CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy III. Nitrogen isotopic ratios in the outer Galaxy

Abstract

Nitrogen isotopic ratios are a key tool for tracing Galactic stellar nucleosynthesis. We present the first study of the ${}^{14}$N/${}^{15}$N abundance ratio in the outer regions of the Milky Way (namely, for galactocentric distances, $R_{\mathrm{GC}}$, from 12 kpc up to 19 kpc), with the aim to study the stellar nucleosynthesis effects in the global Galactic trend. We analysed IRAM 30m 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 ${}^{14}$N/${}^{15}$N ratios from HCN and HNC for 14 and 3 sources, respectively, using the $J$ = 1-0 rotational transition of HN${}^{13}$C, H${}^{15}$NC, H${}^{13}$CN, and HC${}^{15}$N. The results found in the outer Galaxy have been combined with previous measurements obtained in the inner Galaxy. We find an overall linear decreasing H${}^{13}$CN/HC${}^{15}$N ratio with increasing $R_{\mathrm{GC}}$. This translates to a parabolic ${}^{14}$N/${}^{15}$N 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 ${}^{14}$N/${}^{15}$N ratio with $R_{\mathrm{GC}}$ can be naturally explained (i) by a model that assumes novae as the main ${}^{15}$N producers on long timescales ($\ge$1 Gyr) and (ii) by updated stellar yields for low- and intermediate-mass stars.

19 pages, 8 figures, 6 tables, 4 appendix - Accepted for publication in A&A