Letter to the Editor

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

¹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
³ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210033, China

^⋆ Corresponding authors; junzhiwang@gxu.edu.cn; changruan@st.gxu.edu.cn

Received: 2 November 2024
Accepted: 3 February 2025

Abstract

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.