Kinetic Plasma Turbulence: Determining the shape of Type Ia supernova explosions

Kinetic Plasma Turbulence: Determining the shape of Type Ia supernova explosions

Type Ia Supernovae (SNe Ia) are among the most energetic phenomena in the Universe and are important for many different astrophysical research fields (e.g. theories of binary stellar evolution, chemical enrichment of galaxies, and cosmology). These spectacular events are believed to be associated with thermonuclear explosions of white dwarf stars but answers to the questions of when, why and how the explosions are triggered remain unclear.

 Figure 1. Flux and polarization spectra (black) predicted 7 days before maximum for the merger model of Pakmor et al. (2012) viewed from a specific orientation. For comparison, grey lines show observed spectra of SN 2004dt at the same epoch.

In the past few decades spectropolarimetry has 
played a leading role in the search for a comprehensive picture of SNe Ia (Wang & Wheeler 2008). 
The particular value of this technique is its ability to probe the geometry of the explosion: linear polarization, attributed to electron scattering, has been detected in SNe Ia, suggesting that asymmetries 
in the progenitor system and/or explosion mechanism are 
present. However, interpreting polarimetric data is challenging and careful modelling is required to make the connections between observed spectropolarimetry and specific ejecta geometries. Our current research aims to address this challenge by making polarization prediction from modern multi-dimensional explosion models.

We recently developed a new Monte Carlo technique for use in our radiative transfer code (Kromer & Sim 2009) that allows us to calculate polarization signatures for hydrodynamical explosion models (Bulla et al. 2015). Compared to simpler techniques, this method leads to a substantial reduction in the Monte Carlo noise and thus in the required computing time (a factor of ~ 50). The combination of this new technique and access to the super-computing facilities provided by DiRAC has now made it feasible to compute polarization spectra for fully three-dimensional explosion models using simulations that run for only a few days.

In a recent paper (Bulla et al. 2016), we presented polarization calculations for the violent merger of two carbon-oxygen white dwarfs. The violent merger model has received considerable attention in the past few years, but our study is the first to investigate its spectropolarimetric signatures in detail. We demonstrated that the particular merger model of Pakmor et al. (2012) provides a remarkably good match with some highly-polarized SNe Ia (see Fig. 1), but is too asymmetric to reproduce the low levels of polarization commonly seen in many other SNe Ia. We are now extending our studies to alternative explosion scenarios: this will allow us to determine how spectropolarimetry can be most effectively used to distinguish and test explosion theories.

Bibliography: Bulla et al. 2015, MNRAS, 450, 967; Bulla et al. 2016, MNRAS, 455, 1060; Kromer & Sim 2009, MNRAS, 398, 1809; Pakmor et al. 2012, ApJL, 747, L10; Wang & Wheeler 2008, ARA&A, 46, 433.