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

TL;DR

This study measures nitrogen isotopic ratios across the Milky Way's outer regions, revealing a parabolic trend with galactocentric distance that informs models of stellar nucleosynthesis and Galactic chemical evolution.

Contribution

It provides the first measurements of $^{14}$N/$^{15}$N ratios in the outer Galaxy and compares these with models, highlighting the role of novae and stellar yields in nitrogen evolution.

Findings

$^{14}$N/$^{15}$N ratio peaks at 11 kpc from the Galactic center.

Overall ratio decreases linearly with increasing galactocentric distance.

Results support models with novae as primary $^{15}$N producers.

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, $R_{\rm GC}$, 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 $R_{\rm GC}$. 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 $R_{\rm GC}$ 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.