2018A&A...620A.200F

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 ⁵⁶Ni 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 ⁵⁶Ni, different amounts of ⁵⁷Ni and stable ^54,56Fe. We can exclude models that produced only ^54,56Fe or only ⁵⁷Ni in addition to ⁵⁶Ni. If we consider a model that has ⁵⁶Ni, ⁵⁷Ni, and ^54,56Fe then our data imply that these ratios are ^54,56Fe/ ⁵⁶Ni=0.272±0.086 and ⁵⁷Ni/ ⁵⁶Ni=0.032±0.011.