Interstellar Nitrogen Isotope Ratios: New NH 3 Data from the Galactic Center out to the Perseus Arm

Abstract

Our aim is to measure the interstellar 14 N/ 15 N 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 14 NH 3 and 15 NH 3 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 14 NH 3 . Eight of them were also detected in 15 NH 3 . For 10 of the 36 sources with strong NH 3 emission, the Effelsberg 100 m telescope successfully detected their 15 NH 3 (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 14 NH 3 and 15 NH 3 lines. Line and physical parameters for these 15 sources are derived, including optical depths, rotation and kinetic temperatures, and total column densities. 14 N/ 15 N isotope ratios were determined from the 14 NH 3 / 15 NH 3 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. 14 N/ 15 N 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.