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

Abstract

The late-time spectra of Type Ia supernovae (SNe Ia) are powerful probes of\nthe underlying physics of their explosions. We investigate the late-time\noptical and near-infrared spectra of seven SNe Ia obtained at the VLT with\nXShooter at $>$200 d after explosion. At these epochs, the inner Fe-rich ejecta\ncan be studied. We use a line-fitting analysis to determine the relative line\nfluxes, velocity shifts, and line widths of prominent features contributing to\nthe spectra ([Fe II], [Ni II], and [Co III]). By focussing on [Fe II] and [Ni\nII] emission lines in the ~7000-7500 \\AA\\ region of the spectrum, we find that\nthe ratio of stable [Ni II] to mainly radioactively-produced [Fe II] for most\nSNe Ia in the sample is consistent with Chandrasekhar-mass delayed-detonation\nexplosion models, as well as sub-Chandrasekhar mass explosions that have\nmetallicity values above solar. The mean measured Ni/Fe abundance of our sample\nis consistent with the solar value. The more highly ionised [Co III] emission\nlines are found to be more centrally located in the ejecta and have broader\nlines than the [Fe II] and [Ni II] features. Our analysis also strengthens\nprevious results that SNe Ia with higher Si II velocities at maximum light\npreferentially display blueshifted [Fe II] 7155 \\AA\\ lines at late times. Our\ncombined results lead us to speculate that the majority of normal SN Ia\nexplosions produce ejecta distributions that deviate significantly from\nspherical symmetry.\n