SIMBAD references

em Context. Nitrogen isotopic ratios are a key tool for tracing Galactic stellar nucleosynthesis.

em Aims. We present the first study of the ¹⁴N/¹⁵N abundance ratio in the outer regions of the Milky Way (namely, for galactocentric distances, R_GC, from 12 up to 19 kpc), with the aim to study the stellar nucleosynthesis effects in the global Galactic trend. Methods. We analysed IRAM 30 m observations towards a sample of 35 sources in the context of the CHEMical complexity in star-forming regions of the OUTer Galaxy (CHEMOUT) project. We derived the ¹⁴N/¹⁵N ratios from HCN and HNC for 14 and 3 sources, respectively, using the J = 1-0 rotational transition of HN¹³C, H¹⁵NC, H¹³CN, and HC¹⁵N.

em Results. The results found in the outer Galaxy have been combined with previous measurements obtained in the inner Galaxy. We find an overall linear decreasing H¹³CN/HC¹⁵N ratio with increasing R_GC. This translates to a parabolic ¹⁴N/¹⁵N ratio with a peak at 11 kpc. Updated Galactic chemical evolution models have been taken into account and compared with the observations. The parabolic trend of the ¹⁴N/¹⁵N ratio with R_GC can be naturally explained (i) by a model that assumes novae as the main ¹⁵N producers on long timescales (≥1 Gyr) and (ii) by updated stellar yields for low- and intermediate-mass stars.