Nebular Nitrogen Line Emission in Stripped-Envelope Supernovae -- a New Progenitor Mass Diagnostic

TL;DR

This paper introduces a new spectral line diagnostic based on nitrogen emission to determine the progenitor mass of stripped-envelope supernovae, revealing correlations with supernova types and progenitor properties.

Contribution

The study develops a novel method using nitrogen line emission to estimate the He core mass of supernova progenitors, supported by spectral modeling and comparison with observational data.

Findings

Low-mass He cores are linked to Type IIb SNe.

Absence of nitrogen emission in most Type Ib and Ic SNe.

Nitrogen line strength correlates with progenitor He core mass.

Abstract

Nitrogen is produced by CNO-cycling in massive stars, and can be ejected in significant amounts in supernova explosions. While in H-rich SNe, its [\ion{N}{II}] 6548, 6583 emission becomes obscured by strong H$\alpha$, in explosions of He stars, this nitrogen emission becomes more visible. We here explore the formation of this line, using the \texttt{SUMO} code to compute spectra for a grid of 1D models with parameterized mixing informed from new 2D simulations. Because the mass fraction of nitrogen in the ejecta decreases with larger He core masses, as more of the He/N zone gets processed by shell helium burning and is lost to winds, the [\ion{N}{II}] luminosity relative to the overall optical flux probes the He core mass. By comparing to large samples of data, we find that low-mass He cores ($M_{\rm preSN}\lesssim\ 3\ M_\odot$) are exclusively associated with Type IIb SNe, with the exception of Type Ib SN 2007Y. Seeing no strong nitrogen emission in other Type Ib SNe, the implication is either an origin from low-mass stars with the He/N layer (but not the He/C) layer peeled away, or from higher-mass He cores. We also see no clear nitrogen emission in Type Ic SNe. We discuss the diagnostic potential of this new line metric, and also dependencies on mass-loss-rate and metallicity.