CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy

Abstract

Context. Nitrogen isotopic ratios are a key tool for tracing Galactic stellar nucleosynthesis. Aims. We present the first study of the 14N/15N abundance ratio in the outer regions of the Milky Way (namely, for galactocentric distances, RGC, 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. 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 RGC. 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 RGC 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.