¹⁴N/¹⁵N abundance ratio toward massive star-forming regions with different Galactic distances

Context. The abundance ratio of 14 N/ 15 N is, in principle, a powerful tool for tracing stellar nucleosynthesis. Aims. This work aims to measure and analyze ( 14 N/ 15 N) × ( 13 C/ 12 C) and 14 N/ 15 N abundance ratios in massive star-forming regions across a range of galactocentric distances to provide constraints on galactic chemical evolution (GCE) models. Methods. We present H 13 CN and HC 15 N J = 2–1 results toward 51massive star-forming regions obtained with the Institut de Radioastronomie Millimétrique (IRAM) 30 meter telescope. We used these results to derive ( 14 N/ 15 N) × ( 13 C/ 12 C) abundance ratios as well as 14 N/ 15 N ratios using the double isotope method. Results. We find an overall decreasing trend in the ( 14 N/ 15 N) × ( 13 C/ 12 C) abundance ratio and an increasing trend in the 14 N/ 15 N ratio with increasing galactocentric distance ( D GC ), which provides a good constraint for the GCE model based on high signal to noise ratio measurements. While the predicted ( 14 N/ 15 N) × ( 13 C/ 12 C) ratios between 6 and 12 kpc determined using current GCE models are consistent with our observational results, the ratios from models for D GC less than 6 kpc are significantly higher than the observational results, which indicates GCE models for 14 N/ 15 N and/or 13 C/ 12 C ratios need to be updated for at least this range.