Supernova Simulations Point to White Dwarf Mergers

Type Ia supernovae, some of the most violent and luminous explosions in the Universe, have become a handy tool for astronomers to measure the size and expansion of the Universe itself. Because they explode with a rather specific peak luminosity, they can be used as “standard candles” to measure distances. New research presented at the American Astronomical Society meeting this week points to the increased likelihood that the mergers of the stars that create these explosions, white dwarfs, is more likely than previously thought, and could explain the properties of some Type Ia supernovae that are curiously less luminous than expected.

Research presented by Rüdiger Pakmor et al. from the Max-Planck Institute for Astrophysics in Garching, Germany simulated the merger of two white dwarfs in a binary system, and showed that these simulations match previously observed supernovae with odd characteristics, specifically that of 1991bg. That supernova, and others observed since, was curiously less luminous than should have been expected if it were a Type Ia supernovae.

Type Ia supernovae occur when there are two stars orbiting each other in a binary system. In one scenario, one of the stars becomes a white dwarf, a small but very, very dense star, and steals matter from the other, pushing itself over the Chandrasekhar limit – 1.4 times the mass of the Sun – and undergoing a thermonuclear explosion.

Another cause for these types of supernovae could be the merger of both the stars in the system. In the scenario analyzed by these researchers, both stars were white dwarfs of masses just under that of the Sun: .83-0.9 solar masses.

The researchers showed that as the system loses energy due to the emission of gravitational waves, the two white dwarfs approach each other. As they merge, part of the material in one of the stars crashes into the other and heats up the carbon and oxygen, creating a thermonuclear explosion seen in Type Ia supernovae.

You can watch an animation of the simulated merger courtesy of the Max-Planck Institute’s Supernova Research Group right here.

Observations of supernovae like 1991bg show them to burn a smaller amount of nickel 56, about 0.1 solar masses, than regular Type Ia supernovae, which typically burn 0.4-0.9 solar masses of nickel. This makes them less luminous, because the radiative decay of the nickel is one of the phenomenon that gives the luminous display of Type Ia supernovae its punch.

“With our detailed explosion simulations, we could predict observables that indeed closely match actual observations of Type Ia supernovae,” said Friedrich Röpke, a co-author of the paper.

Their simulations show that when the two white dwarfs merge, the density of the system is less than in typical Type Ia supernovae, and thus less nickel is produced. The researchers note in their paper that these types of white dwarf mergers could comprise between 2-11 percent of the Type Ia supernovae observed.

Understanding the mechanisms that create these fantastic explosions is a necessary step in getting a handle on both the extent of our Universe and its expansion, as well as the diversity of Type Ia supernovae themselves.

If you would like to learn more about their research and the details of their computer modeling, the paper is available on Arxiv here. Their results will also be published in the January 7, 2010 edition of Nature.

Source: AAS press release, Arxiv paper