Multi-dimensional Monte Carlo radiative transfer simulations : investigating the double detonation scenario for type Ia supernovae

Type Ia supernovae (SNe Ia) are still poorly understood, despite being widely-studied across many fields of astrophysics. Explosion simulations have shown that sub-Chandrasekhar mass white dwarfs are promising candidates for the progenitors of SNe Ia, and can reproduce a number of the observed features of SNe Ia. The 'double detonation' explosion mechanism is a widely discussed scenario by which an explosion in a sub-Chandrasekhar mass white dwarf may be ignited. In this scenario, helium accreted onto the surface of a carbon-oxygen white dwarf detonates, igniting a secondary carbon detonation in the white dwarf core. However, apparent discrepancies are found between models and observations due to strong absorption caused by the products of the helium shell detonation. In particular, red colours and strong Ti II absorption features are predicted that are not observed in normal SNe Ia. In this work, we present new simulations of the double detonation scenario with the aim of determining whether improved simulation methods can reduce these discrepancies. We have carried out radiative transfer simulations using the 3D code ARTIS to make predictions of the synthetic light curves and spectra for a series of double detonation explosion models. We initially tested whether improvements to the level of sophistication in the hydrodynamic explosion simulations can improve agreement with observations, making use of the standard version of ARTIS (ARTIS-classic), which approximates non-LTE. However, we found that these results were similar to previous work, and discrepancies still remain between models and observations. We tested the full non-LTE implementation of ARTIS (ARTIS-NLTE) at phases around maximum light for the first time. We found that ARTIS-NLTE produces quantitative differences in the predicted synthetic observables compared to ARTIS-classic. These differences were particularly significant for double detonation models. The heavy elements in the ejecta outer layers, produced in the helium shell detonation, are strongly affected by the improved treatment of ionisation in ARTIS-NLTE. The full non-LTE simulations predict significantly less absorption by the products of the helium shell detonation, such that colours become less red, and show better agreement with those of normal SNe Ia. However, we still find strong Ti II spectral features, not observed in normal brightness SNe Ia. While the full non-LTE treatment has reduced the discrepancies between models and observations, further work is required to determine whether the helium detonation products (particularly Ti) can be reduced. We also make predictions of helium spectral features for the double detonation scenario, and compare to observations. These results indicate that the direct detection of helium spectral features is a potential signature of the double detonation.