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

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 ± 0.086 and 57 Ni / 56 Ni = 0.032 ± 0.011.

Sub-Chandrasekhar progenitors favoured for type Ia supernovae: Evidence from late-time spectroscopy★.

“…The additional observations can be used to further the work described in Flörs et al (2018). As has been noted by several authors Flörs et al 2018), the singly ionized lines of Fe, Ni and Co exhibit the same line shift and width. The same holds true for the two additional SNe with nebular phase XShooter observations presented in this work.…”

“…In the optical the rest wavelengths of the 4 659Å [Fe iii], the 5 to radioactive material, which in turn can be compared to predictions of explosion models. A similar study was performed for the NIR line ratio of the 15 470Å [Co ii] to the 15 330 AA [Fe ii] line in Flörs et al (2018). While the NIR nebular spectra are easier to model than the optical spectra, the mass ratio of Co ii to Fe ii changes with time and the number of spectra with NIR coverage is quite limited.…”

“…The red line indicates the mean flux of all fit models at each wavelength while the orange shaded area marks the 68% uncertainty of the fit. Fit results for the previously published spectra of the XShooter sample are shown in Flörs et al (2018). An exemplary zoom into the fit of SN 2017bzc at +215 days is shown in Fig.…”

“…In the NIR, we identify the strongest features as transitions of singly ionized [Fe ii] and [Co ii]. The 10 190Å (a 3 F 4 -b 3 F 4 ) transition of [Co ii] decreases in strength according to the decay of 56 Co to 56 Fe Flörs et al 2018). The emission feature at around 13 000Å is identified as the 12 570Å a 6 D-a 4 D multiplet of [Fe ii].…”

“…We use a one-zone model as described in Flörs et al (2018). We extend the model to include all first and second ionisation stages of iron, nickel and cobalt (see Table 3).…”