ASTROPHYSICS

Summary

In project CHMU14, challenging three-dimensional simulations of thermonuclear explosions of white dwarf stars near the Chandrasekhar-mass limit were conducted. These were followed by radiative transfer simulations that allow to predict observables. A comparison with astronomical data shows that such models can explain the subclass of Type Iax supernovae.

Simulating supernova explosions resulting from explosions of Chandrasekhar-mass white dwarf stars

Type Ia supernovae are bright and can be observed over large parts of the Universe. A unique feature of this class of supernovae is that they seem to be rather homogeneous in their observable properties. This allows to calibrate their peak brightnesses and enables cosmological distance measurements. The spectacular result of these observations was the discovery of the accelerated expansion of the Universe, for which the Nobel Prize in Physics was awarded in 2011.

Despite a wealth of detailed astronomical data, the physical explosion mechanism of Type Ia supernovae remains a puzzle. It is generally agreed that these events are caused by thermonuclear explosions of white dwarf stars consisting of carbon and oxygen. For a consistent physical model, however, it has to be clarified (1) what the mass of the white dwarf at the onset of the explosion is and (2) how the thermonuclear burning initiates. Astronomical observations offer few clues here, as the progenitor stars of the Type Ia supernova are faint (in contrast to the actual explosion event) and direct observation is still pending. Moreover, white dwarf stars are in principle eternally stabilized by a quantum mechanical effect – the Fermi pressure of the degenerate electrons in their material.

A standard scenario addressing issues (1) and (2) is that the white dwarf star accretes material from a companion until it reaches the limit of its stability – the famous Chandrasekhar limit of 1.4 solar masses. This model has been favored for a long time because it seems to naturally set the state of the exploding object to a configuration with a fixed mass and it explains the ignition: Approaching the Chandrasekhar mass, the density in the center of a white dwarf raises steeply and nuclear burning in the carbon-oxygen material of it becomes inevitable. There are, however, several arguments against this scenario accounting for the majority of Type Ia supernovae. In previous accounting periods of project CHMU14, alternative explosion models and their implications were investigated and the results look promising for explaining Type Ia supernovae.

Do we therefore abandon the classical Chandrasekhar-mass explosion scenario? Progress in astronomical observations revealed that the class of Type Ia supernova is not quite as homogeneous as initially thought. Although the majority of observed objects – usually referred to as normal Type Ia supernovae – still shows very similar properties, several peculiar sub-classes have been identified and some of them occur rather frequently.  As previous results from project CHMU14 indicate, spernovae of subclass Type Iax may be reproduced with models that assume explosions of white dwarf stars close to the Chandrasekhar-mass limit. The goal of the reporting period was to follow up on this hypothesis to test its validity in a combination of simulations of the explosion stage and the formation of observables that then can be compared to astronomical observations.

Three-dimensional hydrodynamic simulations of the explosion stage face a particular challenge due to the wide range of involved spatial scales: While the width of thermonuclear combustion fronts is only on the order of millimeters to centimeters, a Chandrasekhar-mass white dwarf star has a radius of about 2000 kilometers and it expands in the explosion. Another challenge arises from the fact that subsonic burning in form of a thermonuclear deflagration (which in many respects resembles chemical flames) is subject to instabilities and interacts with turbulent motions. This accelerates the combustion and can give rise to an explosion. The bright optical emission of thermonuclear supernova explosions arises from the nuclear burning of carbon and oxygen material to iron group elements, particularly the isotope nickel-56. This isotope is unstable and decays releasing gamma rays. In the dense ejecta of the supernova explosion, the gamma photons interact with ions of various elements produced in the explosion and are scattered down to optical wavelengths. Following this radiation transport process numerically is extremely challenging but allows to predict optical observables that can be directly compared to astronomical data.

To address these challenges, the specialized and highly efficient hydrodynamic supernova explosion code LEAFS and the Monte-Carlo based radiation transport code ARTIS were employed in project CHMU14. Both the explosion simulations and the radiative transport calculations were performed in three spatial dimensions. This can only be achieved on massively parallel supercomputers and therefore the simulations were carried out on the JUWELS cluster.

While some brighter objects of the Type Iax supernova class could be reproduced by earlier simulations, the goal was to explore the faint part of the distribution. To this end, simulations were performed of asymmetric explosions, that ignite off-center in the white dwarf star. An example for such a simulation is shown in the Figure.