Limits on stable iron in Type$\,$Ia supernovae from NIR spectroscopy

TL;DR

This study uses optical and near-infrared spectra of Type Ia supernovae at late epochs to analyze iron and cobalt emission lines, constraining explosion models and isotope ratios through NLTE modelling.

Contribution

It provides the first detailed NLTE spectral analysis of late-time SN Ia spectra to derive isotope ratios and test explosion models against observed data.

Findings

Radioactive decay of $^{56}$Ni explains cobalt and iron emission.

Excluded models with only $^{54,56}$Fe or only $^{57}$Ni.

Measured isotope ratios are $^{54,56}$Fe / $^{56}$Ni = 0.272±0.086 and $^{57}$Ni / $^{56}$Ni = 0.032±0.011.

Abstract

We obtained optical and near-infrared spectra of Type$\,$Ia supernovae (SNe$\,$Ia) at epochs ranging from 224 to 496 days after the explosion. The spectra show emission lines from forbidden transitions of singly ionised iron and cobalt atoms. We used non-local thermodynamic equilibrium (NLTE) modelling of the first and second ionisation stages of iron, nickel, and cobalt to fit the spectra using a sampling algorithm allowing us to probe a broad parameter space. We derive velocity shifts, line widths, and abundance ratios for iron and cobalt. The measured line widths and velocity shifts of the singly ionised ions suggest a shared emitting region. Our data are fully compatible with radioactive $^{56}$Ni decay as the origin for cobalt and iron. We compare the measured abundance ratios of iron and cobalt to theoretical predictions of various SN$\,$Ia explosion models. These models include, in addition to $^{56}$Ni, different amounts of $^{57}$Ni and stable $^{54,56}$Fe. We can exclude models that produced only $^{54,56}$Fe or only $^{57}$Ni in addition to $^{56}$Ni. If we consider a model that has $^{56}$Ni, $^{57}$Ni, and $^{54,56}$Fe then our data imply that these ratios are $^{54,56}$Fe / $^{56}$Ni $=0.272\pm0.086$ and $^{57}$Ni / $^{56}$Ni $=0.032\pm0.011$.