Using late-time optical and near-infrared spectra to constrain Type Ia supernova explosion properties

The late-time spectra of Type Ia supernovae (SNe Ia) are powerful probes ofthe underlying physics of their explosions. We investigate the late-timeoptical and near-infrared spectra of seven SNe Ia obtained at the VLT withXShooter at >200 d after explosion. At these epochs, the inner Fe-rich ejectacan be studied. We use a line-fitting analysis to determine the relative linefluxes, velocity shifts, and line widths of prominent features contributing tothe spectra ([Fe II], [Ni II], and [Co III]). By focussing on [Fe II] and [NiII] emission lines in the ~7000-7500 \AA\ region of the spectrum, we find thatthe ratio of stable [Ni II] to mainly radioactively-produced [Fe II] for mostSNe Ia in the sample is consistent with Chandrasekhar-mass delayed-detonationexplosion models, as well as sub-Chandrasekhar mass explosions that havemetallicity values above solar. The mean measured Ni/Fe abundance of our sampleis consistent with the solar value. The more highly ionised [Co III] emissionlines are found to be more centrally located in the ejecta and have broaderlines than the [Fe II] and [Ni II] features. Our analysis also strengthensprevious results that SNe Ia with higher Si II velocities at maximum lightpreferentially display blueshifted [Fe II] 7155 \AA\ lines at late times. Ourcombined results lead us to speculate that the majority of normal SN Iaexplosions produce ejecta distributions that deviate significantly fromspherical symmetry.