹⁴NH₃ and ¹⁵NH₃ were observed with the Shanghai Tianma 65 m radio telescope (TMRT) and the Effelsberg 100 m telescope toward a large sample of 210 sources. One hundred fourty-one of these sources were detected by the TMRT in ¹⁴NH₃. Eight of them were also detected in ¹⁵NH₃. For 10 of the 36 sources with strong NH₃ emission, the Effelsberg 100 m telescope successfully detected their ¹⁵NH₃(1, 1) lines, including 3 sources (G081.7522, W51D, and Orion-KL) with detections by the TMRT telescope. Thus, a total of 15 sources are detected in both the ¹⁴NH₃ and ¹⁵NH₃ lines. Line and physical parameters for these 15 sources are derived, including optical depths, rotation and kinetic temperatures, and total column densities. ¹⁴N/¹⁵N isotope ratios were determined from the ¹⁴NH₃/¹⁵NH₃ abundance ratios. The isotope ratios obtained from both telescopes agree for a given source within the uncertainties, and no dependence on heliocentric distance and kinetic temperature is seen. ¹⁴N/¹⁵N ratios tend to increase with galactocentric distance, confirming a radial nitrogen isotope gradient. This is consistent with results from recent Galactic chemical model calculations, including the impact of superasymptotic giant branch stars and novae.