Supernova Kinetic Yield oN galactic Emission Tracers (SKYNET) I. Contribution of radiative supernova remnants to Galactic N + emission

Abstract

Context. The radiative stage of a supernova remnant (SNR) constitutes approximately 90% of its lifetime. Yet, only a few tens of radiative supernova remnants (r-SNRs) have been observed in the Milky Way, while catalogs of young SNRs in their adiabatic stage contain $\sim$ 300 -400 confirmed sources. This deficit reflects the absence of unambiguous tracers for the radiative phase, and stands as a major obstacle to understanding the impact of SNRs on the energetic balance and the chemical state of the interstellar medium. Aims. We aim to identify spectral tracers of r-SNRs to enable their individual or statistical detection across the Galactic plane, and, more generally, to quantify the collective contribution of the Galactic r-SNR population to interstellar line emission. Methods. We present SKYNET, a new predictive framework that couples a physically motivated model of individual r-SNRs, built upon a dedicated version of the Paris-Durham shock code, with a Galactic model that describes the spatial distribution of r-SNRs and the properties of the medium into which they expand. Together, these components allow SKYNET to predict the cumulative emission of Galactic r-SNRs in nearly one million spectral lines. As a first application, we compare the predictions of SKYNET with an Herschel survey of the two fine-structure lines of singly ionized nitrogen. Results. The Galactic distribution predicted by SKYNET shows that random lines of sight toward the Galactic plane inevitably intercept multiple r-SNRs, whose cumulative emission may be associated with specific spectral tracers. SKYNET successfully reproduces the longitudinal profile of N + emission along the Galactic plane, which results from the projection on the sky of the spiral arms structure. The predicted line-ratio distribution matches the mean, the dispersion, and the tail of the observed distribution, showing that the model naturally explains the unexpectedly narrow range of physical conditions of the medium responsible for the N + emission. Comparison with the observed column densities reveals that SKYNET accounts for $\sim$ 20-25% of the total Galactic N + content, demonstrating that radiative SNRs constitute a significant, and previously underappreciated, source of ionization in the Milky Way. The difference between the observed and predicted column densities could be due to the clustering of supernova remnants into superbubbles, which is currently not taken into account by the model, and to the contribution of non-SNR sources such as H II regions. Conclusions. SKYNET offers a new framework for quantifying the cumulative emission of r-SNRs at Galactic scales that can be directly compared with observational surveys of atomic and molecular lines. The first application reveals that SNRs contribute significantly to the ionization of the ISM, highlighting a role that has largely been overlooked.