An asymmetric explosion mechanism may explain the diversity of Si II linewidths in Type Ia supernovae

Near maximum brightness, the spectra of Type Ia supernovae (SNe Ia) present typical absorption features of Silicon II observed at roughly 6100 and 5750 A. The two-dimensional distribution of the pseudo-equivalent widths (pEWs) of these features is a useful tool for classifying SNe Ia spectra (Branch plot). Comparing the observed distribution of SNe on the Branch plot to results of simulated explosion models, we find that one-dimensional models fail to cover most of the distribution. In contrast, we find that TARDIS radiative transfer simulations of the white dwarf head-on collision models along different lines of sight almost fully cover the distribution. We use several simplified approaches to explain this result. We perform order-of-magnitude analysis and model the opacity of the Si II lines using local thermodynamic equilibrium and non-local thermodynamic equilibrium approximations. Introducing a simple toy model of spectral feature formation, we show that the pEW is a good tracer for the extent of the absorption region in the ejecta. Using radiative transfer simulations of synthetic SN ejecta, we reproduce the observed Branch plot distribution by varying the luminosity of the SN and the Si density profile of the ejecta. We deduce that the success of the collision model in covering the Branch plot is a result of its asymmetry, which allows for a significant range of Si density profiles along different viewing angles, uncorrelated with a range of 56Ni yields that cover the observed range of SN Ia luminosity. We use our results to explain the shape and boundaries of the Branch plot distribution.