Limits on stable iron in Type Ia supernovae from near-infrared spectroscopy

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