Rise and Shine: Type Ia supernova models at early times

October 01, 2017
Type Ia supernovae (SNe Ia) are spectacular explosions in white dwarf stars and play an essential role in astrophysics in general and in cosmological studies in particular. However, many puzzles about the nature and the inherent physical mechanisms in SNe Ia are still waiting to be answered. Robotic surveys of the next decade will provide an unprecedented wealth of observed Type Ia supernovae, detected shortly after explosion. Researchers at MPA examine here whether different explosion models are expected to leave clear imprints in such early observations that could be used in future photometric surveys to help shedding light on the progenitors and explosion mechanism of SNe Ia.

Most likely, you are reading this article using a device whose existence relies on the silicon chip, such as a PC, laptop or mobile phone. Together with a number of other chemical elements such as iron, a significant fraction of the silicon in our Universe today has been forged from lighter elements in the thermonuclear fires raging in cataclysmic events known as "Type Ia supernovae" (SNe Ia). These violent explosions mark the brilliant death of a low mass star. During their evolution, SNe Ia can become incredibly bright – to the point at which they outshine their host galaxies (see for example SN 1994D shown in Figure 1).

This is one of the properties that make SNe Ia ideal for cosmological studies in which they are frequently used as distance indicators mapping out the recent expansion history of the Universe. Specifically, SNe Ia were instrumental in establishing our current cosmological picture which involves a dark energy component responsible for the accelerated expansion. This discovery was recognized by the Nobel prize committee in 2011. However, despite their astrophysical and cosmological significance, astrophysicists are still in the dark about many aspects concerning SNe Ia.

It is broadly accepted that the supernova marks a thermonuclear explosion in a white dwarf made up of mainly carbon and oxygen that has been part of a binary system. White dwarfs are compact objects which are stabilized by electron degeneracy pressure. They are the evolutionary end state of low mass stars after their nuclear fuel has been exhausted. However, it is still heavily debated what the nature of the companion is, whether it is a sun-like or giant star or another white dwarf.

Moreover, the details of how the thermonuclear explosion is triggered and how it proceeds are still under active investigation. In particular, it is not clear if the burning front propagates as a supersonic detonation, as a subsonic turbulent deflagration, or whether a mixture of both modes is realized and the burning starts subsonically and then transitions into a detonation (delayed detonation model).

Related to the previous questions, it is still unclear at which mass the white dwarf explodes, in particular whether the supernova sets in at the theoretical mass limit for systems stabilized by electron degeneracy pressure (about 1.4 times the mass of our sun), or below it. This limit is referred to as "Chandrasekhar mass" and consequently one distinguishes Chandrasekhar mass and sub-Chandrasekhar mass models. In the latter case, the explosion can for example be triggered by a merger with another white dwarf.

Finally, it still has to be firmly established whether one scenario is exclusively responsible for SNe Ia or whether a mixture of the different explosion and progenitor possibilities is realised in nature.

Researchers at MPA performed a theoretical study, developing predictions for the early optical appearance for a number of common explosion models for standard SNe Ia. They focussed specifically on identifying clear signatures in the early light curve, i.e. the time evolution of the emission in a particular passband. Such a signature would make it possible to clearly identify specific explosion scenarios from early photometric observations.

The reason for the interest and focus on early observables is two-fold: currently, the tightest constraints on the nature of SN Ia progenitors come from the earliest data points shortly after explosion. Moreover, upcoming high-cadence surveys and upgrades of existing transient search programmes will drastically increase the number of SNe Ia detected shortly after explosion.

For the main part of the study, the scientists selected two Chandrasekhar mass explosion models, namely the well-known carbon deflagration model W7 and the delayed detonation model N100. In addition, they focussed on three sub-Chandrasekhar models, in particular a merger of two white dwarfs, a double detonation in a carbon-oxygen white dwarf with a helium shell and a pure detonation in a white dwarf core. Using the radiation hydrodynamical code Stella, they followed the supernova ejecta evolution in all these models and calculated colour light curves in various pass bands (see Figure 2). 

While for most scenarios, the light curves of the various models evolve similarly, the double detonation model shows a steep rise and a pronounced first shoulder due to radioactive material located close to the ejecta surface. This material has been synthesized in the first detonation in the Helium shell. Unfortunately, this signature is very similar to the traces left by the interaction between ejecta and a companion star or ejecta and circumstellar material, which have been investigated by other groups, rendering it a challenge to establish a clear link between such a feature in the early observables and the physical properties of the explosion scenario.

Investigating the early light curves in more detail, the researchers found that none of the standard models follow a power-law rise. However, such a behaviour, namely that the emitted luminosity increases proportional to some power of the time since explosion, is often assumed when reconstructing the explosion date from observational data. The scientists demonstrate that this can lead to errors of several days in determining the explosion date without degrading the fidelity of the fits. Potentially, this has severe consequences for estimating the size and nature of the exploding object from early data, which requires a precise determination of the time of explosion.

In summary, the researchers demonstrated that it is very challenging to identify specific explosion scenarios based on early photometric data alone. The additional availability of early spectroscopic information may help to break some of the degeneracy. Unlike typically assumed, they predict an early non-power law rise for all of the investigated standard explosion models. This can lead to serious difficulties in dating the explosion and deriving constraints about the nature of the exploding object.

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