2011MNRAS.416..881M

The optical and near-infrared (near-IR) late-time spectra of the under-luminous Type Ia supernova 2003hv are analysed with a code that computes the nebular emission from a supernova nebula. Synthetic spectra based on the classical explosion model W7 are unable to reproduce the large Fe III/Fe II ratio and the low IR flux at ∼1 year after the explosion, although the optical spectrum of SN 2003hv is reproduced reasonably well for a supernova of luminosity intermediate between normal and subluminous (SN 1991bg-like) ones. A possible solution is that the inner layers of the supernova ejecta (v ≲ 8000 km/s) contain less mass than predicted by classical explosion models like W7. If this inner region contains ∼ 0.5 M_☉ of material, as opposed to ∼ 0.9 M_☉ in Chandrasekhar-mass models developed within the single-degenerate scenario, the low density inhibits recombination, favouring the large Fe III/Fe II ratio observed in the optical, and decreases the flux in the [Fe II] lines which dominate the IR spectrum. The most likely scenario may be an explosion of a sub-Chandrasekhar-mass white dwarf. Alternatively, the violent/dynamical merger of two white dwarfs with the combined mass exceeding the Chandrasekhar limit also shows a reduced inner density.