Monte Carlo radiative transfer for exotic thermonuclear supernovae

The last few years have seen many exciting developments in time-domain astronomy. In particular, thanks to modern surveys that monitor large areas of the night sky on a regular basis, there has been a huge increase in the both the quantity and quality of observational data for a wide variety of astrophysical transients, including supernovae. Such data allow us to test our understanding of astrophysical phenomena in ways, and to a degree of accuracy, that was not previously possible. For example, while the basic picture that supernovae result from the explosive deaths of stars is well established, it has become clear that there is a great deal of variation within the supernova population. This applies even to classes of supernovae that have been traditionally considered rather homogenous, such as the thermonuclear “Type Ia” supernovae (Taubenberger 2017).

Our work focuses on using computer simulations to make predictions for the radiation emitted by supernovae and related stellar explosions. By comparing our results to observations, we hope to test the underlying theoretical models and to help interpret new observational data. Thus, driven by observational advances, a major component of our work in recent years has been understanding how the observed diversity can be reconciled with particular explosion scenarios.

Figure 1. Comparison of our calculated synthetic spectrum for a new double-detonation model (black line) to the observed spectrum of the unusual supernova SN 2016jhr (red) which is clearly different from the spectrum of a ‘normal’ Type Ia supernova such as SN 2011fe (shown in orange). Note that our model provides a reasonable match to both the overall shape and to many of the discreet spectral features (figure adapted from Gronow et al. 2020).

In 2020 we published new results based on simulations of a particular class of white-dwarf explosion and showed that these can provide a plausible match to certain unusual supernovae. Specifically, our work explores the “double-detonation” model in which a white dwarf star, which is composed mainly of carbon and oxygen, is able to accumulate a surface layer of helium via interaction with a companion star in a binary system. If nuclear reactions can ignite in the helium layer, it has been suggested that this may lead first to detonation of the helium which, in turn, may trigger detonation of the underlying carbon-oxygen material. This “double-detonation” scenario predicts certain key signatures owing to the structure where nuclear ash from the carbon-oxygen core is overlaid by material that originated in the helium layer. Our simulations show (see Figure; details published in Gronow et al. 2020) that the theoretical spectra predicted from such a model give a good match to particularly unusual transients, such as SN 2016jhr (Jiang et al. 2017). Such comparisons support the conclusion that transients of this sort may indeed be explainable via this class of theoretical model – one piece for the puzzle of understanding the origins of supernovae. We are now developing further such simulations to explore more fully the range of properties that double-detonation models may be able to explain – it remains to be seen whether this class of model can really account for a large fraction of the observed Type Ia supernova population or whether a mix of scenarios must be invoked to account for the variations that are observed.

This work also used the Cambridge Service for Data Driven Discovery (CSD3), part of which is operated by the University of Cambridge Research Computing on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk).

References:

Taubenberger S., 2017, in Handbook of Supernovae, ed. Alsabti & Murdin (New York Springer), 317;
Jiang J., Doi M., Maeda K., et al. 2017, Nature, 550, 80;
Gronow S., Collins C., Ohlmann S., Pakmor R., Kromer M., Seitenzahl I., Sim S., Röpke F., A&A, 2020, 635, 169.