2025A&A...695L...6R

Context. The abundance ratio of ¹⁴N/¹⁵N is, in principle, a powerful tool for tracing stellar nucleosynthesis.
Aims. This work aims to measure and analyze (¹⁴N/¹⁵N) × (¹³C/¹²C) and ¹⁴N/¹⁵N 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 H¹³CN and HC¹⁵N 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 (¹⁴N/¹⁵N) × (¹³C/¹²C) abundance ratios as well as ¹⁴N/¹⁵N ratios using the double isotope method.
Results. We find an overall decreasing trend in the (¹⁴N/¹⁵N) × (¹³C/¹²C) abundance ratio and an increasing trend in the ¹⁴N/¹⁵N 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 (¹⁴N/¹⁵N) × (¹³C/¹²C) 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 ¹⁴N/¹⁵N and/or ¹³C/¹²C ratios need to be updated for at least this range.